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@ -1,24 +1,15 @@ |
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Frequently Asked Questions (FAQ) for PostgreSQL |
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Last updated: Fri Jun 2 11:32:13 EDT 2000 |
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Last updated: Tue Oct 17 00:21:20 EDT 2000 |
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us) |
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The most recent version of this document can be viewed at the |
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postgreSQL Web site, http://www.PostgreSQL.org. |
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The most recent version of this document can be viewed at |
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http://www.PostgreSQL.org/docs/faq-english.html. |
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Linux-specific questions are answered in |
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http://www.PostgreSQL.org/docs/faq-linux.html. |
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HPUX-specific questions are answered in |
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http://www.PostgreSQL.org/docs/faq-hpux.html. |
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Solaris-specific questions are answered in |
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http://www.postgresql.org/docs/faq-solaris.html. |
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Irix-specific questions are answered in |
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http://www.PostgreSQL.org/docs/faq-irix.html. |
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Platform-specific questions are answered at |
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http://www.PostgreSQL.org/docs/. |
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_________________________________________________________________ |
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General Questions |
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@ -28,9 +19,9 @@ |
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1.3) What Unix platforms does PostgreSQL run on? |
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1.4) What non-unix ports are available? |
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1.5) Where can I get PostgreSQL? |
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1.6) Where can I get support for PostgreSQL? |
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1.7) What is the latest release of PostgreSQL? |
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1.8) What documentation is available for PostgreSQL? |
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1.6) Where can I get support? |
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1.7) What is the latest release? |
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1.8) What documentation is available? |
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1.9) How do I find out about known bugs or missing features? |
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1.10) How can I learn SQL? |
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1.11) Is PostgreSQL Y2K compliant? |
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@ -53,8 +44,8 @@ |
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/usr/local/pgsql? |
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3.3) When I start the postmaster, I get a Bad System Call or core |
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dumped message. Why? |
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3.4) When I try to start the postmaster, I get IpcMemoryCreate |
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errors3. Why? |
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3.4) When I try to start the postmaster, I get IpcMemoryCreate errors. |
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Why? |
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3.5) When I try to start the postmaster, I get IpcSemaphoreCreate |
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errors. Why? |
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3.6) How do I prevent other hosts from accessing my PostgreSQL |
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@ -63,22 +54,22 @@ |
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3.8) Why can't I access the database as the root user? |
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3.9) All my servers crash under concurrent table access. Why? |
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3.10) How do I tune the database engine for better performance? |
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3.11) What debugging features are available in PostgreSQL? |
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3.12) I get 'Sorry, too many clients' when trying to connect. Why? |
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3.13) What are the pg_psort.XXX files in my database directory? |
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3.11) What debugging features are available? |
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3.12) I get "Sorry, too many clients" when trying to connect. Why? |
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3.13) What are the pg_sorttempNNN.NN files in my database directory? |
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Operational Questions |
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4.1) The system seems to be confused about commas, decimal points, and |
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date formats. |
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4.1) Why is the system confused about commas, decimal points, and date |
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formats. |
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4.2) What is the exact difference between binary cursors and normal |
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cursors? |
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4.3) How do I select only the first few rows of a query? |
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4.4) How do I get a list of tables, or other things I can see in psql? |
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4.3) How do I SELECT only the first few rows of a query? |
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4.4) How do I get a list of tables or other things I can see in psql? |
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4.5) How do you remove a column from a table? |
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4.6) What is the maximum size for a row, table, database? |
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4.7) How much database disk space is required to store data from a |
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typical flat file? |
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typical text file? |
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4.8) How do I find out what indices or operations are defined in the |
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database? |
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4.9) My queries are slow or don't make use of the indexes. Why? |
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@ -86,14 +77,14 @@ |
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4.11) What is an R-tree index? |
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4.12) What is Genetic Query Optimization? |
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4.13) How do I do regular expression searches and case-insensitive |
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regexp searching? |
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regular expression searches? |
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4.14) In a query, how do I detect if a field is NULL? |
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4.15) What is the difference between the various character types? |
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4.16.1) How do I create a serial/auto-incrementing field? |
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4.16.2) How do I get the value of a serial insert? |
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4.16.2) How do I get the value of a SERIAL insert? |
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4.16.3) Don't currval() and nextval() lead to a race condition with |
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other concurrent backend processes? |
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4.17) What is an oid? What is a tid? |
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other users? |
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4.17) What is an OID? What is a TID? |
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4.18) What is the meaning of some of the terms used in PostgreSQL? |
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4.19) Why do I get the error "FATAL: palloc failure: memory |
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exhausted?" |
