path: root/doc/src/sgml/pgstattuple.sgml

<!-- doc/src/sgml/pgstattuple.sgml -->

<sect1 id="pgstattuple" xreflabel="pgstattuple">
 <title>pgstattuple</title>

 <indexterm zone="pgstattuple">
  <primary>pgstattuple</primary>
 </indexterm>

 <para>
  The <filename>pgstattuple</filename> module provides various functions to
  obtain tuple-level statistics.
 </para>

 <sect2>
  <title>Functions</title>

  <variablelist>
   <varlistentry>
    <term>
     <indexterm>
      <primary>pgstattuple</primary>
     </indexterm>
     <function>pgstattuple(regclass) returns record</>
    </term>

    <listitem>
     <para>
      <function>pgstattuple</function> returns a relation's physical length,
      percentage of <quote>dead</> tuples, and other info. This may help users
      to determine whether vacuum is necessary or not.  The argument is the
      target relation's name (optionally schema-qualified) or OID.
      For example:
<programlisting>
test=> SELECT * FROM pgstattuple('pg_catalog.pg_proc');
-[ RECORD 1 ]------+-------
table_len          | 458752
tuple_count        | 1470
tuple_len          | 438896
tuple_percent      | 95.67
dead_tuple_count   | 11
dead_tuple_len     | 3157
dead_tuple_percent | 0.69
free_space         | 8932
free_percent       | 1.95
</programlisting>
     The output columns are described in <xref linkend="pgstattuple-columns">.
    </para>

    <table id="pgstattuple-columns">
     <title><function>pgstattuple</function> Output Columns</title>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Column</entry>
        <entry>Type</entry>
        <entry>Description</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><structfield>table_len</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Physical relation length in bytes</entry>
       </row>
       <row>
        <entry><structfield>tuple_count</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Number of live tuples</entry>
       </row>
       <row>
        <entry><structfield>tuple_len</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Total length of live tuples in bytes</entry>
       </row>
       <row>
        <entry><structfield>tuple_percent</structfield></entry>
        <entry><type>float8</type></entry>
        <entry>Percentage of live tuples</entry>
       </row>
       <row>
        <entry><structfield>dead_tuple_count</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Number of dead tuples</entry>
       </row>
       <row>
        <entry><structfield>dead_tuple_len</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Total length of dead tuples in bytes</entry>
       </row>
       <row>
        <entry><structfield>dead_tuple_percent</structfield></entry>
        <entry><type>float8</type></entry>
        <entry>Percentage of dead tuples</entry>
       </row>
       <row>
        <entry><structfield>free_space</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Total free space in bytes</entry>
       </row>
       <row>
        <entry><structfield>free_percent</structfield></entry>
        <entry><type>float8</type></entry>
        <entry>Percentage of free space</entry>
       </row>

      </tbody>
     </tgroup>
    </table>

    <note>
      <para>
        The <literal>table_len</literal> will always be greater than the sum
        of the <literal>tuple_len</literal>, <literal>dead_tuple_len</literal>
        and <literal>free_space</literal>. The difference is accounted for by
        fixed page overhead, the per-page table of pointers to tuples, and
        padding to ensure that tuples are correctly aligned. 
      </para>
    </note>

    <para>
     <function>pgstattuple</function> acquires only a read lock on the
     relation. So the results do not reflect an instantaneous snapshot;
     concurrent updates will affect them.
    </para>

    <para>
     <function>pgstattuple</function> judges a tuple is <quote>dead</> if
     <function>HeapTupleSatisfiesDirty</> returns false.
    </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>pgstattuple(text) returns record</>
    </term>

    <listitem>
     <para>
      This is the same as <function>pgstattuple(regclass)</function>, except
      that the target relation is specified as TEXT. This function is kept
      because of backward-compatibility so far, and will be deprecated in
      some future release.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
    <indexterm>
     <primary>pgstatindex</primary>
    </indexterm>
     <function>pgstatindex(regclass) returns record</>
    </term>

    <listitem>
     <para>
      <function>pgstatindex</function> returns a record showing information
      about a B-tree index.  For example:
<programlisting>
test=> SELECT * FROM pgstatindex('pg_cast_oid_index');
-[ RECORD 1 ]------+------
version            | 2
tree_level         | 0
index_size         | 16384
root_block_no      | 1
internal_pages     | 0
leaf_pages         | 1
empty_pages        | 0
deleted_pages      | 0
avg_leaf_density   | 54.27
leaf_fragmentation | 0
</programlisting>
     </para>

