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Document all the system views created by initdb (several of these were

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never documented anywhere, sigh).  Centralize the detailed documentation
of system views into catalogs.sgml, and provide cross-references.
This commit is contained in:
Tom Lane
2003-10-17 22:38:20 +00:00
parent e5c2c97892
commit 0a4048646b
5 changed files with 858 additions and 334 deletions
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115
doc/src/sgml/perform.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/perform.sgml,v 1.35 2003/10/17 01:14:26 tgl Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/perform.sgml,v 1.36 2003/10/17 22:38:20 tgl Exp $
-->
<chapter id="performance-tips">
@ -439,114 +439,19 @@ SELECT attname, n_distinct, most_common_vals FROM pg_stats WHERE tablename = 'ro
</para>
<para>
<xref linkend="planner-pg-stats-table"> shows the columns that
exist in <structname>pg_stats</structname>.
<structname>pg_stats</structname> is described in detail in
<xref linkend="view-pg-stats">.
</para>
<table id="planner-pg-stats-table">
<title><structname>pg_stats</structname> Columns</title>
<tgroup cols=3>
<thead>
<row>
<entry>Name</entry>
<entry>Data Type</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><literal>schemaname</literal></entry>
<entry><type>name</type></entry>
<entry>Name of the schema containing the table.</entry>
</row>
<row>
<entry><literal>tablename</literal></entry>
<entry><type>name</type></entry>
<entry>Name of the table containing the column.</entry>
</row>
<row>
<entry><literal>attname</literal></entry>
<entry><type>name</type></entry>
<entry>Name of the column described by this row.</entry>
</row>
<row>
<entry><literal>null_frac</literal></entry>
<entry><type>real</type></entry>
<entry>Fraction of column entries that are null.</entry>
</row>
<row>
<entry><literal>avg_width</literal></entry>
<entry><type>integer</type></entry>
<entry>Average width in bytes of the column entries.</entry>
</row>
<row>
<entry><literal>n_distinct</literal></entry>
<entry><type>real</type></entry>
<entry>If greater than zero, the estimated number of distinct values
in the column. If less than zero, the negative of the number of
distinct values divided by the number of rows. (The negated form
is used when <command>ANALYZE</> believes that the number of distinct values
is likely to increase as the table grows; the positive form is used
when the column seems to have a fixed number of possible values.)
For example, -1 indicates a unique column in which the number of
distinct values is the same as the number of rows.
</entry>
</row>
<row>
<entry><literal>most_common_vals</literal></entry>
<entry><type>text[]</type></entry>
<entry>A list of the most common values in the column. (Omitted if
no values seem to be more common than any others.)</entry>
</row>
<row>
<entry><literal>most_common_freqs</literal></entry>
<entry><type>real[]</type></entry>
<entry>A list of the frequencies of the most common values,
i.e., number of occurrences of each divided by total number of rows.
</entry>
</row>
<row>
<entry><literal>histogram_bounds</literal></entry>
<entry><type>text[]</type></entry>
<entry>A list of values that divide the column's values into
groups of approximately equal population. The values in
<structfield>most_common_vals</>, if present, are omitted from this
histogram calculation. (This columns is not filled if the column data type does not have a
<literal>&lt;</> operator or if the <structfield>most_common_vals</>
list accounts for the entire population.)
</entry>
</row>
<row>
<entry><literal>correlation</literal></entry>
<entry><type>real</type></entry>
<entry>Statistical correlation between physical row ordering and
logical ordering of the column values. This ranges from -1 to +1.
When the value is near -1 or +1, an index scan on the column will
be estimated to be cheaper than when it is near zero, due to reduction
of random access to the disk. (This column is not filled if the column data type does
not have a <literal>&lt;</> operator.)
</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The maximum number of entries in the <structfield>most_common_vals</>
and <structfield>histogram_bounds</> arrays can be set on a
The amount of information stored in <structname>pg_statistic</structname>,
in particular the maximum number of entries in the
<structfield>most_common_vals</> and <structfield>histogram_bounds</>
arrays for each column, can be set on a
column-by-column basis using the <command>ALTER TABLE SET STATISTICS</>
command. The default limit is presently 10 entries. Raising the limit
command, or globally by setting the
<varname>default_statistics_target</varname> runtime parameter.
The default limit is presently 10 entries. Raising the limit
may allow more accurate planner estimates to be made, particularly for
columns with irregular data distributions, at the price of consuming
more space in <structname>pg_statistic</structname> and slightly more