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@ -108,13 +99,13 @@ |
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5.1) I wrote a user-defined function. When I run it in psql, why does |
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it dump core? |
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5.2) What does the message: NOTICE:PortalHeapMemoryFree: 0x402251d0 |
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not in alloc set! mean? |
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5.3) How can I contribute some nifty new types and functions for |
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5.2) What does the message "NOTICE:PortalHeapMemoryFree: 0x402251d0 |
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not in alloc set!" mean? |
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5.3) How can I contribute some nifty new types and functions to |
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PostgreSQL? |
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5.4) How do I write a C function to return a tuple? |
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5.5) I have changed a source file. Why does the recompile does not see |
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the change? |
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5.5) I have changed a source file. Why does the recompile not see the |
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change? |
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_________________________________________________________________ |
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General Questions |
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@ -127,14 +118,14 @@ |
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replaces the PostQuel query language with an extended subset of SQL. |
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PostgreSQL is free and the complete source is available. |
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PostgreSQL development is being performed by a team of Internet |
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developers who all subscribe to the PostgreSQL development mailing |
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list. The current coordinator is Marc G. Fournier |
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(scrappy@postgreSQL.org). (See below on how to join). This team is now |
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responsible for all current and future development of PostgreSQL. |
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PostgreSQL development is performed by a team of Internet developers |
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who all subscribe to the PostgreSQL development mailing list. The |
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current coordinator is Marc G. Fournier (scrappy@PostgreSQL.org). (See |
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below on how to join). This team is now responsible for all |
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development of PostgreSQL. |
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The authors of PostgreSQL 1.01 were Andrew Yu and Jolly Chen. Many |
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others have contributed to the porting, testing, debugging and |
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others have contributed to the porting, testing, debugging, and |
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enhancement of the code. The original Postgres code, from which |
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PostgreSQL is derived, was the effort of many graduate students, |
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undergraduate students, and staff programmers working under the |
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@ -149,7 +140,7 @@ |
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1.2) What's the copyright on PostgreSQL? |
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PostgreSQL is subject to the following COPYRIGHT. |
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PostgreSQL is subject to the following COPYRIGHT: |
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PostgreSQL Data Base Management System |
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@ -204,36 +195,32 @@ |
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A file win31.mak is included in the distribution for making a Win32 |
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libpq library and psql. |
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The database server is now working on Windows NT using the Cygnus |
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Unix/NT porting library. See pgsql/doc/README.NT in the distribution. |
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There is also a web page at |
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http://www.freebsd.org/~kevlo/postgres/portNT.html. There is another |
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port using U/Win at http://surya.wipro.com/uwin/ported.html. |
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The database server is now working on Windows NT using Cygwin, the |
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Cygnus Unix/NT porting library. See pgsql/doc/FAQ_NT in the |
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distribution. It does not work on MS Windows 9X because Cygwin does |
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not support the features we need on those platforms. |
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1.5) Where can I get PostgreSQL? |
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The primary anonymous ftp site for PostgreSQL is |
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ftp://ftp.postgreSQL.org/pub |
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ftp://ftp.PostgreSQL.org/pub. For mirror sites, see our main Web site. |
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For mirror sites, see our main web site. |
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1.6) Where can I get support for PostgreSQL? |
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1.6) Where can I get support? |
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There is no official support for PostgreSQL from the University of |
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California, Berkeley. It is maintained through volunteer effort. |
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There is no support for PostgreSQL from the University of California, |
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Berkeley. It is maintained through volunteer effort. |
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The main mailing list is: pgsql-general@postgreSQL.org. It is |
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The main mailing list is: pgsql-general@PostgreSQL.org. It is |
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available for discussion of matters pertaining to PostgreSQL. To |
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subscribe, send a mail with the lines in the body (not the subject |
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line) |
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subscribe, send mail with the following lines in the body (not the |
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subject line) |
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subscribe |
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end |
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to pgsql-general-request@postgreSQL.org. |
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to pgsql-general-request@PostgreSQL.org. |
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There is also a digest list available. To subscribe to this list, send |
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email to: pgsql-general-digest-request@postgreSQL.org with a BODY of: |
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email to: pgsql-general-digest-request@PostgreSQL.org with a body of: |
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subscribe |
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end |
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@ -241,14 +228,14 @@ |
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has received around 30k of messages. |
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The bugs mailing list is available. To subscribe to this list, send |
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email to bugs-request@postgreSQL.org with a BODY of: |
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email to pgsql-bugs-request@PostgreSQL.org with a body of: |
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subscribe |
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end |
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There is also a developers discussion mailing list available. To |
