This lock mode is also acquired by any
         <command>UPDATE</command> that does not acquire a <literal>FOR UPDATE</literal> lock.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>
        <literal>FOR SHARE</literal>
       </term>
       <listitem>
        <para>
         Behaves similarly to <literal>FOR NO KEY UPDATE</literal>, except that it
         acquires a shared lock rather than exclusive lock on each retrieved
         row.  A shared lock blocks other transactions from performing
         <command>UPDATE</command>, <command>DELETE</command>,
         <command>SELECT FOR UPDATE</command> or
         <command>SELECT FOR NO KEY UPDATE</command> on these rows, but it does not
         prevent them from performing <command>SELECT FOR SHARE</command> or
         <command>SELECT FOR KEY SHARE</command>.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>
        <literal>FOR KEY SHARE</literal>
       </term>
       <listitem>
        <para>
         Behaves similarly to <literal>FOR SHARE</literal>, except that the
         lock is weaker: <literal>SELECT FOR UPDATE</literal> is blocked, but not
         <literal>SELECT FOR NO KEY UPDATE</literal>.  A key-shared lock blocks
         other transactions from performing <command>DELETE</command> or
         any <command>UPDATE</command> that changes the key values, but not
         other <command>UPDATE</command>, and neither does it prevent
         <command>SELECT FOR NO KEY UPDATE</command>, <command>SELECT FOR SHARE</command>,
         or <command>SELECT FOR KEY SHARE</command>.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

    <para>
     <productname>PostgreSQL</productname> doesn't remember any
     information about modified rows in memory, so there is no limit on
     the number of rows locked at one time.  However, locking a row
     might cause a disk write, e.g., <command>SELECT FOR
     UPDATE</command> modifies selected rows to mark them locked, and so
     will result in disk writes.
    </para>

    <table tocentry="1" id="row-lock-compatibility">
     <title>Conflicting Row-Level Locks</title>
     <tgroup cols="5">
      <colspec colname="col1"    colwidth="1.5*"/>
      <colspec colname="lockst"  colwidth="1*"/>
      <colspec colname="col3"    colwidth="1*"/>
      <colspec colname="col4"    colwidth="1*"/>
      <colspec colname="lockend" colwidth="1*"/>
      <spanspec namest="lockst" nameend="lockend" spanname="lockreq"/>
      <thead>
       <row>
        <entry morerows="1">Requested Lock Mode</entry>
        <entry spanname="lockreq">Current Lock Mode</entry>
       </row>
       <row>
        <entry>FOR KEY SHARE</entry>
        <entry>FOR SHARE</entry>
        <entry>FOR NO KEY UPDATE</entry>
        <entry>FOR UPDATE</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>FOR KEY SHARE</entry>
        <entry align="center"></entry>
        <entry align="center"></entry>
        <entry align="center"></entry>
        <entry align="center">X</entry>
       </row>
       <row>
        <entry>FOR SHARE</entry>
        <entry align="center"></entry>
        <entry align="center"></entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
       </row>
       <row>
        <entry>FOR NO KEY UPDATE</entry>
        <entry align="center"></entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
        <entry align="center">X</entry>
       </row>
       <row>
        <entry>FOR UPDATE</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-pages">
    <title>Page-Level Locks</title>

    <para>
     In addition to table and row locks, page-level share/exclusive