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@ -1,7 +1,7 @@ |
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Frequently Asked Questions (FAQ) for PostgreSQL |
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Last updated: Sun Aug 30 00:01:22 EDT 1998 |
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Last updated: Sat Oct 24 00:12:23 EDT 1998 |
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Current maintainer: Bruce Momjian (maillist@candle.pha.pa.us) |
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@ -15,10 +15,8 @@ |
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http://postgreSQL.org/docs/faq-irix.shtml. |
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_________________________________________________________________ |
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Questions answered: |
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1) General questions |
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General questions |
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1.1) What is PostgreSQL? |
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1.2) What does PostgreSQL run on? |
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1.3) Where can I get PostgreSQL? |
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@ -37,8 +35,8 @@ Questions answered: |
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1.14) How can I learn SQL? |
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1.15) What languages are available to communicate with PostgreSQL? |
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2) Installation/Configuration questions |
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Installation/Configuration questions |
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2.1) initdb doesn't run |
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2.2) when I start up the postmaster, I get "FindBackend: could not |
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find a backend to execute..." "postmaster: could not find backend to |
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@ -54,17 +52,17 @@ Questions answered: |
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change? |
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2.8) How do I prevent other hosts from accessing my PostgreSQL |
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database? |
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2.9) I can't access the database as the 'root' user. |
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2.9) I can't access the database as the root user. |
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2.10) All my servers crash under concurrent table access. Why? |
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2.11) How do I tune the database engine for better performance? |
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2.12) What debugging features are available in PostgreSQL? |
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2.13) How do I enable more than 64 concurrent backends? 2.14) What |
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non-unix ports are available? |
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2.13) How do I enable more than 64 concurrent backends? |
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2.14) What non-unix ports are available? |
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3) Operational questions |
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Operational questions |
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3.1) Does PostgreSQL support nested subqueries? |
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3.2) How can I write client applications to PostgreSQL? |
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3.2) How can I write client applications for PostgreSQL? |
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3.3) How do I set up a pg_group? |
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3.4) What is the exact difference between binary cursors and normal |
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cursors? |
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@ -82,13 +80,13 @@ Questions answered: |
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3.13) How do I create a serial/auto-incrementing field? |
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3.14) What are the pg_psort.XXX files in my database directory? |
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3.15) Why can't I connect to my database from another machine? |
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3.16) How do I find out what indexes or operations are defined in the |
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3.16) How do I find out what indices or operations are defined in the |
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database? |
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3.17) What is an oid? What is a tid? |
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3.18) What is the meaning of some of the terms used in PostgreSQL? |
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3.19) What is Genetic Query Optimization? |
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3.20) How do you remove a column from a table? |
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3.21) How do SELECT only the first few rows of a query? |
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3.21) How do I select only the first few rows of a query? |
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3.22) How much database disk space is required to store data from a |
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typical flat file? |
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3.23) How do I get a list of tables, or other things I can see in |
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@ -97,8 +95,8 @@ Questions answered: |
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exhausted?" |
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3.25) How do I tell what PostgreSQL version I am running? |
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4) Questions about extending PostgreSQL |
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Questions about extending PostgreSQL |
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4.1) I wrote a user-defined function and when I run it in psql, it |
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dumps core. |
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4.2) I get messages of the type NOTICE:PortalHeapMemoryFree: |
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@ -106,8 +104,8 @@ Questions answered: |
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4.3) I've written some nifty new types and functions for PostgreSQL. |
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4.4) How do I write a C function to return a tuple? |
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5) Bugs |
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Bugs |
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5.1) How do I make a bug report? |
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_________________________________________________________________ |
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@ -146,7 +144,7 @@ Section 1: General Questions |
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* aix - IBM on AIX 3.2.5 or 4.x |
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* alpha - DEC Alpha AXP on Digital Unix 2.0, 3.2, 4.0 |
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* BSD44_derived - OSs derived from 4.4-lite BSD (NetBSD, FreeBSD) |
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* bsdi - BSD/OS 2.0, 2.01, 2.1, 3.0 |
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* bsdi - BSD/OS 2.x, 3.x, 4.x |
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* dgux - DG/UX 5.4R4.11 |
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* hpux - HP PA-RISC on HP-UX 9.0, 10 |
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* i386_solaris - i386 Solaris |
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@ -159,9 +157,6 @@ Section 1: General Questions |
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* svr4 - Intel x86 on Intel SVR4 and MIPS |
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* ultrix4 - DEC MIPS on Ultrix 4.4 |
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The following platforms have known problems/bugs: |
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* nextstep - Motorola MC68K or Intel x86 on NeXTSTEP 3.2 |
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1.3) Where can I get PostgreSQL? |
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The primary anonymous ftp site for PostgreSQL is: |
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@ -203,9 +198,8 @@ Section 1: General Questions |
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1.5) Support for PostgreSQL |
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There is no official support for PostgreSQL from the original |
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maintainers or from University of California, Berkeley. It is |
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maintained through volunteer effort only. |