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subscribe to this list, send email to hackers-request@postgreSQL.org |
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with a BODY of: |
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subscribe to this list, send email to |
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pgsql-hackers-request@PostgreSQL.org with a body of: |
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subscribe |
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end |
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@ -256,43 +243,42 @@ |
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Additional mailing lists and information about PostgreSQL can be found |
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via the PostgreSQL WWW home page at: |
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http://postgreSQL.org |
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http://www.PostgreSQL.org |
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There is also an IRC channel on EFNet, channel #PostgreSQL. I use the |
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unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net |
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unix command irc -c '#PostgreSQL' "$USER" irc.phoenix.net. |
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Commercial support for PostgreSQL is available at |
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http://www.pgsql.com/ |
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http://www.pgsql.com/. |
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1.7) What is the latest release of PostgreSQL? |
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1.7) What is the latest release? |
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The latest release of PostgreSQL is version 7.0.2. |
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We plan to have major releases every four months. |
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1.8) What documentation is available for PostgreSQL? |
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1.8) What documentation is available? |
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Several manuals, manual pages, and some small test examples are |
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included in the distribution. See the /doc directory. You can also |
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browse the manual on-line at http://www.postgresql.org/docs/postgres. |
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in the distribution. |
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browse the manual online at http://www.PostgreSQL.org/docs/postgres. |
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There is a PostgreSQL book availiable at |
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http://www.postgresql.org/docs/awbook.html |
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There is a PostgreSQL book available at |
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http://www.PostgreSQL.org/docs/awbook.html. |
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psql has some nice \d commands to show information about types, |
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operators, functions, aggregates, etc. |
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The web site contains even more documentation. |
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Our Web site contains even more documentation. |
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1.9) How do I find out about known bugs or missing features? |
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PostgreSQL supports an extended subset of SQL-92. See our TODO for a |
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list of known bugs, missing features, and future plans. |
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PostgreSQL supports an extended subset of SQL-92. See our TODO list |
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for known bugs, missing features, and future plans. |
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1.10) How can I learn SQL? |
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The PostgreSQL book at http://www.postgresql.org/docs/awbook.html |
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The PostgreSQL book at http://www.PostgreSQL.org/docs/awbook.html |
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teaches SQL. There is a nice tutorial at |
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http://w3.one.net/~jhoffman/sqltut.htm and at |
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http://ourworld.compuserve.com/homepages/graeme_birchall/HTM_COOK.HTM. |
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@ -300,9 +286,9 @@ |
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Another one is "Teach Yourself SQL in 21 Days, Second Edition" at |
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http://members.tripod.com/er4ebus/sql/index.htm |
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Many of our users like The Practical SQL Handbook, Bowman et al., |
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Addison Wesley. Others like The Complete Reference SQL, Groff et al., |
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McGraw-Hill. |
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Many of our users like The Practical SQL Handbook, Bowman, Judith S., |
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et al., Addison-Wesley. Others like The Complete Reference SQL, Groff |
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et al., McGraw-Hill. |
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1.11) Is PostgreSQL Y2K compliant? |
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@ -310,22 +296,23 @@ |
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1.12) How do I join the development team? |
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First, download the latest sources and read the PostgreSQL Developers |
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documentation on our web site, or in the distribution. Second, |
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First, download the latest source and read the PostgreSQL Developers |
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documentation on our Web site, or in the distribution. Second, |
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subscribe to the pgsql-hackers and pgsql-patches mailing lists. Third, |
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submit high-quality patches to pgsql-patches. |
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There are about a dozen people who have COMMIT privileges to the |
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PostgreSQL CVS archive. All of them have submitted so many |
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high-quality patches that it was a pain for the existing committers to |
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keep up, and we had confidence that patches they committed were likely |
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to be of high quality. |
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There are about a dozen people who have commit privileges to the |
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PostgreSQL CVS archive. They each have submitted so many high-quality |
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patches that it was impossible for the existing committers to keep up, |
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and we had confidence that patches they committed were of high |
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quality. |
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1.13) How do I submit a bug report? |
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Fill out the "bug-template" file and send it to: bugs@postgreSQL.org |
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Fill out the "bug-template" file and send it to: |
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pgsql-bugs@PostgreSQL.org |
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Also check out our ftp site ftp://ftp.postgreSQL.org/pub to see if |
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Also check out our ftp site ftp://ftp.PostgreSQL.org/pub to see if |
|
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|
|
there is a more recent PostgreSQL version or patches. |
|
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|
|
1.14) How does PostgreSQL compare to other DBMS's? |
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|
@ -340,7 +327,7 @@ |
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|
some features they don't have, like user-defined types, |
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|
|
inheritance, rules, and multi-version concurrency control to |
|
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|
|
reduce lock contention. We don't have outer joins, but are |
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|
|
working on them for our next release. |
