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<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/set_transaction.sgml,v 1.9 2002/08/04 04:31:44 momjian Exp $ -->
<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 query or data-modification
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 query or data-modification 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
<programlisting>
SET default_transaction_isolation = '<replaceable>value</replaceable>'
</programlisting>
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>
<option>SERIALIZABLE</option> is the default level in
<acronym>SQL</acronym>. <productname>PostgreSQL</productname> does
not provide the isolation levels <option>READ UNCOMMITTED</option>
and <option>REPEATABLE READ</option>. Because of multiversion
concurrency control, the <option>SERIALIZABLE</option> 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
<productname>PostgreSQL</productname>.
</para>
</refsect2>
</refsect1>
</refentry>
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