doc/src/sgml/ref/savepoint.sgml - master - githubmirror/postgres

doc/src/sgml/ref/savepoint.sgml
165 строк4 KB
Bruce Momjian
doc: clarify the behavior of identically-named savepoints
14 июл 2022, 22:44
14 июл 2022, 22:44ec1fe23
<!--
doc/src/sgml/ref/savepoint.sgml
PostgreSQL documentation
-->

<refentry id="sql-savepoint">
 <indexterm zone="sql-savepoint">
  <primary>SAVEPOINT</primary>
 </indexterm>

 <indexterm zone="sql-savepoint">
  <primary>savepoints</primary>
  <secondary>defining</secondary>
 </indexterm>

 <refmeta>
  <refentrytitle>SAVEPOINT</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>SAVEPOINT</refname>
  <refpurpose>define a new savepoint within the current transaction</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
SAVEPOINT <replaceable>savepoint_name</replaceable>
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>SAVEPOINT</command> establishes a new savepoint within
   the current transaction.
  </para>

  <para>
   A savepoint is a special mark inside a transaction that allows all commands
   that are executed after it was established to be rolled back, restoring
   the transaction state to what it was at the time of the savepoint.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable>savepoint_name</replaceable></term>
    <listitem>
     <para>
      The name to give to the new savepoint.  If savepoints with the
      same name already exist, they will be inaccessible until newer
      identically-named savepoints are released.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   Use <link linkend="sql-rollback-to"><command>ROLLBACK TO</command></link> to
   rollback to a savepoint.  Use <link linkend="sql-release-savepoint"><command>RELEASE SAVEPOINT</command></link>
   to destroy a savepoint, keeping
   the effects of commands executed after it was established.
  </para>

  <para>
   Savepoints can only be established when inside a transaction block.
   There can be multiple savepoints defined within a transaction.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   To establish a savepoint and later undo the effects of all commands executed
   after it was established:
<programlisting>
BEGIN;
    INSERT INTO table1 VALUES (1);
    SAVEPOINT my_savepoint;
    INSERT INTO table1 VALUES (2);
    ROLLBACK TO SAVEPOINT my_savepoint;
    INSERT INTO table1 VALUES (3);
COMMIT;
</programlisting>
   The above transaction will insert the values 1 and 3, but not 2.
  </para>

  <para>
   To establish and later destroy a savepoint:
<programlisting>
BEGIN;
    INSERT INTO table1 VALUES (3);
    SAVEPOINT my_savepoint;
    INSERT INTO table1 VALUES (4);
    RELEASE SAVEPOINT my_savepoint;
COMMIT;
</programlisting>
   The above transaction will insert both 3 and 4.
  </para>

  <para>
  To use a single savepoint name:
<programlisting>
BEGIN;
    INSERT INTO table1 VALUES (1);
    SAVEPOINT my_savepoint;
    INSERT INTO table1 VALUES (2);
    SAVEPOINT my_savepoint;
    INSERT INTO table1 VALUES (3);

    -- rollback to the second savepoint
    ROLLBACK TO SAVEPOINT my_savepoint;
    SELECT * FROM table1;               -- shows rows 1 and 2

    -- release the second savepoint
    RELEASE SAVEPOINT my_savepoint;

    -- rollback to the first savepoint
    ROLLBACK TO SAVEPOINT my_savepoint;
    SELECT * FROM table1;               -- shows only row 1
COMMIT;
</programlisting>
  The above transaction shows row 3 being rolled back first, then row 2.
  </para>

 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   SQL requires a savepoint to be destroyed automatically when another
   savepoint with the same name is established.  In
   <productname>PostgreSQL</productname>, the old savepoint is kept, though only the more
   recent one will be used when rolling back or releasing.  (Releasing the
   newer savepoint with <command>RELEASE SAVEPOINT</command> will cause the older one
   to again become accessible to <command>ROLLBACK TO SAVEPOINT</command> and
   <command>RELEASE SAVEPOINT</command>.) Otherwise, <command>SAVEPOINT</command> is
   fully SQL conforming.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-begin"/></member>
   <member><xref linkend="sql-commit"/></member>
   <member><xref linkend="sql-release-savepoint"/></member>
   <member><xref linkend="sql-rollback"/></member>
   <member><xref linkend="sql-rollback-to"/></member>
  </simplelist>
 </refsect1>
</refentry>