|
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|
|
working on them. |
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|
|
Performance |
|
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|
|
PostgreSQL runs in two modes. Normal fsync mode flushes every |
|
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|
@ -353,14 +340,13 @@ |
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|
though in this mode, an OS crash could cause data corruption. |
|
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|
|
We are working to provide an intermediate mode that suffers |
|
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|
|
less performance overhead than full fsync mode, and will allow |
|
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|
|
data integrity within 30 seconds of an OS crash. The mode is |
|
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|
|
select-able by the database administrator. |
|
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|
|
data integrity within 30 seconds of an OS crash. |
|
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|
|
In comparison to MySQL or leaner database systems, we are |
|
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|
|
slower on inserts/updates because we have transaction overhead. |
|
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|
|
Of course, MySQL doesn't have any of the features mentioned in |
|
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|
|
the Features section above. We are built for flexibility and |
|
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|
|
features, though we continue to improve performance through |
|
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|
|
profiling and source code analysis. There is an interesting web |
|
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|
|
profiling and source code analysis. There is an interesting Web |
|
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|
|
page comparing PostgreSQL to MySQL at |
|
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|
|
http://openacs.org/why-not-mysql.html |
|
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|
|
We handle each user connection by creating a Unix process. |
|
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|
@ -399,7 +385,7 @@ |
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|
There are two ODBC drivers available, PsqlODBC and OpenLink ODBC. |
|
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|
|
PsqlODBC is included in the distribution. More information about it |
|
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|
|
can be gotten from: ftp://ftp.postgresql.org/pub/odbc/index.html |
|
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|
|
can be gotten from ftp://ftp.PostgreSQL.org/pub/odbc/. |
|
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|
OpenLink ODBC can be gotten from http://www.openlinksw.com. It works |
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|
|
with their standard ODBC client software so you'll have PostgreSQL |
|
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|
@ -419,13 +405,12 @@ |
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|
There is also one at http://www.phone.net/home/mwm/hotlist/. |
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|
For web integration, PHP is an excellent interface. It is at: |
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|
For Web integration, PHP is an excellent interface. It is at |
|
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|
|
http://www.php.net |
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|
PHP is great for simple stuff, but for more complex cases, many use |
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|
the perl interface and CGI.pm. |
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|
For complex cases, many use the Perl interface and CGI.pm. |
|
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|
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|
|
A WWW gateway based on WDB using perl can be downloaded from |
|
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|
|
A WWW gateway based on WDB using Perl can be downloaded from |
|
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|
|
http://www.eol.ists.ca/~dunlop/wdb-p95 |
|
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|
2.3) Does PostgreSQL have a graphical user interface? A report generator? |
|
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|
@ -433,7 +418,7 @@ |
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|
We have a nice graphical user interface called pgaccess, which is |
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|
|
shipped as part of the distribution. Pgaccess also has a report |
|
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|
|
generator. The web page is http://www.flex.ro/pgaccess |
|
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|
|
generator. The Web page is http://www.flex.ro/pgaccess |
|
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|
We also include ecpg, which is an embedded SQL query language |
|
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|
|
interface for C. |
|
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|
@ -441,27 +426,31 @@ |
|
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|
|
2.4) What languages are available to communicate with PostgreSQL? |
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|
We have: |
|
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|
|
* C(libpq) |
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|
* C++(libpq++) |
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|
* Embedded C(ecpg) |
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|
* Java(jdbc) |
|
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|
|
* Perl(perl5) |
|
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|
|
* ODBC(odbc) |
|
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|
|
* Python(PyGreSQL) |
|
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|
|
* TCL(libpgtcl) |
|
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|
|
* C Easy API(libpgeasy) |
|
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|
|
* Embedded HTML(PHP from http://www.php.net) |
|
|
|
|
* C (libpq) |
|
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|
|
* C++ (libpq++) |
|
|
|
|
* Embedded C (ecpg) |
|
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|
|
* Java (jdbc) |
|
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|
|
* Perl (perl5) |
|
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|
|
* ODBC (odbc) |
|
|
|
|
* Python (PyGreSQL) |
|
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|
|
* TCL (libpgtcl) |
|
|
|
|
* C Easy API (libpgeasy) |
|
|
|
|
* Embedded HTML (PHP from http://www.php.net) |
|
|
|
|
_________________________________________________________________ |
|
|
|
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|
|
|
|
Administrative Questions |
|
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|
|
|
|
|
|
3.1) Why does initdb fail? |
|
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|
|
|
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|
|
Try these: |
|
|
|
|
* check that you don't have any of the previous version's binaries |
|
|
|
|
in your path |
|
|
|
|
* check to see that you have the proper paths set |
|
|
|
|
* check that the postgres user owns the proper files |
|
|
|
|
|
|
|
|
|
If you see an error message about oidvector, you definately have a |
|
|
|
|
version mismatch. |
|
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|
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|
|
3.2) How do I install PostgreSQL somewhere other than /usr/local/pgsql? |
|
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|
|
|
|
|
The simplest way is to specify the --prefix option when running |
|
|
|
|
@ -473,15 +462,15 @@ |
|
|
|
|
message. Why? |
|
|
|
|
|
|
|
|
|
It could be a variety of problems, but first check to see that you |
|
|
|
|
have system V extensions installed in your kernel. PostgreSQL requires |
|
|
|
|
have System V extensions installed in your kernel. PostgreSQL requires |
|
|
|
|
kernel support for shared memory and semaphores. |
|
|
|
|
|
|
|
|
|
3.4) When I try to start the postmaster, I get IpcMemoryCreate errors. Why? |
|
|
|
|
|
|
|
|
|
You either do not have shared memory configured properly in kernel or |
|
|
|
|
you need to enlarge the shared memory available in the kernel. The |
|
|
|
|
exact amount you need depends on your architecture and how many |
|
|
|
|
buffers and backend processes you configure postmaster to run with. |
|
|
|
|
You either do not have shared memory configured properly in your |
|
|
|
|
kernel or you need to enlarge the shared memory available in the |
|
|
|
|
kernel. The exact amount you need depends on your architecture and how |
|
|
|
|
many buffers and backend processes you configure for the postmaster. |
|
|
|
|
For most systems, with default numbers of buffers and processes, you |
|
|
|
|
need a minimum of ~1MB. |
|
|
|
|
|
|
|
|
|
@ -512,14 +501,13 @@ |
|
|
|
|
The default configuration allows only unix domain socket connections |
