postgresql/doc/src/sgml/pgrowlocks.sgml

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

<sect1 id="pgrowlocks" xreflabel="pgrowlocks">
 <title>pgrowlocks &mdash; show a table's row locking information</title>

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

 <para>
  The <filename>pgrowlocks</filename> module provides a function to show row
  locking information for a specified table.
 </para>

 <para>
  By default use is restricted to superusers, roles with privileges of the
  <literal>pg_stat_scan_tables</literal> role, and users with
  <literal>SELECT</literal> permissions on the table.
 </para>


 <sect2 id="pgrowlocks-overview">
  <title>Overview</title>

  <indexterm>
   <primary>pgrowlocks</primary>
  </indexterm>

<synopsis>
pgrowlocks(text) returns setof record
</synopsis>

  <para>
   The parameter is the name of a table.  The result is a set of records,
   with one row for each locked row within the table.  The output columns
   are shown in <xref linkend="pgrowlocks-columns"/>.
  </para>

  <table id="pgrowlocks-columns">
   <title><function>pgrowlocks</function> Output Columns</title>

   <tgroup cols="3">
    <thead>
     <row>
      <entry>Name</entry>
      <entry>Type</entry>
      <entry>Description</entry>
     </row>
    </thead>
    <tbody>

     <row>
      <entry><structfield>locked_row</structfield></entry>
      <entry><type>tid</type></entry>
      <entry>Tuple ID (TID) of locked row</entry>
     </row>
     <row>
      <entry><structfield>locker</structfield></entry>
      <entry><type>xid</type></entry>
      <entry>Transaction ID of locker, or multixact ID if
      multitransaction;  see <xref linkend="transaction-id"/></entry>
     </row>
     <row>
      <entry><structfield>multi</structfield></entry>
      <entry><type>boolean</type></entry>
      <entry>True if locker is a multitransaction</entry>
     </row>
     <row>
      <entry><structfield>xids</structfield></entry>
      <entry><type>xid[]</type></entry>
      <entry>Transaction IDs of lockers (more than one if multitransaction)</entry>
     </row>
     <row>
      <entry><structfield>modes</structfield></entry>
      <entry><type>text[]</type></entry>
      <entry>Lock mode of lockers (more than one if multitransaction),
       an array of <literal>For Key Share</literal>, <literal>For Share</literal>,
       <literal>For No Key Update</literal>, <literal>No Key Update</literal>,
       <literal>For Update</literal>, <literal>Update</literal>.</entry>
     </row>

     <row>
      <entry><structfield>pids</structfield></entry>
      <entry><type>integer[]</type></entry>
      <entry>Process IDs of locking backends (more than one if multitransaction)</entry>
     </row>

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

  <para>
   <function>pgrowlocks</function> takes <literal>AccessShareLock</literal> for the
   target table and reads each row one by one to collect the row locking
   information.  This is not very speedy for a large table.  Note that:
  </para>

  <orderedlist>
   <listitem>
    <para>
     If an <literal>ACCESS EXCLUSIVE</literal> lock is taken on the table,
     <function>pgrowlocks</function> will be blocked.
    </para>
   </listitem>
   <listitem>
    <para>
     <function>pgrowlocks</function> is not guaranteed to produce a
     self-consistent snapshot.  It is possible that a new row lock is taken,
     or an old lock is freed, during its execution.
    </para>
   </listitem>
  </orderedlist>

  <para>
   <function>pgrowlocks</function> does not show the contents of locked
   rows. If you want to take a look at the row contents at the same time, you
   could do something like this:

<programlisting>
SELECT * FROM accounts AS a, pgrowlocks('accounts') AS p
  WHERE p.locked_row = a.ctid;
</programlisting>

   Be aware however that such a query will be very inefficient.
  </para>
 </sect2>

 <sect2 id="pgrowlocks-sample-output">
  <title>Sample Output</title>

  <screen>
=# SELECT * FROM pgrowlocks('t1');
 locked_row | locker | multi | xids  |     modes      |  pids
------------+--------+-------+-------+----------------+--------
 (0,1)      |    609 | f     | {609} | {"For Share"}  | {3161}
 (0,2)      |    609 | f     | {609} | {"For Share"}  | {3161}
 (0,3)      |    607 | f     | {607} | {"For Update"} | {3107}
 (0,4)      |    607 | f     | {607} | {"For Update"} | {3107}
(4 rows)
</screen>
 </sect2>

 <sect2 id="pgrowlocks-author">
  <title>Author</title>

  <para>
   Tatsuo Ishii
  </para>
 </sect2>

</sect1>