    <para>
     The output columns are:

    <informaltable>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Column</entry>
        <entry>Type</entry>
        <entry>Description</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><structfield>version</structfield></entry>
        <entry><type>integer</type></entry>
        <entry>B-tree version number</entry>
       </row>

       <row>
        <entry><structfield>tree_level</structfield></entry>
        <entry><type>integer</type></entry>
        <entry>Tree level of the root page</entry>
       </row>

       <row>
        <entry><structfield>index_size</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Total index size in bytes</entry>
       </row>

       <row>
        <entry><structfield>root_block_no</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Location of root page (zero if none)</entry>
       </row>

       <row>
        <entry><structfield>internal_pages</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Number of <quote>internal</> (upper-level) pages</entry>
       </row>

       <row>
        <entry><structfield>leaf_pages</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Number of leaf pages</entry>
       </row>

       <row>
        <entry><structfield>empty_pages</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Number of empty pages</entry>
       </row>

       <row>
        <entry><structfield>deleted_pages</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Number of deleted pages</entry>
       </row>

       <row>
        <entry><structfield>avg_leaf_density</structfield></entry>
        <entry><type>float8</type></entry>
        <entry>Average density of leaf pages</entry>
       </row>

       <row>
        <entry><structfield>leaf_fragmentation</structfield></entry>
        <entry><type>float8</type></entry>
        <entry>Leaf page fragmentation</entry>
       </row>

      </tbody>
     </tgroup>
    </informaltable>
    </para>

    <para>
     The reported <literal>index_size</> will normally correspond to one more
     page than is accounted for by <literal>internal_pages + leaf_pages +
     empty_pages + deleted_pages</literal>, because it also includes the
     index's metapage.
    </para>

    <para>
     As with <function>pgstattuple</>, the results are accumulated
     page-by-page, and should not be expected to represent an
     instantaneous snapshot of the whole index.
    </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>pgstatindex(text) returns record</>
    </term>

    <listitem>
     <para>
      This is the same as <function>pgstatindex(regclass)</function>, except
      that the target index is specified as TEXT. This function is kept
      because of backward-compatibility so far, and will be deprecated in
      some future release.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <indexterm>
      <primary>pgstatginindex</primary>
     </indexterm>
     <function>pgstatginindex(regclass) returns record</>
    </term>

    <listitem>
     <para>
      <function>pgstatginindex</function> returns a record showing information
      about a GIN index.  For example:
<programlisting>
test=> SELECT * FROM pgstatginindex('test_gin_index');
-[ RECORD 1 ]--+--
version        | 1
pending_pages  | 0
pending_tuples | 0
</programlisting>
     </para>

    <para>
     The output columns are:

    <informaltable>
     <tgroup cols="3">
      <thead>
       <row>
        <entry>Column</entry>
        <entry>Type</entry>
        <entry>Description</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><structfield>version</structfield></entry>
        <entry><type>integer</type></entry>
        <entry>GIN version number</entry>
       </row>

       <row>
        <entry><structfield>pending_pages</structfield></entry>
        <entry><type>integer</type></entry>
        <entry>Number of pages in the pending list</entry>
       </row>

       <row>
        <entry><structfield>pending_tuples</structfield></entry>
        <entry><type>bigint</type></entry>
        <entry>Number of tuples in the pending list</entry>
       </row>

      </tbody>
     </tgroup>
    </informaltable>
    </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <indexterm>
      <primary>pg_relpages</primary>
     </indexterm>
     <function>pg_relpages(regclass) returns bigint</>
    </term>

    <listitem>
     <para>
      <function>pg_relpages</function> returns the number of pages in the
      relation.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>pg_relpages(text) returns bigint</>
    </term>

    <listitem>
     <para>
      This is the same as <function>pg_relpages(regclass)</function>, except
      that the target relation is specified as TEXT. This function is kept
      because of backward-compatibility so far, and will be deprecated in
      some future release.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <indexterm>
      <primary>pgstattuple_approx</primary>
     </indexterm>
     <function>pgstattuple_approx(regclass) returns record</>
    </term>