|
|
|
|
from the local machine. To enable TCP/IP connections, make sure the |
|
|
|
|
postmaster has been started with the -i option, and add an appropriate |
|
|
|
|
host entry to the file pgsql/data/pg_hba.conf. See the pg_hba.conf |
|
|
|
|
manual page. |
|
|
|
|
host entry to the file pgsql/data/pg_hba.conf. |
|
|
|
|
|
|
|
|
|
3.8) Why can't I access the database as the root user? |
|
|
|
|
|
|
|
|
|
You should not create database users with user id 0 (root). They will |
|
|
|
|
be unable to access the database. This is a security precaution |
|
|
|
|
because of the ability of any user to dynamically link object modules |
|
|
|
|
because of the ability of users to dynamically link object modules |
|
|
|
|
into the database engine. |
|
|
|
|
|
|
|
|
|
3.9) All my servers crash under concurrent table access. Why? |
|
|
|
|
@ -534,20 +522,20 @@ |
|
|
|
|
indices are being used. |
|
|
|
|
|
|
|
|
|
If you are doing a lot of INSERTs, consider doing them in a large |
|
|
|
|
batch using the COPY command. This is much faster than single |
|
|
|
|
individual INSERTS. Second, statements not in a BEGIN WORK/COMMIT |
|
|
|
|
transaction block are considered to be in their own transaction. |
|
|
|
|
Consider performing several statements in a single transaction block. |
|
|
|
|
This reduces the transaction overhead. Also consider dropping and |
|
|
|
|
batch using the COPY command. This is much faster than individual |
|
|
|
|
INSERTS. Second, statements not in a BEGIN WORK/COMMIT transaction |
|
|
|
|
block are considered to be in their own transaction. Consider |
|
|
|
|
performing several statements in a single transaction block. This |
|
|
|
|
reduces the transaction overhead. Also consider dropping and |
|
|
|
|
recreating indices when making large data changes. |
|
|
|
|
|
|
|
|
|
There are several tuning things that can be done. You can disable |
|
|
|
|
fsync() by starting the postmaster with a -o -F option. This will |
|
|
|
|
prevent fsync()'s from flushing to disk after every transaction. |
|
|
|
|
There are several tuning options. You can disable fsync() by starting |
|
|
|
|
the postmaster with a -o -F option. This will prevent fsync()'s from |
|
|
|
|
flushing to disk after every transaction. |
|
|
|
|
|
|
|
|
|
You can also use the postmaster -B option to increase the number of |
|
|
|
|
shared memory buffers used by the backend processes. If you make this |
|
|
|
|
parameter too high, the postmaster may not start up because you've |
|
|
|
|
parameter too high, the postmaster may not start because you've |
|
|
|
|
exceeded your kernel's limit on shared memory space. Each buffer is 8K |
|
|
|
|
and the default is 64 buffers. |
|
|
|
|
|
|
|
|
|
@ -555,10 +543,10 @@ |
|
|
|
|
of memory used by the backend process for temporary sorts. The -S |
|
|
|
|
value is measured in kilobytes, and the default is 512 (ie, 512K). |
|
|
|
|
|
|
|
|
|
You can also use the CLUSTER command to group data in base tables to |
|
|
|
|
match an index. See the cluster(l) manual page for more details. |
|
|
|
|
You can also use the CLUSTER command to group data in tables to match |
|
|
|
|
an index. See the CLUSTER manual page for more details. |
|
|
|
|
|
|
|
|
|
3.11) What debugging features are available in PostgreSQL? |
|
|
|
|
3.11) What debugging features are available? |
|
|
|
|
|
|
|
|
|
PostgreSQL has several features that report status information that |
|
|
|
|
can be valuable for debugging purposes. |
|
|
|
|
@ -610,9 +598,9 @@ |
|
|
|
|
You need to increase the postmaster's limit on how many concurrent |
|
|
|
|
backend processes it can start. |
|
|
|
|
|
|
|
|
|
In Postgres 6.5 and up, the default limit is 32 processes. You can |
|
|
|
|
In PostgreSQL 6.5 and up, the default limit is 32 processes. You can |
|
|
|
|
increase it by restarting the postmaster with a suitable -N value. |
|
|
|
|
With the default configuration you can set -N as large as 1024; if you |
|
|
|
|
With the default configuration you can set -N as large as 1024. If you |
|
|
|
|
need more, increase MAXBACKENDS in include/config.h and rebuild. You |
|
|
|
|
can set the default value of -N at configuration time, if you like, |
|
|
|
|
using configure's --with-maxbackends switch. |
|
|
|
|
@ -622,38 +610,37 @@ |
|
|
|
|
should be more than that for best performance. For large numbers of |
|
|
|
|
backend processes, you are also likely to find that you need to |
|
|
|
|
increase various Unix kernel configuration parameters. Things to check |
|
|
|
|
include the maximum size of shared memory blocks, SHMMAX, the maximum |
|
|
|
|
number of semaphores, SEMMNS and SEMMNI, the maximum number of |
|
|
|
|
processes, NPROC, the maximum number of processes per user, MAXUPRC, |
|
|
|
|
include the maximum size of shared memory blocks, SHMMAX; the maximum |
|
|
|
|
number of semaphores, SEMMNS and SEMMNI; the maximum number of |
|
|
|
|
processes, NPROC; the maximum number of processes per user, MAXUPRC; |
|
|
|
|
and the maximum number of open files, NFILE and NINODE. The reason |
|
|
|
|
that Postgres has a limit on the number of allowed backend processes |
|
|
|
|
is so that you can ensure that your system won't run out of resources. |
|
|
|
|
that PostgreSQL has a limit on the number of allowed backend processes |
|
|
|
|
is so your system won't run out of resources. |
|
|
|
|
|
|
|
|
|
In Postgres versions prior to 6.5, the maximum number of backends was |
|
|
|
|
64, and changing it required a rebuild after altering the MaxBackendId |
|
|
|
|
constant in include/storage/sinvaladt.h. |
|
|
|
|
In PostgreSQL versions prior to 6.5, the maximum number of backends |
|
|
|
|
was 64, and changing it required a rebuild after altering the |
|
|
|
|
MaxBackendId constant in include/storage/sinvaladt.h. |
|
|
|
|
|
|
|
|
|
3.13) What are the pg_tempNNN.NN files in my database directory? |
|
|
|
|
3.13) What are the pg_sorttempNNN.NN files in my database directory? |
|
|
|
|
|
|
|
|
|
They are temporary files generated by the query executor. For example, |
|
|
|
|
if a sort needs to be done to satisfy an ORDER BY, and the sort |
|
|
|
|
requires more space than the backend's -S parameter allows, then temp |
|
|
|
|
files are created to hold the extra data. |
|
|
|
|
requires more space than the backend's -S parameter allows, then |
|
|
|
|
temporary files are created to hold the extra data. |
|
|
|
|
|
|
|
|
|
The temp files should go away automatically, but might not if a |
|
|
|
|
backend crashes during a sort. If you have no transactions running at |
|
|
|
|
The temporary files should be deleted automatically, but might not if |
|
|
|
|
a backend crashes during a sort. If you have no backends running at |
|
|
|
|
the time, it is safe to delete the pg_tempNNN.NN files. |
|
|
|
|
_________________________________________________________________ |
|
|
|
|
|
|
|
|
|
Operational Questions |
|
|
|
|
|
|
|
|
|
4.1) The system seems to be confused about commas, decimal points, and date |
|
|
|
|
formats. |
|
|
|
|
4.1) Why is system confused about commas, decimal points, and date formats. |
|
|
|
|
|
|
|
|
|
Check your locale configuration. PostgreSQL uses the locale settings |
|
|
|
|
of the user that ran the postmaster process. There are postgres and |
|
|
|
|
psql SET commands to control the date format. Set those accordingly |
|
|
|
|
for your operating environment. |
|
|
|
|
Check your locale configuration. PostgreSQL uses the locale setting of |
|
|
|
|
the user that ran the postmaster process. There are postgres and psql |
|
|
|
|
SET commands to control the date format. Set those accordingly for |
|
|
|
|
your operating environment. |
|
|
|
|
|
|
|
|
|
4.2) What is the exact difference between binary cursors and normal |
|
|
|
|
cursors? |
|
|
|
|
@ -670,12 +657,13 @@ |
|
|
|
|
only the first few records requested, or the entire query may have to |
|
|
|
|
be evaluated until the desired rows have been generated. |
|
|
|
|
|
|
|
|
|
4.4) How do I get a list of tables, or other information I see in psql? |
|
|
|
|
4.4) How do I get a list of tables or other things I can see in psql? |
|
|
|
|
|
|
|
|
|
You can read the source code for psql, file pgsql/src/bin/psql/psql.c. |
|
|
|
|
It contains SQL commands that generate the output for psql's backslash |
|
|
|
|
commands. You can also start psql with the -E option so that it will |
|
|
|
|
print out the queries it uses to execute the commands you give. |
|
|
|
|
You can read the source code for psql in file |
|
|
|
|
pgsql/src/bin/psql/psql.c. It contains SQL commands that generate the |
|
|
|
|
output for psql's backslash commands. You can also start psql with the |
|
|
|
|
-E option so it will print out the queries it uses to execute the |
|
|
|
|
commands you give. |
|
|
|
|
|
|
|
|
|
4.5) How do you remove a column from a table? |
|
|
|
|
|
|
|
|
|
@ -693,7 +681,7 @@ Maximum size for a database? unlimited (60GB databases exist) |
|
|
|
|
Maximum size for a table? unlimited on all operating systems |
|
|
|
|
Maximum size for a row? 8k, configurable to 32k |
|
|
|
|
Maximum number of rows in a table? unlimited |
|
|
|
|
Maximum number of columns table? unlimited |
|
