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3c49c6facb
Since some preparation work had already been done, the only source changes left were changing empty-element tags like <xref linkend="foo"> to <xref linkend="foo"/>, and changing the DOCTYPE. The source files are still named *.sgml, but they are actually XML files now. Renaming could be considered later. In the build system, the intermediate step to convert from SGML to XML is removed. Everything is build straight from the source files again. The OpenSP (or the old SP) package is no longer needed. The documentation toolchain instructions are updated and are much simpler now. Peter Eisentraut, Alexander Lakhin, Jürgen Purtz
630 lines
18 KiB
Plaintext
630 lines
18 KiB
Plaintext
<!-- doc/src/sgml/pgstattuple.sgml -->
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<sect1 id="pgstattuple" xreflabel="pgstattuple">
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<title>pgstattuple</title>
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<indexterm zone="pgstattuple">
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<primary>pgstattuple</primary>
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</indexterm>
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<para>
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The <filename>pgstattuple</filename> module provides various functions to
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obtain tuple-level statistics.
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</para>
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<para>
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Because these functions return detailed page-level information, access is
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restricted by default. By default, only the
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role <literal>pg_stat_scan_tables</literal> has <literal>EXECUTE</literal>
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privilege. Superusers of course bypass this restriction. After the
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extension has been installed, users may issue <command>GRANT</command>
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commands to change the privileges on the functions to allow others to
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execute them. However, it might be preferable to add those users to
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the <literal>pg_stat_scan_tables</literal> role instead.
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</para>
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<sect2>
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<title>Functions</title>
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<variablelist>
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<varlistentry>
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<term>
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<indexterm>
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<primary>pgstattuple</primary>
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</indexterm>
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<function>pgstattuple(regclass) returns record</function>
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</term>
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<listitem>
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<para>
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<function>pgstattuple</function> returns a relation's physical length,
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percentage of <quote>dead</quote> tuples, and other info. This may help users
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to determine whether vacuum is necessary or not. The argument is the
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target relation's name (optionally schema-qualified) or OID.
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For example:
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<programlisting>
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test=> SELECT * FROM pgstattuple('pg_catalog.pg_proc');
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-[ RECORD 1 ]------+-------
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table_len | 458752
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tuple_count | 1470
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tuple_len | 438896
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tuple_percent | 95.67
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dead_tuple_count | 11
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dead_tuple_len | 3157
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dead_tuple_percent | 0.69
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free_space | 8932
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free_percent | 1.95
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</programlisting>
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The output columns are described in <xref linkend="pgstattuple-columns"/>.
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</para>
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<table id="pgstattuple-columns">
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<title><function>pgstattuple</function> Output Columns</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Column</entry>
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<entry>Type</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><structfield>table_len</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Physical relation length in bytes</entry>
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</row>
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<row>
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<entry><structfield>tuple_count</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of live tuples</entry>
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</row>
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<row>
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<entry><structfield>tuple_len</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Total length of live tuples in bytes</entry>
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</row>
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<row>
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<entry><structfield>tuple_percent</structfield></entry>
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<entry><type>float8</type></entry>
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<entry>Percentage of live tuples</entry>
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</row>
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<row>
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<entry><structfield>dead_tuple_count</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of dead tuples</entry>
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</row>
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<row>
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<entry><structfield>dead_tuple_len</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Total length of dead tuples in bytes</entry>
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</row>
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<row>
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<entry><structfield>dead_tuple_percent</structfield></entry>
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<entry><type>float8</type></entry>
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<entry>Percentage of dead tuples</entry>
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</row>
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<row>
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<entry><structfield>free_space</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Total free space in bytes</entry>
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</row>
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<row>
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<entry><structfield>free_percent</structfield></entry>
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<entry><type>float8</type></entry>
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<entry>Percentage of free space</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<note>
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<para>
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The <literal>table_len</literal> will always be greater than the sum
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of the <literal>tuple_len</literal>, <literal>dead_tuple_len</literal>
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and <literal>free_space</literal>. The difference is accounted for by
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fixed page overhead, the per-page table of pointers to tuples, and
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padding to ensure that tuples are correctly aligned.
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</para>
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</note>
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<para>
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<function>pgstattuple</function> acquires only a read lock on the
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relation. So the results do not reflect an instantaneous snapshot;
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concurrent updates will affect them.