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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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The main mailing list is: pgsql-general@postgreSQL.org. It is |
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available for discussion of matters pertaining to PostgreSQL, For info |
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@ -247,7 +241,7 @@ Section 1: General Questions |
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1.6) Latest release of PostgreSQL |
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The latest release of PostgreSQL is version 6.4 beta. |
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The latest release of PostgreSQL is version 6.4. |
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We plan to have major releases every four months. |
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@ -255,32 +249,31 @@ Section 1: General Questions |
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Illustra Information Technology (a wholly owned subsidiary of Informix |
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Software, Inc.) sells an object-relational DBMS called Illustra that |
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was originally based on postgres. Illustra has cosmetic similarities |
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to PostgreSQL but has more features, is more robust, performs better, |
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and offers real support. On the flip side, it costs money. For more |
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information, contact sales@illustra.com |
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was originally based on Postgres. For more information, contact |
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sales@illustra.com |
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1.8) What documentation is available for PostgreSQL? |
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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. The pgintro, |
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sql, and pgbuiltin manual pages are particularly important. pgintro |
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contains a list of all available manual pages. |
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included in the distribution. See the /doc directory. |
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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 www page contains pointers to an implementation guide and five |
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papers written about Postgres design concepts and features. |
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The web page contains even more documentation. |
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1.9) What version of SQL does PostgreSQL use? |
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PostgreSQL supports an extended subset of SQL-92. |
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It is Y2K compliant. |
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1.10) Does PostgreSQL work with databases from earlier versions of |
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PostgreSQL? |
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Upgrading to 6.4 from earlier releases requires a dump and restore. |
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Upgrading to 6.4 from release 6.3.* can be accomplished using the new |
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pg_upgrade utility. Those upgrading from earlier releases require a |
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dump and restore. |
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Those ugrading from versions earlier than 1.09 must upgrade to 1.09 |
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first without a dump/reload, then dump the data from 1.09, and then |
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@ -313,9 +306,9 @@ Section 1: General Questions |
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PHP is great for simple stuff, but for more complex stuff, some still |
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use the perl interface and CGI.pm. |
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An WWW gatway 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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An WWW gatway 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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1.13) Does PostgreSQL have a graphical user interface? A report generator? A |
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embedded query language interface? |
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@ -323,8 +316,8 @@ Section 1: General Questions |
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shipped as part of the distribtion. Pgaccess also has a report |
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generator. |
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We also have ecpg, which is an embedded SQL query language interface |
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for C. This is also included. |
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The web page is http://www.flex.ro/pgaccess We also include ecpg, |
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which is an embedded SQL query language interface for C. |
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1.14) How can I learn SQL? |
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@ -354,17 +347,15 @@ Section 2: Installation Questions |
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2.1) initdb doesn't run |
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* check that you don't have any of the previous version's binaries |
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in your path |
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in your path (If you see the message WARN:heap_modifytuple: repl |
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is \ 9, this is the problem.) |
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* check to see that you have the proper paths set |
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* check that the 'postgres' user owns all the right files |
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* ensure that there are files in $PGDATA/files, and that they are |
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non-empty. If they aren't, then "gmake install" failed for some |
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reason |
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* check that the postgres user owns the proper files |
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2.2) when I start up the postmaster, I get "FindBackend: could not find a |
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backend to execute..." "postmaster: could not find backend to execute..." |
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You probably do not have the right path set up. The 'postgres' |
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You probably do not have the right path set up. The postgres |
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executable needs to be in your path. |
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2.3) The system seems to be confused about commas, decimal points, and date |
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@ -383,7 +374,7 @@ Section 2: Installation Questions |
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It could be a variety of problems, but first check to see that you |
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have system V extensions installed on your kernel. PostgreSQL requires |
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kernel support for shared memory. |
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kernel support for shared memory and semaphores. |
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2.6) When I try to start the postmaster, I get IpcMemoryCreate errors. |
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@ -396,16 +387,16 @@ Section 2: Installation Questions |
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2.7) I have changed a source file, but a recompile does not see the change? |
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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'. |
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You have to do a make clean and then another make. |
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2.8) How do I prevent other hosts from accessing my PostgreSQL database? |
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By default, PostgreSQL only allows connections from the local machine |
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using unix domain. You must add the -i flag to the postmaster, and |
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enable host-based authentication by modifying the file $PGDATA/pg_hba |
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accordingly. |
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using unix domain sockets. You must add the -i flag to the postmaster, |