|
|
|
Maximum number of columns in a table? unlimited |
|
|
|
|
Maximum number of indexes on a table? unlimited |
|
|
|
|
|
|
|
|
|
Of course, these are not actually unlimited, but limited to available |
|
|
|
|
@ -703,13 +691,13 @@ Maximum number of indexes on a table? unlimited |
|
|
|
|
BLCKSZ. To use attributes larger than 8K, you can also use the large |
|
|
|
|
object interface. |
|
|
|
|
|
|
|
|
|
Row length limit will be removed in 7.1. |
|
|
|
|
The row length limit will be removed in 7.1. |
|
|
|
|
|
|
|
|
|
4.7)How much database disk space is required to store data from a typical |
|
|
|
|
flat file? |
|
|
|
|
4.7) How much database disk space is required to store data from a typical |
|
|
|
|
text file? |
|
|
|
|
|
|
|
|
|
A Postgres database can require about six and a half times the disk |
|
|
|
|
space required to store the data in a flat file. |
|
|
|
|
A PostgreSQL database may need six-and-a-half times the disk space |
|
|
|
|
required to store the data in a flat file. |
|
|
|
|
|
|
|
|
|
Consider a file of 300,000 lines with two integers on each line. The |
|
|
|
|
flat file is 2.4MB. The size of the PostgreSQL database file |
|
|
|
|
@ -732,7 +720,7 @@ Maximum number of indexes on a table? unlimited |
|
|
|
|
|
|
|
|
|
1755 database pages * 8192 bytes per page = 14,376,960 bytes (14MB) |
|
|
|
|
|
|
|
|
|
Indexes do not contain as much overhead, but do contain the data that |
|
|
|
|
Indexes do not require as much overhead, but do contain the data that |
|
|
|
|
is being indexed, so they can be large also. |
|
|
|
|
|
|
|
|
|
4.8) How do I find out what indices or operations are defined in the |
|
|
|
|
@ -747,15 +735,15 @@ Maximum number of indexes on a table? unlimited |
|
|
|
|
|
|
|
|
|
4.9) My queries are slow or don't make use of the indexes. Why? |
|
|
|
|
|
|
|
|
|
PostgreSQL does not automatically maintain statistics. One has to make |
|
|
|
|
an explicit VACUUM call to update the statistics. After statistics are |
|
|
|
|
updated, the optimizer knows how many rows in the table, and can |
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better decide if it should use indices. Note that the optimizer does |
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not use indices in cases when the table is small because a sequential |
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scan would be faster. |
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PostgreSQL does not automatically maintain statistics. VACUUM must be |
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run to update the statistics. After statistics are updated, the |
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optimizer knows how many rows in the table, and can better decide if |
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it should use indices. Note that the optimizer does not use indices in |
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cases when the table is small because a sequential scan would be |
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faster. |
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For column-specific optimization statistics, use VACUUM ANALYZE. |
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VACUUM ANALYZE is important for complex multi-join queries, so the |
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VACUUM ANALYZE is important for complex multijoin queries, so the |
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optimizer can estimate the number of rows returned from each table, |
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and choose the proper join order. The backend does not keep track of |
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column statistics on its own, so VACUUM ANALYZE must be run to collect |
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@ -776,41 +764,38 @@ Maximum number of indexes on a table? unlimited |
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4.11) What is an R-tree index? |
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An r-tree index is used for indexing spatial data. A hash index can't |
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An R-tree index is used for indexing spatial data. A hash index can't |
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handle range searches. A B-tree index only handles range searches in a |
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single dimension. R-tree's can handle multi-dimensional data. For |
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example, if an R-tree index can be built on an attribute of type |
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point, the system can more efficient answer queries like select all |
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points within a bounding rectangle. |
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point, the system can more efficiently answer queries such as "select |
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all points within a bounding rectangle." |
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The canonical paper that describes the original R-Tree design is: |
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The canonical paper that describes the original R-tree design is: |
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Guttman, A. "R-Trees: A Dynamic Index Structure for Spatial |
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Guttman, A. "R-trees: A Dynamic Index Structure for Spatial |
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Searching." Proc of the 1984 ACM SIGMOD Int'l Conf on Mgmt of Data, |
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45-57. |
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You can also find this paper in Stonebraker's "Readings in Database |
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Systems" |
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Systems". |
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Builtin R-Trees can handle polygons and boxes. In theory, R-trees can |
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Built-in R-trees can handle polygons and boxes. In theory, R-trees can |
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be extended to handle higher number of dimensions. In practice, |
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extending R-trees require a bit of work and we don't currently have |
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extending R-trees requires a bit of work and we don't currently have |
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any documentation on how to do it. |
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4.12) What is Genetic Query Optimization? |
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The GEQO module in PostgreSQL is intended to solve the query |
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optimization problem of joining many tables by means of a Genetic |
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Algorithm (GA). It allows the handling of large join queries through |
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non-exhaustive search. |
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For further information see the documentation. |
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The GEQO module speeds query optimization when joining many tables by |
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means of a Genetic Algorithm (GA). It allows the handling of large |
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join queries through nonexhaustive search. |
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4.13) How do I do regular expression searches and case-insensitive regexp |
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searching? |
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4.13) How do I do regular expression searches and case-insensitive regular |
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expression searches? |
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The ~ operator does regular-expression matching, and ~* does |
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case-insensitive regular-expression matching. There is no |
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The ~ operator does regular expression matching, and ~* does |
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case-insensitive regular expression matching. There is no |
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case-insensitive variant of the LIKE operator, but you can get the |
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effect of case-insensitive LIKE with this: |
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WHERE lower(textfield) LIKE lower(pattern) |
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@ -832,7 +817,7 @@ BYTEA bytea variable-length array of bytes |
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You will see the internal name when examining system catalogs and in |
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some error messages. |