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</para>
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<para>
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<function>pgstattuple</function> judges a tuple is <quote>dead</quote> if
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<function>HeapTupleSatisfiesDirty</function> returns false.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>pgstattuple(text) returns record</function>
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</term>
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<listitem>
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<para>
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This is the same as <function>pgstattuple(regclass)</function>, except
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that the target relation is specified as TEXT. This function is kept
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because of backward-compatibility so far, and will be deprecated in
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some future release.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<indexterm>
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<primary>pgstatindex</primary>
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</indexterm>
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<function>pgstatindex(regclass) returns record</function>
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</term>
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<listitem>
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<para>
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<function>pgstatindex</function> returns a record showing information
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about a B-tree index. For example:
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<programlisting>
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test=> SELECT * FROM pgstatindex('pg_cast_oid_index');
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-[ RECORD 1 ]------+------
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version | 2
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tree_level | 0
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index_size | 16384
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root_block_no | 1
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internal_pages | 0
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leaf_pages | 1
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empty_pages | 0
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deleted_pages | 0
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avg_leaf_density | 54.27
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leaf_fragmentation | 0
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</programlisting>
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</para>
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<para>
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The output columns are:
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<informaltable>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Column</entry>
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<entry>Type</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><structfield>version</structfield></entry>
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<entry><type>integer</type></entry>
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<entry>B-tree version number</entry>
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</row>
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<row>
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<entry><structfield>tree_level</structfield></entry>
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<entry><type>integer</type></entry>
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<entry>Tree level of the root page</entry>
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</row>
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<row>
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<entry><structfield>index_size</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Total index size in bytes</entry>
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</row>
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<row>
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<entry><structfield>root_block_no</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Location of root page (zero if none)</entry>
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</row>
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<row>
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<entry><structfield>internal_pages</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of <quote>internal</quote> (upper-level) pages</entry>
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</row>
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<row>
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<entry><structfield>leaf_pages</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of leaf pages</entry>
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</row>
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<row>
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<entry><structfield>empty_pages</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of empty pages</entry>
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</row>
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<row>
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<entry><structfield>deleted_pages</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of deleted pages</entry>
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</row>
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<row>
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<entry><structfield>avg_leaf_density</structfield></entry>
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<entry><type>float8</type></entry>
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<entry>Average density of leaf pages</entry>
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</row>
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<row>
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<entry><structfield>leaf_fragmentation</structfield></entry>
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<entry><type>float8</type></entry>
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<entry>Leaf page fragmentation</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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</para>
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<para>
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The reported <literal>index_size</literal> will normally correspond to one more
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page than is accounted for by <literal>internal_pages + leaf_pages +
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empty_pages + deleted_pages</literal>, because it also includes the
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index's metapage.
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</para>
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<para>
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As with <function>pgstattuple</function>, the results are accumulated
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page-by-page, and should not be expected to represent an
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instantaneous snapshot of the whole index.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>pgstatindex(text) returns record</function>
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</term>
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<listitem>
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<para>
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This is the same as <function>pgstatindex(regclass)</function>, except
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that the target index is specified as TEXT. This function is kept
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because of backward-compatibility so far, and will be deprecated in
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some future release.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<indexterm>
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<primary>pgstatginindex</primary>
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</indexterm>
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<function>pgstatginindex(regclass) returns record</function>
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</term>
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<listitem>
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<para>
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<function>pgstatginindex</function> returns a record showing information
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about a GIN index. For example:
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<programlisting>
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test=> SELECT * FROM pgstatginindex('test_gin_index');
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-[ RECORD 1 ]--+--
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version | 1
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pending_pages | 0
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pending_tuples | 0
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</programlisting>
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</para>
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<para>
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The output columns are:
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<informaltable>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Column</entry>
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<entry>Type</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><structfield>version</structfield></entry>
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<entry><type>integer</type></entry>
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<entry>GIN version number</entry>
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</row>
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<row>
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<entry><structfield>pending_pages</structfield></entry>
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<entry><type>integer</type></entry>
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<entry>Number of pages in the pending list</entry>
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</row>
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<row>
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<entry><structfield>pending_tuples</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of tuples in the pending list</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<indexterm>
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<primary>pgstathashindex</primary>
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</indexterm>
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<function>pgstathashindex(regclass) returns record</function>
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</term>
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<listitem>
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<para>
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<function>pgstathashindex</function> returns a record showing information
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about a HASH index. For example:
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<programlisting>
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test=> select * from pgstathashindex('con_hash_index');
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-[ RECORD 1 ]--+-----------------
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version | 4
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bucket_pages | 33081
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overflow_pages | 0
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bitmap_pages | 1
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unused_pages | 32455
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live_items | 10204006
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dead_items | 0
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free_percent | 61.8005949100872
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</programlisting>
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</para>
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<para>
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The output columns are:
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<informaltable>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Column</entry>
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<entry>Type</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><structfield>version</structfield></entry>
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<entry><type>integer</type></entry>
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<entry>HASH version number</entry>
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</row>
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<row>
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<entry><structfield>bucket_pages</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of bucket pages</entry>
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</row>
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<row>
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<entry><structfield>overflow_pages</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of overflow pages</entry>
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</row>
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<row>
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<entry><structfield>bitmap_pages</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of bitmap pages</entry>
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</row>
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<row>
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<entry><structfield>unused_pages</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of unused pages</entry>
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</row>
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<row>
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<entry><structfield>live_items</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of live tuples</entry>
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</row>
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<row>
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<entry><structfield>dead_tuples</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of dead tuples</entry>
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</row>
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<row>
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<entry><structfield>free_percent</structfield></entry>
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<entry><type>float</type></entry>
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<entry>Percentage of free space</entry>
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</row>
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</tbody>
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</tgroup>
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</informaltable>
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<indexterm>
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<primary>pg_relpages</primary>
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</indexterm>
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<function>pg_relpages(regclass) returns bigint</function>
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</term>
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<listitem>
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<para>
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<function>pg_relpages</function> returns the number of pages in the
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relation.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<function>pg_relpages(text) returns bigint</function>
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</term>
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<listitem>
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<para>
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This is the same as <function>pg_relpages(regclass)</function>, except
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that the target relation is specified as TEXT. This function is kept
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because of backward-compatibility so far, and will be deprecated in
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some future release.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<indexterm>
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<primary>pgstattuple_approx</primary>
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</indexterm>
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<function>pgstattuple_approx(regclass) returns record</function>
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</term>
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<listitem>
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<para>
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<function>pgstattuple_approx</function> is a faster alternative to
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<function>pgstattuple</function> that returns approximate results.
