postgresql/doc/src/sgml/ref/set_transaction.sgml

<!-- doc/src/sgml/ref/set_transaction.sgml -->
<refentry id="sql-set-transaction">
 <indexterm zone="sql-set-transaction">
  <primary>SET TRANSACTION</primary>
 </indexterm>

 <indexterm>
  <primary>transaction isolation level</primary>
  <secondary>setting</secondary>
 </indexterm>

 <indexterm>
  <primary>read-only transaction</primary>
  <secondary>setting</secondary>
 </indexterm>

 <indexterm>
  <primary>deferrable transaction</primary>
  <secondary>setting</secondary>
 </indexterm>

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

 <refnamediv>
  <refname>SET TRANSACTION</refname>
  <refpurpose>set the characteristics of the current transaction</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
SET TRANSACTION <replaceable class="parameter">transaction_mode</replaceable> [, ...]
SET TRANSACTION SNAPSHOT <replaceable class="parameter">snapshot_id</replaceable>
SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transaction_mode</replaceable> [, ...]

<phrase>where <replaceable class="parameter">transaction_mode</replaceable> is one of:</phrase>

    ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
    READ WRITE | READ ONLY
    [ NOT ] DEFERRABLE
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   The <command>SET TRANSACTION</command> command sets the
   characteristics of the current transaction. It has no effect on any
   subsequent transactions.  <command>SET SESSION
   CHARACTERISTICS</command> sets the default transaction
   characteristics for subsequent transactions of a session.  These
   defaults can be overridden by <command>SET TRANSACTION</command>
   for an individual transaction.
  </para>

  <para>
   The available transaction characteristics are the transaction
   isolation level, the transaction access mode (read/write or
   read-only), and the deferrable mode.
   In addition, a snapshot can be selected, though only for the current
   transaction, not as a session default.
  </para>

  <para>
   The isolation level of a transaction determines what data the
   transaction can see when other transactions are running concurrently:

   <variablelist>
    <varlistentry>
     <term><literal>READ COMMITTED</literal></term>
     <listitem>
      <para>
       A statement can only see rows committed before it began. This
       is the default.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>REPEATABLE READ</literal></term>
     <listitem>
      <para>
       All statements of the current transaction can only see rows committed
       before the first query or data-modification statement was executed in
       this transaction.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>SERIALIZABLE</literal></term>
     <listitem>
      <para>
       All statements of the current transaction can only see rows committed
       before the first query or data-modification statement was executed in
       this transaction.  If a pattern of reads and writes among concurrent
       serializable transactions would create a situation which could not
       have occurred for any serial (one-at-a-time) execution of those
       transactions, one of them will be rolled back with a
       <literal>serialization_failure</literal> error.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

   The SQL standard defines one additional level, <literal>READ
   UNCOMMITTED</literal>.
   In <productname>PostgreSQL</productname> <literal>READ
   UNCOMMITTED</literal> is treated as <literal>READ COMMITTED</literal>.
  </para>

  <para>
   The transaction isolation level cannot be changed after the first query or
   data-modification statement (<command>SELECT</command>,
   <command>INSERT</command>, <command>DELETE</command>,
   <command>UPDATE</command>, <command>FETCH</command>, or
   <command>COPY</command>) of a transaction has been executed.  See
   <xref linkend="mvcc"> for more information about transaction
   isolation and concurrency control.
  </para>

  <para>
   The transaction access mode determines whether the transaction is
   read/write or read-only.  Read/write is the default.  When a
   transaction is read-only, the following SQL commands are
   disallowed: <literal>INSERT</literal>, <literal>UPDATE</literal>,
   <literal>DELETE</literal>, and <literal>COPY FROM</literal> if the
   table they would write to is not a temporary table; all
   <literal>CREATE</literal>, <literal>ALTER</literal>, and
   <literal>DROP</literal> commands; <literal>COMMENT</literal>,
   <literal>GRANT</literal>, <literal>REVOKE</literal>,
   <literal>TRUNCATE</literal>; and <literal>EXPLAIN ANALYZE</literal>
   and <literal>EXECUTE</literal> if the command they would execute is
   among those listed.  This is a high-level notion of read-only that
   does not prevent all writes to disk.
  </para>

  <para>
   The <literal>DEFERRABLE</literal> transaction property has no effect
   unless the transaction is also <literal>SERIALIZABLE</literal> and
   <literal>READ ONLY</literal>.  When all three of these properties are
   selected for a
   transaction, the transaction may block when first acquiring its snapshot,
   after which it is able to run without the normal overhead of a
   <literal>SERIALIZABLE</literal> transaction and without any risk of
   contributing to or being canceled by a serialization failure.  This mode
   is well suited for long-running reports or backups.
  </para>