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The last four types above are "varlena" types (i.e. the first four |
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The last four types above are "varlena" types (i.e., the first four |
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bytes are the length, followed by the data). char(#) allocates the |
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maximum number of bytes no matter how much data is stored in the |
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field. text, varchar(#), and bytea all have variable length on the |
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@ -842,14 +827,14 @@ BYTEA bytea variable-length array of bytes |
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4.16.1) How do I create a serial/auto-incrementing field? |
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PostgreSQL supports SERIAL data type. It auto-creates a sequence and |
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|
index on the column. For example, this... |
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PostgreSQL supports a SERIAL data type. It auto-creates a sequence and |
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|
index on the column. For example, this: |
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CREATE TABLE person ( |
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|
id SERIAL, |
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name TEXT |
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); |
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...is automatically translated into this... |
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is automatically translated into this: |
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CREATE SEQUENCE person_id_seq; |
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CREATE TABLE person ( |
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id INT4 NOT NULL DEFAULT nextval('person_id_seq'), |
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|
@ -858,69 +843,73 @@ BYTEA bytea variable-length array of bytes |
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CREATE UNIQUE INDEX person_id_key ON person ( id ); |
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See the create_sequence manual page for more information about |
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sequences. You can also use each row's oid field as a unique value. |
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|
sequences. You can also use each row's OID field as a unique value. |
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|
However, if you need to dump and reload the database, you need to use |
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|
pg_dump's -o option or COPY WITH OIDS option to preserve the oids. |
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|
pg_dump's -o option or COPY WITH OIDS option to preserve the OIDs. |
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|
For more details, see Bruce Momjian's chapter on Numbering Rows. |
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|
|
Numbering Rows. |
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|
4.16.2) How do I get the back the generated SERIAL value after an insert? |
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|
4.16.2) How do I get the value of a SERIAL insert? |
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|
Probably the simplest approach is to to retrieve the next SERIAL value |
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|
|
from the sequence object with the nextval() function before inserting |
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|
and then insert it explicitly. Using the example table in 4.16.1, that |
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|
might look like this: |
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|
|
One approach is to to retrieve the next SERIAL value from the sequence |
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|
|
object with the nextval() function before inserting and then insert it |
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|
explicitly. Using the example table in 4.16.1, that might look like |
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|
|
this: |
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|
$newSerialID = nextval('person_id_seq'); |
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|
INSERT INTO person (id, name) VALUES ($newSerialID, 'Blaise Pascal'); |
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|
You would then also have the new value stored in $newSerialID for use |
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|
in other queries (e.g., as a foreign key to the person table). Note |
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|
|
that the name of the automatically-created SEQUENCE object will be |
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|
|
that the name of the automatically created SEQUENCE object will be |
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|
|
named <table>_<serialcolumn>_seq, where table and serialcolumn are the |
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|
|
names of your table and your SERIAL column, respectively. |
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|
Similarly, you could retrieve the just-assigned SERIAL value with the |
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|
|
Alternatively, you could retrieve the assigned SERIAL value with the |
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|
|
currval() function after it was inserted by default, e.g., |
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|
|
INSERT INTO person (name) VALUES ('Blaise Pascal'); |
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|
|
$newID = currval('person_id_seq'); |
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|
Finally, you could use the oid returned from the INSERT statement to |
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|
|
lookup the default value, though this is probably the least portable |
|
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|
|
approach. In perl, using DBI with Edmund Mergl's DBD::Pg module, the |
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|
|
Finally, you could use the OID returned from the INSERT statement to |
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|
|
look up the default value, though this is probably the least portable |
|
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|
|
approach. In Perl, using DBI with Edmund Mergl's DBD::Pg module, the |
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|
|
oid value is made available via $sth->{pg_oid_status} after |
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|
|
$sth->execute(). |
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|
4.16.3) Don't currval() and nextval() lead to a race condition with other |
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|
|
concurrent backend processes? |
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|
|
users? |
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|
No. That has been handled by the backends. |
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|
No. This is handled by the backends. |
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|
4.17) What is an oid? What is a tid? |
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|
4.17) What is an OID? What is a TID? |
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|
|
Oids are PostgreSQL's answer to unique row ids. Every row that is |
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|
|
created in PostgreSQL gets a unique oid. All oids generated during |
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|
|
OIDs are PostgreSQL's answer to unique row ids. Every row that is |
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|
|
created in PostgreSQL gets a unique OID. All OIDs generated during |
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|
|
initdb are less than 16384 (from backend/access/transam.h). All |
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|
|
user-created oids are equal or greater that this. By default, all |
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|
|
these oids are unique not only within a table, or database, but unique |
|
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|
|
user-created OIDs are equal to or greater than this. By default, all |
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|
|
these OIDs are unique not only within a table or database, but unique |
|
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|
|
within the entire PostgreSQL installation. |
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|
|
PostgreSQL uses oids in its internal system tables to link rows |
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|
|
between tables. These oids can be used to identify specific user rows |
|
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|
|
and used in joins. It is recommended you use column type oid to store |
|
