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>