Frequently Asked Questions (FAQ) for PostgreSQL
- Last updated: Thu Feb 14 12:14:47 EST 2002
+ Last updated: Tue Mar 5 01:28:16 EST 2002
Current maintainer: Bruce Momjian (pgman@candle.pha.pa.us)
browse the manual online at
http://www.PostgreSQL.org/users-lounge/docs/.
- There is a PostgreSQL book available at
- http://www.PostgreSQL.org/docs/awbook.html.
+ There are two PostgreSQL books available online at
+ http://www.PostgreSQL.org/docs/awbook.html and
+ http://www.commandprompt.com/ppbook/. There is a list of PostgreSQL
+ books available for purchase at http://www.postgresql.org/books/.
+ There is also a collection of PostgreSQL technical articles at
+ http://techdocs.postgresql.org/.
psql has some nice \d commands to show information about types,
operators, functions, aggregates, etc.
4.5) What is the maximum size for a row, a table, and a database?
These are the limits:
- Maximum size for a database? unlimited (60 GB databases exist)
+ Maximum size for a database? unlimited (500 GB databases exist)
Maximum size for a table? 16 TB
Maximum size for a row? unlimited in 7.1 and later
Maximum size for a field? 1 GB in 7.1 and later
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 characters in length. The flat file would be 2.8 MB. The size
- of the PostgreSQL database file containing this data can be estimated
- as 6.6 MB:
+ 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:
36 bytes: each row header (approximate)
- 26 bytes: two int fields @ 4 bytes each
+ 24 bytes: one int field and one text filed
+ 4 bytes: pointer on page to tuple
----------------------------------------
- 66 bytes per row
+ 64 bytes per row
The data page size in PostgreSQL is 8192 bytes (8 KB), so:
8192 bytes per page
- ------------------- = 124 rows per database page (rounded down)
- 66 bytes per row
+ ------------------- = 128 rows per database page (rounded down)
+ 64 bytes per row
100000 data rows
- -------------------- = 807 database pages (rounded up)
- 124 rows per page
+ -------------------- = 782 database pages (rounded up)
+ 128 rows per page
-807 database pages * 8192 bytes per page = 6,610,944 bytes (6.6 MB)
+782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
Indexes do not require as much overhead, but do contain the data that
is being indexed, so they can be large also.
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 query 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: Thu Feb 14 12:14:47 EST 2002</P>
+ <P>Last updated: Tue Mar 5 01:28:16 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>
can also browse the manual online at <A href=
"http://www.PostgreSQL.org/users-lounge/docs/">http://www.PostgreSQL.org/users-lounge/docs/</A>.</P>
- <P>There is a PostgreSQL book available at <A href=
- "http://www.PostgreSQL.org/docs/awbook.html">http://www.PostgreSQL.org/docs/awbook.html</A>.</P>
+ <P>There are two PostgreSQL books available online at <A href=
+ "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=
+ "http://www.postgresql.org/books/">http://www.postgresql.org/books/</A>.
+ 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
types, operators, functions, aggregates, etc.</P>
<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=
<P>These are the limits:</P>
<PRE>
- Maximum size for a database? unlimited (60 GB databases exist)
+ Maximum size for a database? unlimited (500 GB databases exist)
Maximum size for a table? 16 TB
Maximum size for a row? unlimited in 7.1 and later
Maximum size for a field? 1 GB in 7.1 and later
<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 characters in length. The flat file would be 2.8 MB. The size
- of the PostgreSQL database file containing this data can be
- estimated as 6.6 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)
- 26 bytes: two int fields @ 4 bytes each
+ 24 bytes: one int field and one text filed
+ 4 bytes: pointer on page to tuple
----------------------------------------
- 66 bytes per row
+ 64 bytes per row
The data page size in PostgreSQL is 8192 bytes (8 KB), so:
8192 bytes per page
- ------------------- = 124 rows per database page (rounded down)
- 66 bytes per row
+ ------------------- = 128 rows per database page (rounded down)
+ 64 bytes per row
100000 data rows
- -------------------- = 807 database pages (rounded up)
- 124 rows per page
+ -------------------- = 782 database pages (rounded up)
+ 128 rows per page
-807 database pages * 8192 bytes per page = 6,610,944 bytes (6.6 MB)
+782 database pages * 8192 bytes per page = 6,406,144 bytes (6.4 MB)
</PRE>
<P>Indexes do not require as much overhead, but do contain the data
<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 query
+ 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>
projects / postgresql / commitdiff