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|
|
oid values. See the sql(l) manual page to see the other internal |
|
|
|
|
columns. You can create an index on the oid field for faster access. |
|
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|
|
PostgreSQL uses OIDs in its internal system tables to link rows |
|
|
|
|
between tables. These OIDs can be used to identify specific user rows |
|
|
|
|
and used in joins. It is recommended you use column type OID to store |
|
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|
|
OID values. You can create an index on the OID field for faster |
|
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|
|
access. |
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|
Oids are assigned to all new rows from a central area that is used by |
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|
|
all databases. If you want to change the oid to something else, or if |
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|
|
you want to make a copy of the table, with the original oid's, there |
|
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|
|
all databases. If you want to change the OID to something else, or if |
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|
|
you want to make a copy of the table, with the original OID's, there |
|
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|
|
is no reason you can't do it: |
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|
|
CREATE TABLE new_table(old_oid oid, mycol int); |
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|
|
SELECT INTO new SELECT old_oid, mycol FROM old; |
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|
|
SELECT old_oid, mycol INTO new FROM old; |
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|
|
COPY new TO '/tmp/pgtable'; |
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|
|
DELETE FROM new; |
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|
|
COPY new WITH OIDS FROM '/tmp/pgtable'; |
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|
|
Tids are used to identify specific physical rows with block and offset |
|
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|
|
OIDs are stored as 4-byte integers, and will overflow at 4 billion. No |
|
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|
|
one has reported this ever happening, and we plan to have the limit |
|
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|
|
removed before anyone does. |
|
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|
|
TIDs are used to identify specific physical rows with block and offset |
|
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|
|
values. Tids change after rows are modified or reloaded. They are used |
|
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|
|
by index entries to point to physical rows. |
|
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|
|
@ -934,10 +923,13 @@ BYTEA bytea variable-length array of bytes |
|
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|
|
* retrieve, select |
|
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|
|
* replace, update |
|
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|
|
* append, insert |
|
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|
|
* oid, serial value |
|
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|
|
* OID, serial value |
|
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|
|
* portal, cursor |
|
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|
|
* range variable, table name, table alias |
|
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|
|
A list of general database terms can be found at: |
|
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|
|
http://www.comptechnews.com/~reaster/dbdesign.html |
|
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|
|
4.19) Why do I get the error "FATAL: palloc failure: memory exhausted?" |
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|
|
It is possible you have run out of virtual memory on your system, or |
|
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|
|
@ -962,12 +954,11 @@ BYTEA bytea variable-length array of bytes |
|
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|
|
You need to put BEGIN WORK and COMMIT around any use of a large object |
|
|
|
|
handle, that is, surrounding lo_open ... lo_close. |
|
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|
|
Current PostgreSQL enforces the rule by closing large object handles |
|
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|
|
at transaction commit, which will be instantly upon completion of the |
|
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|
|
lo_open command if you are not inside a transaction. So the first |
|
|
|
|
attempt to do anything with the handle will draw invalid large obj |
|
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|
|
descriptor. So code that used to work (at least most of the time) will |
|
|
|
|
now generate that error message if you fail to use a transaction. |
|
|
|
|
Currently PostgreSQL enforces the rule by closing large object handles |
|
|
|
|
at transaction commit. So the first attempt to do anything with the |
|
|
|
|
handle will draw invalid large obj descriptor. So code that used to |
|
|
|
|
work (at least most of the time) will now generate that error message |
|
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|
|
if you fail to use a transaction. |
|
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|
|
|
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|
|
If you are using a client interface like ODBC you may need to set |
|
|
|
|
auto-commit off. |
|
|
|
|
@ -975,13 +966,13 @@ BYTEA bytea variable-length array of bytes |
|
|
|
|
4.22) How do I create a column that will default to the current time? |
|
|
|
|
|
|
|
|
|
Use now(): |
|
|
|
|
CREATE TABLE test (x int, modtime timestamp default now() ); |
|
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|
|
CREATE TABLE test (x int, modtime timestamp DEFAULT now() ); |
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|
|
4.23) Why are my subqueries using IN so slow? |
|
|
|
|
|
|
|
|
|
Currently, we join subqueries to outer queries by sequential scanning |
|
|
|
|
the result of the subquery for each row of the outer query. A |
|
|
|
|
workaround is to replace IN with EXISTS. For example, change: |
|
|
|
|
Currently, we join subqueries to outer queries by sequentially |
|
|
|
|
scanning the result of the subquery for each row of the outer query. A |
|
|
|
|
workaround is to replace IN with EXISTS: |
|
|
|
|
SELECT * |
|
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|
|
FROM tab |
|
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|
|
WHERE col1 IN (SELECT col2 FROM TAB2) |
|
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|
|
@ -1015,16 +1006,15 @@ BYTEA bytea variable-length array of bytes |
|
|
|
|
dump core? |
|
|
|
|
|
|
|
|
|
The problem could be a number of things. Try testing your user-defined |
|
|
|
|
function in a stand alone test program first. |
|
|
|
|
function in a stand-alone test program first. |
|
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|
|
5.2) What does the message: NOTICE:PortalHeapMemoryFree: 0x402251d0 not in |
|
|
|
|
alloc set! mean? |
|
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|
|
5.2) What does the message "NOTICE:PortalHeapMemoryFree: 0x402251d0 not in |
|
|
|
|
alloc set!" mean? |
|
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|
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|
|
You are pfree'ing something that was not palloc'ed. Beware of mixing |
|
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|
|
malloc/free and palloc/pfree. |
|
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|
|
5.3) How can I contribute some nifty new types and functions for |
|
|
|
|
PostgreSQL? |
|
|
|
|
5.3) How can I contribute some nifty new types and functions to PostgreSQL? |
|
|
|
|
|
|
|
|
|
Send your extensions to the pgsql-hackers mailing list, and they will |
|
|
|
|
eventually end up in the contrib/ subdirectory. |
|
|
|
|
@ -1034,9 +1024,8 @@ BYTEA bytea variable-length array of bytes |
|
|
|
|
This requires wizardry so extreme that the authors have never tried |
|
|
|
|
it, though in principle it can be done. |
|
|
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|
|
5.5) I have changed a source file. Why does the recompile does not see the |
|
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|
|
5.5) I have changed a source file. Why does the recompile not see the |
|
|
|
|
change? |
|
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|
|
|
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|
|
The Makefiles do not have the proper dependencies for include files. |
|
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|
|
You have to do a make clean and then another make. You have to do a |
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|
|
make clean and then another make. |
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|
|
You have to do a make clean and then another make. |
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|
|
|
Bruce Momjian
25 years ago