pgrowlocks.sgml

<sect1 id="pgrowlocks">
 <title>pgrowlocks</title>
 
 <indexterm zone="pgrowlocks">
  <primary>pgrowlocks</primary>
 </indexterm>

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

 <sect2>
  <title>Overview</title>
  <programlisting>
pgrowlocks(text) RETURNS pgrowlocks_type
  </programlisting>
  <para>
   The parameter is a name of table. And <literal>pgrowlocks_type</literal> is 
   defined as:
  </para>
  <programlisting>
CREATE TYPE pgrowlocks_type AS (
	locked_row TID,		-- row TID
	lock_type TEXT,		-- lock type
	locker XID,		-- locking XID
	multi bool,		-- multi XID?
	xids xid[],		-- multi XIDs
	pids INTEGER[]		-- locker's process id
);
  </programlisting>

  <table>
   <title>pgrowlocks_type</title>
   <tgroup cols="2">
    <tbody>
     <row>
      <entry>locked_row</entry>
      <entry>tuple ID(TID) of each locked rows</entry>
     </row>
     <row>
      <entry>lock_type</entry>
      <entry>"Shared" for shared lock, "Exclusive" for exclusive lock</entry>
     </row>
     <row>
      <entry>locker</entry>
      <entry>transaction ID of locker (Note 1)</entry>
     </row>
     <row>
      <entry>multi</entry>
      <entry>"t" if locker is a multi transaction, otherwise "f"</entry>
     </row>
     <row>
      <entry>xids</entry>
      <entry>XIDs of lockers (Note 2)</entry>
     </row>
     <row>
      <entry>pids</entry>
      <entry>process ids of locking backends</entry>
     </row>
    </tbody>
   </tgroup>
  </table>
  <para>
   Note1: If the locker is multi transaction, it represents the multi ID.
  </para>
  <para>
   Note2: If the locker is multi, multiple data are shown.
  </para>

  <para>
   The calling sequence for <literal>pgrowlocks</literal> is as follows:
   <literal>pgrowlocks</literal> grabs AccessShareLock for the target table and 
   reads each row one by one to get the row locking information. You should
   notice that:
  </para>
  <orderedlist>
   <listitem>
    <para>
    if the table is exclusive locked by someone else, 
    <literal>pgrowlocks</literal> will be blocked.
    </para>
   </listitem>
   <listitem>
    <para>
     <literal>pgrowlocks</literal> may show incorrect information if there's a 
     new lock or a lock is freeed while its execution.
    </para>
   </listitem>
  </orderedlist>
  <para>
   <literal>pgrowlocks</literal> 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:
  </para>
  <programlisting>
SELECT * FROM accounts AS a, pgrowlocks('accounts') AS p WHERE p.locked_ row = a.ctid;
  </programlisting>
 </sect2>

 <sect2>
  <title>Example</title>
  <para>
   <literal>pgrowlocks</literal> returns the following data type:
  </para>
  <para>
   Here is a sample execution of pgrowlocks:
  </para>
  <programlisting>
test=# SELECT * FROM pgrowlocks('t1');
 locked_row | lock_type | locker | multi |   xids    |     pids      
------------+-----------+--------+-------+-----------+---------------
      (0,1) | Shared    |     19 | t     | {804,805} | {29066,29068}
      (0,2) | Shared    |     19 | t     | {804,805} | {29066,29068}
      (0,3) | Exclusive |    804 | f     | {804}     | {29066}
      (0,4) | Exclusive |    804 | f     | {804}     | {29066}
(4 rows)
  </programlisting>

 </sect2>
</sect1>