    <listitem>
     <para>
      <function>pgstattuple_approx</function> is a faster alternative to
      <function>pgstattuple</function> that returns approximate results.
      The argument is the target relation's name or OID.
      For example:
<programlisting>
test=> SELECT * FROM pgstattuple_approx('pg_catalog.pg_proc'::regclass);
-[ RECORD 1 ]--------+-------
table_len            | 573440
scanned_percent      | 2
approx_tuple_count   | 2740
approx_tuple_len     | 561210
approx_tuple_percent | 97.87
dead_tuple_count     | 0
dead_tuple_len       | 0
dead_tuple_percent   | 0
approx_free_space    | 11996
approx_free_percent  | 2.09
</programlisting>
      The output columns are described in <xref linkend="pgstatapprox-columns">.
     </para>

     <para>
      Whereas <function>pgstattuple</function> always performs a
      full-table scan and returns an exact count of live and dead tuples
      (and their sizes) and free space, <function>pgstattuple_approx</function>
      tries to avoid the full-table scan and returns exact dead tuple
      statistics along with an approximation of the number and
      size of live tuples and free space.
     </para>

     <para>
      It does this by skipping pages that have only visible tuples
      according to the visibility map (if a page has the corresponding VM
      bit set, then it is assumed to contain no dead tuples). For such
      pages, it derives the free space value from the free space map, and
      assumes that the rest of the space on the page is taken up by live
      tuples.
     </para>

     <para>
      For pages that cannot be skipped, it scans each tuple, recording its
      presence and size in the appropriate counters, and adding up the
      free space on the page. At the end, it estimates the total number of
      live tuples based on the number of pages and tuples scanned (in the
      same way that VACUUM estimates pg_class.reltuples).
     </para>

     <table id="pgstatapprox-columns">
      <title><function>pgstattuple_approx</function> Output Columns</title>
      <tgroup cols="3">
       <thead>
        <row>
         <entry>Column</entry>
         <entry>Type</entry>
         <entry>Description</entry>
        </row>
       </thead>

       <tbody>
        <row>
         <entry><structfield>table_len</structfield></entry>
         <entry><type>bigint</type></entry>
         <entry>Physical relation length in bytes (exact)</entry>
        </row>
        <row>
         <entry><structfield>scanned_percent</structfield></entry>
         <entry><type>float8</type></entry>
         <entry>Percentage of table scanned</entry>
        </row>
        <row>
         <entry><structfield>approx_tuple_count</structfield></entry>
         <entry><type>bigint</type></entry>
         <entry>Number of live tuples (estimated)</entry>
        </row>
        <row>
         <entry><structfield>approx_tuple_len</structfield></entry>
         <entry><type>bigint</type></entry>
         <entry>Total length of live tuples in bytes (estimated)</entry>
        </row>
        <row>
         <entry><structfield>approx_tuple_percent</structfield></entry>
         <entry><type>float8</type></entry>
         <entry>Percentage of live tuples</entry>
        </row>
        <row>
         <entry><structfield>dead_tuple_count</structfield></entry>
         <entry><type>bigint</type></entry>
         <entry>Number of dead tuples (exact)</entry>
        </row>
        <row>
         <entry><structfield>dead_tuple_len</structfield></entry>
         <entry><type>bigint</type></entry>
         <entry>Total length of dead tuples in bytes (exact)</entry>
        </row>
        <row>
         <entry><structfield>dead_tuple_percent</structfield></entry>
         <entry><type>float8</type></entry>
         <entry>Percentage of dead tuples</entry>
        </row>
        <row>
         <entry><structfield>approx_free_space</structfield></entry>
         <entry><type>bigint</type></entry>
         <entry>Total free space in bytes (estimated)</entry>
        </row>
        <row>
         <entry><structfield>approx_free_percent</structfield></entry>
         <entry><type>float8</type></entry>
         <entry>Percentage of free space</entry>
        </row>

       </tbody>
      </tgroup>
     </table>

     <para>
      In the above output, the free space figures may not match the
      <function>pgstattuple</function> output exactly, because the free
      space map gives us an exact figure, but is not guaranteed to be
      accurate to the byte.
     </para>

    </listitem>
   </varlistentry>

  </variablelist>
 </sect2>

 <sect2>
  <title>Authors</title>

  <para>
   Tatsuo Ishii, Satoshi Nagayasu and Abhijit Menon-Sen
  </para>
 </sect2>

</sect1>