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@ -14,7 +14,7 @@ |
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alink="#0000ff"> |
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<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1> |
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<P>Last updated: Tue Feb 12 12:18:09 EST 2002</P> |
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<P>Last updated: Thu Feb 14 12:14:47 EST 2002</P> |
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<P>Current maintainer: Bruce Momjian (<A href= |
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"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR> |
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@ -72,16 +72,13 @@ |
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get <I>IpcMemoryCreate</I> errors. Why?<BR> |
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<A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I |
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get <I>IpcSemaphoreCreate</I> errors. Why?<BR> |
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<A href="#3.5">3.5</A>) How do I prevent other hosts from |
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accessing my PostgreSQL database?<BR> |
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<A href="#3.6">3.6</A>) Why can't I connect to my database from |
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another machine?<BR> |
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<A href="#3.7">3.7</A>) How do I tune the database engine for |
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<A href="#3.5">3.5</A>) How do I control connections from other hosts?<BR> |
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<A href="#3.6">3.6</A>) How do I tune the database engine for |
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better performance?<BR> |
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<A href="#3.8">3.8</A>) What debugging features are available?<BR> |
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<A href="#3.9">3.9</A>) Why do I get <I>"Sorry, too many |
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<A href="#3.7">3.7</A>) What debugging features are available?<BR> |
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<A href="#3.8">3.8</A>) Why do I get <I>"Sorry, too many |
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clients"</I> when trying to connect?<BR> |
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<A href="#3.10">3.10</A>) What are the <I>pg_sorttempNNN.NN</I> |
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<A href="#3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I> |
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files in my database directory?<BR> |
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@ -633,8 +630,8 @@ |
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PostgreSQL Administrator's Guide for more detailed information |
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about shared memory and semaphores.</P> |
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<H4><A name="3.5">3.5</A>) How do I prevent other hosts from |
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accessing my PostgreSQL database?</H4> |
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<H4><A name="3.5">3.5</A>) How do I control connections from other |
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hosts?</H4> |
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<P>By default, PostgreSQL only allows connections from the local |
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machine using Unix domain sockets. Other machines will not be able |
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@ -643,16 +640,7 @@ |
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<I>$PGDATA/pg_hba.conf</I> accordingly. This will allow TCP/IP |
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connections.</P> |
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<H4><A name="3.6">3.6</A>) Why can't I connect to my database from |
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another machine?</H4> |
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<P>The default configuration allows only Unix domain socket |
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connections from the local machine. To enable TCP/IP connections, |
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make sure <I>postmaster</I> has been started with the <I>-i</I> |
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option, and add an appropriate host entry to the file |
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<I>pgsql/data/pg_hba.conf</I>.</P> |
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<H4><A name="3.7">3.7</A>) How do I tune the database engine for |
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<H4><A name="3.6">3.6</A>) How do I tune the database engine for |
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better performance?</H4> |
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<P>Certainly, indexes can speed up queries. The |
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@ -689,7 +677,7 @@ |
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data in tables to match an index. See the <SMALL>CLUSTER</SMALL> |
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manual page for more details.</P> |
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<H4><A name="3.8">3.8</A>) What debugging features are |
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<H4><A name="3.7">3.7</A>) What debugging features are |
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available?</H4> |
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<P>PostgreSQL has several features that report status information |
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@ -745,7 +733,7 @@ |
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in the <I>pgsql/data/base/dbname</I> directory. The client profile |
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file will be put in the client's current directory.</P> |
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<H4><A name="3.9">3.9</A>) Why do I get <I>"Sorry, too many |
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<H4><A name="3.8">3.8</A>) Why do I get <I>"Sorry, too many |
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clients"</I> when trying to connect?</H4> |
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<P>You need to increase <I>postmaster</I>'s limit on how many |
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@ -775,7 +763,7 @@ |
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the MaxBackendId constant in |
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<I>include/storage/sinvaladt.h</I>.</P> |
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<H4><A name="3.10">3.10</A>) What are the <I>pg_sorttempNNN.NN</I> |
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<H4><A name="3.9">3.9</A>) What are the <I>pg_sorttempNNN.NN</I> |
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files in my database directory?</H4> |
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<P>They are temporary files generated by the query executor. For |
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@ -862,30 +850,32 @@ |
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<H4><A name="4.6">4.6</A>) How much database disk space is required |
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to store data from a typical text file?</H4> |
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<P>A PostgreSQL database may need six-and-a-half times the disk |
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space required to store the data in a flat file.</P> |
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<P>A PostgreSQL database may require up to five times the disk space |
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to store data from a text file.</P> |
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<P>Consider a file of 300,000 lines with two integers on each line. |
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The flat file is 2.4 MB. The size of the PostgreSQL database file |
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containing this data can be estimated at 14 MB:</P> |
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<P>As an example, consider a file of 100,000 lines with an integer |
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and text description on each line. Suppose the text string avergages |
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twenty characters in length. The flat file would be 2.8 MB. The size |
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of the PostgreSQL database file containing this data can be |
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estimated as 6.6 MB:</P> |
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<PRE> |
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36 bytes: each row header (approximate) |
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+ 8 bytes: two int fields @ 4 bytes each |
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26 bytes: two int fields @ 4 bytes each |
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+ 4 bytes: pointer on page to tuple |
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---------------------------------------- |
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48 bytes per row |
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66 bytes per row |
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The data page size in PostgreSQL is 8192 bytes (8 KB), so: |
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8192 bytes per page |
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------------------- = 171 rows per database page (rounded up) |
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48 bytes per row |
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------------------- = 124 rows per database page (rounded down) |
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66 bytes per row |
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300000 data rows |
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-------------------- = 1755 database pages |
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171 rows per page |
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100000 data rows |
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-------------------- = 807 database pages (rounded up) |
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124 rows per page |
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1755 database pages * 8192 bytes per page = 14,376,960 bytes (14 MB) |
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807 database pages * 8192 bytes per page = 6,610,944 bytes (6.6 MB) |
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</PRE> |
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<P>Indexes do not require as much overhead, but do contain the data |
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Bruce Momjian
24 years ago