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postgresql
17th chunk of `doc/src/sgml/mvcc.sgml`
d863ce22c6be43ef1d4144e681193d075cdcea92fad7ebdd0000000100000fa0
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        <entry align="center"></entry>
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        <entry align="center"></entry>
        <entry align="center"></entry>
        <entry align="center"></entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
       </row>
       <row>
        <entry><literal>ROW EXCL.</literal></entry>
        <entry align="center"></entry>
        <entry align="center"></entry>
        <entry align="center"></entry>
        <entry align="center"></entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
       </row>
       <row>
        <entry><literal>SHARE UPDATE EXCL.</literal></entry>
        <entry align="center"></entry>
        <entry align="center"></entry>
        <entry align="center"></entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
       </row>
       <row>
        <entry><literal>SHARE</literal></entry>
        <entry align="center"></entry>
        <entry align="center"></entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center"></entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
       </row>
       <row>
        <entry><literal>SHARE ROW EXCL.</literal></entry>
        <entry align="center"></entry>
        <entry align="center"></entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
       </row>
       <row>
        <entry><literal>EXCL.</literal></entry>
        <entry align="center"></entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
       </row>
       <row>
        <entry><literal>ACCESS EXCL.</literal></entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </sect2>

   <sect2 id="locking-rows">
    <title>Row-Level Locks</title>

    <para>
     In addition to table-level locks, there are row-level locks, which
     are listed as below with the contexts in which they are used
     automatically by <productname>PostgreSQL</productname>.  See
     <xref linkend="row-lock-compatibility"/> for a complete table of
     row-level lock conflicts.  Note that a transaction can hold
     conflicting locks on the same row, even in different subtransactions;
     but other than that, two transactions can never hold conflicting locks
     on the same row.  Row-level locks do not affect data querying; they
     block only <emphasis>writers and lockers</emphasis> to the same
     row.  Row-level locks are released at transaction end or during
     savepoint rollback, just like table-level locks.

    </para>

     <variablelist>
      <title>Row-Level Lock Modes</title>
      <varlistentry>
       <term>
        <literal>FOR UPDATE</literal>
       </term>
       <listitem>
        <para>
         <literal>FOR UPDATE</literal> causes the rows retrieved by the
         <command>SELECT</command> statement to be locked as though for
         update.  This prevents them from being locked, modified or
Title: Lock Modes and Row-Level Locking in PostgreSQL
Summary
This section describes the different lock modes in PostgreSQL, including table-level locks and row-level locks, and explains how they are used to prevent conflicts between transactions, with details on the contexts in which row-level locks are used automatically and the rules for releasing locks at the end of a transaction or during savepoint rollback.