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<!-- $Header: /cvsroot/pgsql/doc/src/sgml/ref/set_transaction.sgml,v 1.17 2003/09/11 21:42:20 momjian Exp $ -->
<refentry id="SQL-SET-TRANSACTION">
<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>
<indexterm zone="sql-set-transaction">
<primary>SET TRANSACTION</primary>
</indexterm>
<refsynopsisdiv>
<synopsis>
SET TRANSACTION
[ ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE } ] [ READ WRITE | READ ONLY ]
SET SESSION CHARACTERISTICS AS TRANSACTION
[ ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE } ] [ READ WRITE | READ ONLY ]
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
The <command>SET TRANSACTION</command> command sets the transaction
characteristics of the current transaction. It has no effect on any
subsequent transactions. <command>SET SESSION
CHARACTERISTICS</command> sets the default transaction
characteristics for each transaction of a session. <command>SET
TRANSACTION</command> can override it for an individual
transaction.
</para>
<para>
The available transaction characteristics are the transaction
isolation level and the transaction access mode (read/write or
read-only).
</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>SERIALIZABLE</literal></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>
The transaction isolation level cannot be set 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. 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 TO</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 writes to disk.
</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 <xref linkend="runtime-config"> for more
information.
</para>
</refsect1>
<refsect1 id="R1-SQL-SET-TRANSACTION-3">
<title>Compatibility</title>
<para>
Both commands are defined in the <acronym>SQL</acronym> standard.
<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
described above. <productname>PostgreSQL</productname> does not
provide the isolation levels <literal>READ UNCOMMITTED</literal>
and <literal>REPEATABLE READ</literal>. Because of multiversion
concurrency control, the <literal>SERIALIZABLE</literal> level is
not truly serializable. See <xref linkend="mvcc"> for details.
</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 only for use in embedded SQL.
</para>
</refsect1>
</refentry>
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