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and enable host-based authentication by modifying the file |
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$PGDATA/pg_hba accordingly. |
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2.9) I can't access the database as the 'root' user. |
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2.9) I can't access the database as the root user. |
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You should not create database users with user id 0(root). They will |
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be unable to access the database. This is a security precaution |
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@ -419,28 +410,32 @@ Section 2: Installation Questions |
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2.11) How do I tune the database engine for better performance? |
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If you are doing a lot of INSERTs, consider doing them in a large |
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batch using the COPY command. This is much faster than single |
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individual INSERTs. Second, statements not in a BEGIN WORK/COMMIT |
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Certainly, indices can speed up queries. The explain command allows |
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you to see how PostgreSQL is interpreting your query, and which |
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indices are being used. |
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If you are doing a lot of inserts, consider doing them in a large |
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batch using the copy command. This is much faster than single |
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individual inserts. Second, statements not in a begin work/commit |
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transaction block are considered to be their in their own transaction. |
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Consider performing several statements in a single transaction block. |
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This reduces the transaction overhead. Also consider dropping and |
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recreating indexes when making large data changes. |
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recreating indices when making large data changes. |
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There are several tuning things that can be done. You can disable |
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fsync() by starting the postmaster with a '-o -F' option. This will |
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fsync() by starting the postmaster with a -o -F option. This will |
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prevent fsync()'s from flushing to disk after every transaction. |
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You can also use the postmaster -B option to increase the number of |
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shared memory buffers shared among the backend processes. If you make |
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this parameter too high, the process will not start or crash |
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unexpectedly. Each buffer is 8K and the defualt is 64 buffers. |
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shared memory buffers used by the backend processes. If you make this |
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parameter too high, the backends will not start or crash unexpectedly. |
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Each buffer is 8K and the default is 64 buffers. |
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You can also use the postgres -S option to increase the maximum amount |
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of memory used by each backend process for temporary sorts. Each |
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buffer is 1K and the defualt is 512 buffers. |
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You can also use the CLUSTER command to group data in base tables to |
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You can also use the cluster command to group data in base tables to |
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match an index. See the cluster(l) manual page for more details. |
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2.12) What debugging features are available in PostgreSQL? |
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@ -460,14 +455,14 @@ Section 2: Installation Questions |
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./bin/postmaster >server.log 2>&1 & |
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This will put a server.log file in the top-level PostgreSQL directory. |
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This file can contain useful information about problems or errors |
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This file contains useful information about problems or errors |
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encountered by the server. Postmaster has a -d option that allows even |
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more detailed information to be reported. The -d option takes a number |
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1-3 that specifies the debug level. Be warned that a debug level of 3 |
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that specifies the debug level. Be warned that high debug level values |
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generates large log files. |
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You can actuall run the postgres backend from the command line, and |
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type your SQL statement directly. This is recommended ONLY for |
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You can actually run the postgres backend from the command line, and |
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type your SQL statement directly. This is recommended only for |
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debugging purposes. Note that a newline terminates the query, not a |
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semicolon. If you have compiled with debugging symbols, you can use a |
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debugger to see what is happening. Because the backend was not started |
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@ -484,9 +479,6 @@ Section 2: Installation Questions |
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pgsql/data/base/dbname directory. The client profile file will be put |
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in the current directory. |
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The EXPLAIN command (see this FAQ) allows you to see how PostgreSQL is |
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interpreting your query. |
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2.13) How do I enable more than 64 concurrent backends? |
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Edit include/storage/sinvaladt.h, and change the value of |
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@ -503,24 +495,22 @@ Section 2: Installation Questions |
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A file win32.mak is included in the distributiion for making a Win32 |
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libpq library and psql. |
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People have attempted to port our PostgreSQL database server to |
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Windows NT using the Cygnus Unix/NT porting library, but no one has |
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succeeded yet. |
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Someone is attempting to port our PostgreSQL database server to |
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Windows NT using the Cygnus Unix/NT porting library. He has gotten it |
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compiled, but initdb is currently failing. |
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_________________________________________________________________ |
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Section 3: PostgreSQL Features |
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3.1) Does PostgreSQL support nested subqueries? |
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Yes. |
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Yes, fully supported, but only in the where clause, not in the target |
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list. |
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3.2) How can I write client applications to PostgreSQL? |
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3.2) How can I write client applications for PostgreSQL? |
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PostgreSQL supports a C-callable library interface called libpq as |
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well as many others. See the /src/interfaces directory. |