  <para>
   The <literal>SET TRANSACTION SNAPSHOT</literal> command allows a new
   transaction to run with the same <firstterm>snapshot</firstterm> as an existing
   transaction.  The pre-existing transaction must have exported its snapshot
   with the <literal>pg_export_snapshot</literal> function (see <xref
   linkend="functions-snapshot-synchronization">).  That function returns a
   snapshot identifier, which must be given to <literal>SET TRANSACTION
   SNAPSHOT</literal> to specify which snapshot is to be imported.  The
   identifier must be written as a string literal in this command, for example
   <literal>'000003A1-1'</literal>.
   <literal>SET TRANSACTION SNAPSHOT</literal> can only be executed at the
   start of a transaction, before the first query or
   data-modification statement (<command>SELECT</command>,
   <command>INSERT</command>, <command>DELETE</command>,
   <command>UPDATE</command>, <command>FETCH</command>, or
   <command>COPY</command>) of the transaction.  Furthermore, the transaction
   must already be set to <literal>SERIALIZABLE</literal> or
   <literal>REPEATABLE READ</literal> isolation level (otherwise, the snapshot
   would be discarded immediately, since <literal>READ COMMITTED</literal> mode takes
   a new snapshot for each command).  If the importing transaction uses
   <literal>SERIALIZABLE</literal> isolation level, then the transaction that
   exported the snapshot must also use that isolation level.  Also, a
   non-read-only serializable transaction cannot import a snapshot from a
   read-only transaction.
  </para>

 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   If <command>SET TRANSACTION</command> is executed without a prior
   <command>START TRANSACTION</command> or <command>BEGIN</command>,
   it emits a warning and otherwise has no effect.
  </para>

  <para>
   It is possible to dispense with <command>SET TRANSACTION</command>
   by instead specifying the desired <replaceable
   class="parameter">transaction_modes</replaceable> in
   <command>BEGIN</command> or <command>START TRANSACTION</command>.
   But that option is not available for <command>SET TRANSACTION
   SNAPSHOT</command>.
  </para>

  <para>
   The session default transaction modes can also be set by setting the
   configuration parameters <xref linkend="guc-default-transaction-isolation">,
   <xref linkend="guc-default-transaction-read-only">, and
   <xref linkend="guc-default-transaction-deferrable">.
   (In fact <command>SET SESSION CHARACTERISTICS</command> is just a
   verbose equivalent for setting these variables with <command>SET</command>.)
   This means the defaults can be set in the configuration file, via
   <command>ALTER DATABASE</command>, etc.  Consult <xref linkend="runtime-config">
   for more information.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   To begin a new transaction with the same snapshot as an already
   existing transaction, first export the snapshot from the existing
   transaction. That will return the snapshot identifier, for example:

<programlisting>
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SELECT pg_export_snapshot();
 pg_export_snapshot
---------------------
 00000003-0000001B-1
(1 row)
</programlisting>

   Then give the snapshot identifier in a <command>SET TRANSACTION
   SNAPSHOT</command> command at the beginning of the newly opened
   transaction:

<programlisting>
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SET TRANSACTION SNAPSHOT '00000003-0000001B-1';
</programlisting></para>
 </refsect1>

 <refsect1 id="r1-sql-set-transaction-3">
  <title>Compatibility</title>

  <para>
   These commands are defined in the <acronym>SQL</acronym> standard,
   except for the <literal>DEFERRABLE</literal> transaction mode
   and the <command>SET TRANSACTION SNAPSHOT</command> form, which are
   <productname>PostgreSQL</productname> extensions.
  </para>

  <para>
   <literal>SERIALIZABLE</literal> is the default transaction
   isolation level in the standard.  In
   <productname>PostgreSQL</productname> the default is ordinarily
   <literal>READ COMMITTED</literal>, but you can change it as
   mentioned above.
  </para>

  <para>
   In the SQL standard, there is one other transaction characteristic
   that can be set with these commands: the size of the diagnostics
   area.  This concept is specific to embedded SQL, and therefore is
   not implemented in the <productname>PostgreSQL</productname> server.
  </para>

  <para>
   The SQL standard requires commas between successive <replaceable
   class="parameter">transaction_modes</replaceable>, but for historical
   reasons <productname>PostgreSQL</productname> allows the commas to be
   omitted.
  </para>
 </refsect1>
</refentry>