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The argument is the target relation's name or OID.
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For example:
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<programlisting>
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test=> SELECT * FROM pgstattuple_approx('pg_catalog.pg_proc'::regclass);
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-[ RECORD 1 ]--------+-------
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table_len | 573440
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scanned_percent | 2
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approx_tuple_count | 2740
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approx_tuple_len | 561210
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approx_tuple_percent | 97.87
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dead_tuple_count | 0
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dead_tuple_len | 0
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dead_tuple_percent | 0
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approx_free_space | 11996
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approx_free_percent | 2.09
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</programlisting>
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The output columns are described in <xref linkend="pgstatapprox-columns"/>.
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</para>
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<para>
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Whereas <function>pgstattuple</function> always performs a
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full-table scan and returns an exact count of live and dead tuples
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(and their sizes) and free space, <function>pgstattuple_approx</function>
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tries to avoid the full-table scan and returns exact dead tuple
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statistics along with an approximation of the number and
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size of live tuples and free space.
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</para>
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<para>
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It does this by skipping pages that have only visible tuples
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according to the visibility map (if a page has the corresponding VM
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bit set, then it is assumed to contain no dead tuples). For such
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pages, it derives the free space value from the free space map, and
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assumes that the rest of the space on the page is taken up by live
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tuples.
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</para>
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<para>
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For pages that cannot be skipped, it scans each tuple, recording its
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presence and size in the appropriate counters, and adding up the
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free space on the page. At the end, it estimates the total number of
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live tuples based on the number of pages and tuples scanned (in the
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same way that VACUUM estimates pg_class.reltuples).
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</para>
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<table id="pgstatapprox-columns">
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<title><function>pgstattuple_approx</function> Output Columns</title>
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<tgroup cols="3">
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<thead>
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<row>
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<entry>Column</entry>
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<entry>Type</entry>
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<entry>Description</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><structfield>table_len</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Physical relation length in bytes (exact)</entry>
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</row>
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<row>
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<entry><structfield>scanned_percent</structfield></entry>
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<entry><type>float8</type></entry>
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<entry>Percentage of table scanned</entry>
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</row>
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<row>
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<entry><structfield>approx_tuple_count</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of live tuples (estimated)</entry>
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</row>
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<row>
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<entry><structfield>approx_tuple_len</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Total length of live tuples in bytes (estimated)</entry>
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</row>
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<row>
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<entry><structfield>approx_tuple_percent</structfield></entry>
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<entry><type>float8</type></entry>
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<entry>Percentage of live tuples</entry>
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</row>
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<row>
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<entry><structfield>dead_tuple_count</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Number of dead tuples (exact)</entry>
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</row>
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<row>
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<entry><structfield>dead_tuple_len</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Total length of dead tuples in bytes (exact)</entry>
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</row>
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<row>
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<entry><structfield>dead_tuple_percent</structfield></entry>
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<entry><type>float8</type></entry>
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<entry>Percentage of dead tuples</entry>
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</row>
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<row>
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<entry><structfield>approx_free_space</structfield></entry>
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<entry><type>bigint</type></entry>
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<entry>Total free space in bytes (estimated)</entry>
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</row>
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<row>
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<entry><structfield>approx_free_percent</structfield></entry>
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<entry><type>float8</type></entry>
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<entry>Percentage of free space</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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In the above output, the free space figures may not match the
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<function>pgstattuple</function> output exactly, because the free
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space map gives us an exact figure, but is not guaranteed to be
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accurate to the byte.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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<sect2>
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<title>Authors</title>
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<para>
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Tatsuo Ishii, Satoshi Nagayasu and Abhijit Menon-Sen
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</para>
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</sect2>
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</sect1>
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