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Others have contributed a perl interface and a WWW gateway to |
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PostgreSQL. See the PostgreSQL home pages for more details. |
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well as many others. See the above list of supported languages. |
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3.3) How do I set up a pg_group? |
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@ -552,7 +542,7 @@ Section 3: PostgreSQL Features |
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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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point, the system can more efficient answer queries like select all |
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points within a bounding rectangle. |
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The canonical paper that describes the original R-Tree design is: |
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@ -582,23 +572,19 @@ Section 3: PostgreSQL Features |
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3.7) I defined indices but my queries don't seem to make use of them. Why? |
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PostgreSQL does not automatically maintain statistics. One has to make |
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an explicit 'vacuum' call to update the statistics. After statistics |
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are updated, the optimizer has a better shot at using indices. Note |
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that the optimizer is limited and does not use indices in some |
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circumstances (such as OR clauses). For column-specific optimization |
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statistics, use 'vacuum analyze'. |
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If the system still does not see the index, it is probably because you |
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have created an index on a field with the improper *_ops type. For |
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example, you have created a CHAR(4) field, but have specified a |
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char_ops index type_class. |
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See the create_index manual page for information on what type classes |
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are available. It must match the field type. |
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an explicit vacuum call to update the statistics. After statistics are |
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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 sequentail |
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scan would be faster. For column-specific optimization statistics, use |
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vacuum analyze. |
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PostgreSQL does not warn the user when the improper index is created. |
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Indexes are not used for order by operations. |
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Indexes not used for ORDER BY operations. |
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When using wildcard operators like LIKE or ~, indices can only be used |
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if the beginning of the search is anchored to the start of the string. |
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So, to use indices, LIKE searches can should not begin with %, and |
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~(regular expression searches) should start with ^. |
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3.8) How do I do regular expression searches? case-insensitive regexp |
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searching? |
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@ -623,11 +609,11 @@ BYTEA bytea variable-length array of bytes |
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You need to use the internal name when doing internal operations. |
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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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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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field. text, varchar(#), and bytea all have variable length on the |
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disk, and because of this, there is a small performance penalty for |
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using them. Specifically, the penalty is for access to any columns |
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using them. Specifically, the penalty is for access to all columns |
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after the first column of this type. |
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3.11) In a query, how do I detect if a field is NULL? |
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@ -640,39 +626,17 @@ BYTEA bytea variable-length array of bytes |
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3.13) How do I create a serial/auto-incrementing field? |
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PostgreSQL does not allow the user to specifiy a user column as type |
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SERIAL. Instead, you can 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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pgdump's -o option or COPY's WITH OIDS option to preserver the oids. |
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We also have a SEQUENCE function that is very similar to SERIAL. See |
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the create_sequence manual page. |
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Another valid way of doing this is to create a function: |
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create table my_oids (f1 int4); |
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insert into my_oids values (1); |
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create function new_oid () returns int4 as |
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'update my_oids set f1 = f1 + 1; select f1 from my_oids; ' |
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language 'sql'; |
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then: |
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create table my_stuff (my_key int4, value text); |
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insert into my_stuff values (new_oid(), 'hello'); |
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However, keep in mind there is a race condition here where one server |
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could do the update, then another one do an update, and they both |
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could select the same new id. This statement should be performed |
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within a transaction. |
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Yet another way is to use general trigger function autoinc() from |
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contrib/spi/autoinc.c. |
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PostgreSQL supports a serial data type. It auto-creates a sequence and |
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index on the column. See the create_sequence manual page for more |
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information about sequences. You can also use each row's oid field as |
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a unique value. However, if you need to dump and reload the database, |
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you need to use pgdump's -o option or copy with oids option to |
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preserve the oids. |
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3.14) What are the pg_psort.XXX files in my database directory? |
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They are temporary sort files generated by the query executor. For |
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example, if a sort needs to be done to satisfy an ORDER BY, some temp |
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example, if a sort needs to be done to satisfy an order by, some temp |
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files are generated as a result of the sort. |
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If you have no transactions or sorts running at the time, it is safe |
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@ -685,28 +649,27 @@ BYTEA bytea variable-length array of bytes |
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postmaster -i option You need to add a host entry to the file |
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pgsql/data/pg_hba. See the pg_hba.conf manual page. |
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3.16) How do I find out what indexes or operations are defined in the |
