Frequently Asked Questions (FAQ) for PostgreSQL
- Last updated: Tue Feb 26 23:52:13 EST 2002
+ Last updated: Sun Mar 3 11:02:16 EST 2002
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
4.8) My queries are slow or don't make use of the indexes. Why?
- PostgreSQL does not automatically maintain statistics. VACUUM must be
- run to update the statistics. After statistics are updated, the
- optimizer knows how many rows in the table, and can better decide if
- it should use indexes. Note that the optimizer does not use indexes in
- cases when the table is small because a sequential scan would be
- faster.
-
- For column-specific optimization statistics, use VACUUM ANALYZE.
- VACUUM ANALYZE is important for complex multijoin queries, so the
- optimizer can estimate the number of rows returned from each table,
- and choose the proper join order. The backend does not keep track of
- column statistics on its own, so VACUUM ANALYZE must be run to collect
- them periodically.
-
- Indexes are usually not used for ORDER BY or joins. A sequential scan
- followed by an explicit sort is faster than an indexscan of all tuples
- of a large table. This is because random disk access is very slow.
+ Indexes are not automatically used by every query. Indexes are only
+ used if the table is larger than a minimum size, and the index selects
+ only a small percentage of the rows in the table. This is because the
+ random disk access caused by an index scan is sometimes slower than a
+ straight read through the table, or sequential scan.
+
+ To determine if an index should be used, PostgreSQL must have
+ statistics about the table. These statistics are collected using
+ VACUUM ANALYZE, or simply ANALYZE. Using statistics, the optimizer
+ knows how many rows are in the table, and can better determine if
+ indexes should be used. Statistics are also valuable in determining
+ optimal join order and join methods. Statistics collection should be
+ performed periodically as the contents of the table change.
+
+ Indexes are normally not used for ORDER BY or to perform joins. A
+ sequential scan followed by an explicit sort is usually faster than an
+ index scan of a large table.
+ However, LIMIT combined with ORDER BY often will use an index because
+ only a small portion of the table is returned.
When using wild-card operators such as LIKE or ~, indexes can only be
used if the beginning of the search is anchored to the start of the
- string. So, to use indexes, LIKE searches should not begin with %, and
- ~(regular expression searches) should start with ^.
+ string. Therefore, to use indexes, LIKE patterns must not start with
+ %, and ~(regular expression) patterns must start with ^.
4.9) How do I see how the query optimizer is evaluating my query?
alink="#0000ff">
<H1>Frequently Asked Questions (FAQ) for PostgreSQL</H1>
- <P>Last updated: Tue Feb 26 23:52:13 EST 2002</P>
+ <P>Last updated: Sun Mar 3 11:02:16 EST 2002</P>
<P>Current maintainer: Bruce Momjian (<A href=
"mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</A>)<BR>
get <I>IpcMemoryCreate</I> errors. Why?<BR>
<A href="#3.4">3.4</A>) When I try to start <I>postmaster</I>, I
get <I>IpcSemaphoreCreate</I> errors. Why?<BR>
- <A href="#3.5">3.5</A>) How do I control connections from other hosts?<BR>
+ <A href="#3.5">3.5</A>) How do I control connections from other
+ hosts?<BR>
<A href="#3.6">3.6</A>) How do I tune the database engine for
better performance?<BR>
<A href="#3.7">3.7</A>) What debugging features are available?<BR>
<SMALL>SERIAL</SMALL> insert?<BR>
<A href="#4.15.3">4.15.3</A>) Don't <I>currval()</I> and
<I>nextval()</I> lead to a race condition with other users?<BR>
- <A href="#4.15.4">4.15.4</A>) Why aren't my sequence numbers reused
- on transaction abort? Why are there gaps in the numbering of my
- sequence/SERIAL column?<BR>
+ <A href="#4.15.4">4.15.4</A>) Why aren't my sequence numbers
+ reused on transaction abort? Why are there gaps in the numbering of
+ my sequence/SERIAL column?<BR>
<A href="#4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is a
<SMALL>TID</SMALL>?<BR>
<A href="#4.17">4.17</A>) What is the meaning of some of the terms
UNIVERSITY OF CALIFORNIA HAS NO OBLIGATIONS TO PROVIDE MAINTENANCE,
SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.</P>
- <P>The above is the BSD license, the classic open-source license. It
- has no restrictions on how the source code may be used. We like it
- and have no intention of changing it.</P>
+ <P>The above is the BSD license, the classic open-source license.
+ It has no restrictions on how the source code may be used. We like
+ it and have no intention of changing it.</P>
<H4><A name="1.3">1.3</A>) What Unix platforms does PostgreSQL run
on?</H4>
"http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
and <A href=
"http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook/</A>.
- There is a list of PostgreSQL books available for purchase at <A href=
+ There is a list of PostgreSQL books available for purchase at <A
+ href=
"http://www.postgresql.org/books/">http://www.postgresql.org/books/</A>.
