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

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<refentry id="SQL-SET-TRANSACTION">
 <docinfo>
  <date>2000-11-24</date>
 </docinfo>

 <refmeta>
  <refentrytitle id="SQL-SET-TRANSACTION-TITLE">SET TRANSACTION</refentrytitle>
  <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 ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE }
SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE }
  </synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   This command sets the transaction isolation level.  The
   <command>SET TRANSACTION</command> command sets the characteristics
   for the current SQL-transaction. It has no effect on any subsequent
   transactions.  This command cannot be used after the first DML
   statement (<command>SELECT</command>, <command>INSERT</command>,
   <command>DELETE</command>, <command>UPDATE</command>,
   <command>FETCH</command>, <command>COPY</command>) of a transaction
   has been executed.  <command>SET SESSION CHARACTERISTICS</command>
   sets the default transaction isolation level for each transaction
   for a session.  <command>SET TRANSACTION</command> can override it
   for an individual transaction.
  </para>

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

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

    <varlistentry>
     <term>SERIALIZABLE</term>
     <listitem>
      <para>
       The current transaction can only see rows committed before
       first DML statement was executed in this transaction.
      </para>
      <tip>
       <para>
        Intuitively, serializable means that two concurrent
        transactions will leave the database in the same state as if
        the two has been executed strictly after one another in either
        order.
       </para>
      </tip>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   The session default transaction isolation level can also be set
   with the command <literal>SET default_transaction_isolation =
   '<replaceable>value</replaceable>'</literal> and in the
   configuration file.  Consult the <citetitle>Administrator's
   Guide</citetitle> for more information.
  </para>
 </refsect1>

 <refsect1 id="R1-SQL-SET-TRANSACTION-3">
  <title>Compatibility</title>

  <refsect2 id="R2-SQL-SET-TRANSACTION-4">
   <title>SQL92, SQL99</title>

   <para>
    SERIALIZABLE is the default level in <acronym>SQL</acronym>.
    Postgres does not provide the isolation levels <option>READ
     UNCOMMITTED</option> and <option>REPEATABLE READ</option>. Because
    of multi-version concurrency control, the serializable level is not
    truly serializable. See the <citetitle>User's Guide</citetitle> for
    details.
   </para>

   <para>
    In <acronym>SQL</acronym> there are two other transaction
    characteristics that can be set with these commands: whether the
    transaction is read-only and the size of the diagnostics area.
    Neither of these concepts are supported in Postgres.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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