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3.16) How do I find out what indices or operations are defined in the |
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database? |
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psql has a variety of backslash commands to show such information. Use |
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\? to see them. |
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Also try the file pgsql/src/tutorial/syscat.source. It illustrates |
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many of the 'select's needed to get information out of the database |
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many of the selects needed to get information out of the database |
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system tables. |
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3.17) What is an oid? What is a tid? |
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Oids are PostgreSQL's answer to unique row ids or serial columns. |
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Every row that is created in PostgreSQL gets a unique oid. All oids |
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generated by initdb are less than 16384 (from |
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backend/access/transam.h). All post-initdb (user-created) oids are |
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equal or greater that this. By default, all these oids are unique not |
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only within a table, or database, but unique within the entire |
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PostgreSQL installation. |
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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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within the entire PostgreSQL installation. |
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PostgreSQL uses oids in its internal system tables to link rows in |
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separate tables. These oids can be used to identify specific user rows |
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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 |
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columns. You can create an index on the oid field for faster access. |
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@ -720,8 +683,7 @@ BYTEA bytea variable-length array of bytes |
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Tids are used to indentify specific physical rows with block and |
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offset values. Tids change after rows are modified or reloaded. They |
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are used by index entries to point to physical rows. They can not be |
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accessed through sql. |
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are used by index entries to point to physical rows. |
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3.18) What is the meaning of some of the terms used in PostgreSQL? |
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@ -737,8 +699,6 @@ BYTEA bytea variable-length array of bytes |
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* portal, cursor |
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* range variable, table name, table alias |
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Please let me know if you think of any more. |
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3.19) What is Genetic Query Optimization? |
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The GEQO module in PostgreSQL is intended to solve the query |
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@ -750,7 +710,7 @@ BYTEA bytea variable-length array of bytes |
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3.20) How do you remove a column from a table? |
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We do not support ALTER TABLE DROP COLUMN, but do this: |
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We do not support alter table drop column, but do this: |
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SELECT ... -- select all columns but the one you want to remove |
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INTO TABLE new_table |
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@ -758,15 +718,14 @@ BYTEA bytea variable-length array of bytes |
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DROP TABLE old_table; |
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ALTER TABLE new_table RENAME TO old_table; |
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3.21) How do SELECT only the first few rows of a query? |
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3.21) How do I select only the first few rows of a query? |
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See the fetch manual page. |
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This only prevents all row results from being transfered to the |
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client. The entire query must be evaluated, even if you only want just |
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the first few rows. Consider a query that has and an ORDER BY. There |
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is no way to return any rows until the entire query is evaluated and |
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sorted. |
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the first few rows. Consider a query that has an order by. There is no |
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way to return any rows until the entire query is evaluated and sorted. |
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3.22)How much database disk space is required to store data from a typical |
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flat file? |
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@ -812,10 +771,10 @@ being indexed, so they can be large also. |
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Depending on your shell, only one of these may succeed, but it will |
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set your process data segment limit much higher and perhaps allow the |
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query to complete. This command applies the current process, and all |
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subprocesses created after the command is run. If are having a problem |
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with the SQL client because the backend is returning too much data, |
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try it before starting the client. |
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query to complete. This command applies to the current process, and |
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all subprocesses created after the command is run. If are having a |
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problem with the SQL client because the backend is returning too much |
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data, try it before starting the client. |
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3.25) How do I tell what PostgreSQL version I am running? |
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@ -848,9 +807,8 @@ Section 4: Extending PostgreSQL |
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4.4) How do I write a C function to return a tuple? |
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This requires extreme wizardry, so extreme that the authors have not |
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ever tried it, though in principle it can be done. The short answer is |
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... you can't. This capability is forthcoming in the future. |
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This requires extreme wizardry so extreme that the authors have not |
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ever tried it, though in principle it can be done. |
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_________________________________________________________________ |
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Section 5: Bugs |
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@ -860,9 +818,7 @@ Section 5: Bugs |
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Check the current FAQ at http://postgreSQL.org |
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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. |
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there is a more recent PostgreSQL version or patches. |
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You can also fill out the "bug-template" file and send it to: |
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* bugs@postgreSQL.org |
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This is the address of the developers mailing list. |
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bugs@postgreSQL.org |
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Bruce Momjian
27 years ago