Update FAQ.
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Bruce Momjian
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doc/FAQ
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doc/FAQ
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@ -1,7 +1,7 @@
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Frequently Asked Questions (FAQ) for PostgreSQL
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Last updated: Tue Feb 12 12:18:09 EST 2002
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Last updated: Thu Feb 14 12:14:47 EST 2002
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Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
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@ -48,13 +48,11 @@
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Why?
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3.4) When I try to start postmaster, I get IpcSemaphoreCreate errors.
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Why?
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3.5) How do I prevent other hosts from accessing my PostgreSQL
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database?
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3.6) Why can't I connect to my database from another machine?
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3.7) How do I tune the database engine for better performance?
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3.8) What debugging features are available?
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3.9) Why do I get "Sorry, too many clients" when trying to connect?
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3.10) What are the pg_sorttempNNN.NN files in my database directory?
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3.5) How do I control connections from other hosts?
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3.6) How do I tune the database engine for better performance?
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3.7) What debugging features are available?
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3.8) Why do I get "Sorry, too many clients" when trying to connect?
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3.9) What are the pg_sorttempNNN.NN files in my database directory?
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Operational Questions
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@ -484,7 +482,7 @@
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Administrator's Guide for more detailed information about shared
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memory and semaphores.
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3.5) How do I prevent other hosts from accessing my PostgreSQL database?
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3.5) How do I control connections from other hosts?
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By default, PostgreSQL only allows connections from the local machine
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using Unix domain sockets. Other machines will not be able to connect
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@ -492,14 +490,7 @@
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authentication by modifying the file $PGDATA/pg_hba.conf accordingly.
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This will allow TCP/IP connections.
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3.6) Why can't I connect to my database from another machine?
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The default configuration allows only Unix domain socket connections
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from the local machine. To enable TCP/IP connections, make sure
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postmaster has been started with the -i option, and add an appropriate
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host entry to the file pgsql/data/pg_hba.conf.
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3.7) How do I tune the database engine for better performance?
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3.6) How do I tune the database engine for better performance?
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Certainly, indexes 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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@ -530,7 +521,7 @@
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You can also use the CLUSTER command to group data in tables to match
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an index. See the CLUSTER manual page for more details.
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3.8) What debugging features are available?
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3.7) What debugging features are available?
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PostgreSQL has several features that report status information that
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can be valuable for debugging purposes.
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@ -577,7 +568,7 @@
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pgsql/data/base/dbname directory. The client profile file will be put
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in the client's current directory.
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3.9) Why do I get "Sorry, too many clients" when trying to connect?
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3.8) Why do I get "Sorry, too many clients" when trying to connect?
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You need to increase postmaster's limit on how many concurrent backend
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processes it can start.
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@ -601,7 +592,7 @@
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was 64, and changing it required a rebuild after altering the
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MaxBackendId constant in include/storage/sinvaladt.h.
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3.10) What are the pg_sorttempNNN.NN files in my database directory?
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3.9) What are the pg_sorttempNNN.NN files in my database directory?
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They are temporary files generated by the query executor. For example,
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if a sort needs to be done to satisfy an ORDER BY, and the sort
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@ -671,29 +662,31 @@
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4.6) How much database disk space is required to store data from a typical
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text file?
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A PostgreSQL database may need six-and-a-half times the disk space
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required to store the data in a flat file.
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A PostgreSQL database may require up to five times the disk space to
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store data from a text file.
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Consider a file of 300,000 lines with two integers on each line. The
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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:
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As an example, consider a file of 100,000 lines with an integer and
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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 estimated
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as 6.6 MB:
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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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Indexes do not require as much overhead, but do contain the data that
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is being indexed, so they can be large also.
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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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