- There is also a collection of PostgreSQL technical articles at <A href=
+ There is also a collection of PostgreSQL technical articles at <A
+ href=
"http://techdocs.postgresql.org/">http://techdocs.postgresql.org/</A>.</P>
<P><I>psql</I> has some nice \d commands to show information about
<P>The PostgreSQL book at <A href=
"http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>
- teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at
- <A href="http://www.commandprompt.com/ppbook/">
- http://www.commandprompt.com/ppbook.</A>
+ teaches <SMALL>SQL</SMALL>. There is another PostgreSQL book at <A
+ href=
+ "http://www.commandprompt.com/ppbook/">http://www.commandprompt.com/ppbook.</A>
There is a nice tutorial at <A href=
"http://www.intermedia.net/support/sql/sqltut.shtm">http://www.intermedia.net/support/sql/sqltut.shtm,</A>
at <A href=
<H4><A name="4.6">4.6</A>) How much database disk space is required
to store data from a typical text file?</H4>
- <P>A PostgreSQL database may require up to five times the disk space
- to store data from a text file.</P>
+ <P>A PostgreSQL database may require up to five times the disk
+ space to store data from a text file.</P>
<P>As an example, consider a file of 100,000 lines with an integer
- and text description on each line. Suppose the text string avergages
- twenty bytes in length. The flat file would be 2.8 MB. The size
- of the PostgreSQL database file containing this data can be
- estimated as 6.4 MB:</P>
+ and text description on each line. Suppose the text string
+ avergages twenty bytes in length. The flat file would be 2.8 MB.
+ The size of the PostgreSQL database file containing this data can
+ be estimated as 6.4 MB:</P>
<PRE>
36 bytes: each row header (approximate)
24 bytes: one int field and one text filed
<H4><A name="4.8">4.8</A>) My queries are slow or don't make use of
the indexes. Why?</H4>
-
- <P>PostgreSQL does not automatically maintain statistics.
- V<SMALL>ACUUM</SMALL> must be run to update the statistics. After
- statistics are updated, the optimizer knows how many rows in the
- table, and can better decide if it should use indexes. Note that
- the optimizer does not use indexes in cases when the table is small
- because a sequential scan would be faster.</P>
-
- <P>For column-specific optimization statistics, use <SMALL>VACUUM
- ANALYZE.</SMALL> V<SMALL>ACUUM ANALYZE</SMALL> is important for
- complex multijoin queries, so the optimizer can estimate the number
- of rows returned from each table, and choose the proper join order.
- The backend does not keep track of column statistics on its own, so
- <SMALL>VACUUM ANALYZE</SMALL> must be run to collect them
- periodically.</P>
-
- <P>Indexes are usually not used for <SMALL>ORDER BY</SMALL> or
- joins. A sequential scan followed by an explicit sort is faster
- than an indexscan of all tuples of a large table. This is because
- random disk access is very slow.</P>
+ Indexes are not automatically used by every query. Indexes are only
+ used if the table is larger than a minimum size, and the index
+ selects only a small percentage of the rows in the table. This is
+ because the random disk access caused by an index scan is sometimes
+ slower than a straight read through the table, or sequential scan.
+
+ <P>To determine if an index should be used, PostgreSQL must have
+ statistics about the table. These statistics are collected using
+ <SMALL>VACUUM ANALYZE</SMALL>, or simply <SMALL>ANALYZE</SMALL>.
+ Using statistics, the optimizer knows how many rows are in the
+ table, and can better determine if indexes should be used.
+ Statistics are also valuable in determining optimal join order and
+ join methods. Statistics collection should be performed
+ periodically as the contents of the table change.</P>
+
+ <P>Indexes are normally not used for <SMALL>ORDER BY</SMALL> or to
+ perform joins. A sequential scan followed by an explicit sort is
+ usually faster than an index scan of a large table.</P>
+ However, <SMALL>LIMIT</SMALL> combined with <SMALL>ORDER BY</SMALL>
+ often will use an index because only a small portion of the table
+ is returned.
<P>When using wild-card operators such as <SMALL>LIKE</SMALL> or
<I>~</I>, indexes can only be used if the beginning of the search
- is anchored to the start of the string. So, to use indexes,
- <SMALL>LIKE</SMALL> searches should not begin with <I>%</I>, and
- <I>~</I>(regular expression searches) should start with
- <I>^</I>.</P>
+ is anchored to the start of the string. Therefore, to use indexes,
+ <SMALL>LIKE</SMALL> patterns must not start with <I>%</I>, and
+ <I>~</I>(regular expression) patterns must start with <I>^</I>.</P>
<H4><A name="4.9">4.9</A>) How do I see how the query optimizer is
evaluating my query?</H4>
<P>No. Currval() returns the current value assigned by your
backend, not by all users.</P>
- <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers reused
- on transaction abort? Why are there gaps in the numbering of my
- sequence/SERIAL column?</H4>
+ <H4><A name="4.15.4">4.15.4</A>) Why aren't my sequence numbers
+ reused on transaction abort? Why are there gaps in the numbering of
+ my sequence/SERIAL column?</H4>
<P>To improve concurrency, sequence values are given out to running
transactions as needed and are not locked until the transaction
- completes. This causes gaps in numbering from aborted transactions.
+ completes. This causes gaps in numbering from aborted
+ transactions.</P>
<H4><A name="4.16">4.16</A>) What is an <SMALL>OID</SMALL>? What is
a <SMALL>TID</SMALL>?</H4>
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