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<!-- doc/src/sgml/ref/set_transaction.sgml -->
<refentry id="SQL-SET-TRANSACTION">
<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>
<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>
<refsynopsisdiv>
<synopsis>
SET TRANSACTION <replaceable class="parameter">transaction_mode</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.
</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> <literal>SQLSTATE</literal>.
</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 of these properties are set on 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>
</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 will appear to have no effect, since the transaction will immediately
end.
</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>.
</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</>.)
This means the defaults can be set in the configuration file, via
<command>ALTER DATABASE</>, etc. 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
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 <literal>DEFERRABLE</literal>
<replaceable class="parameter">transaction_mode</replaceable>
is a <productname>PostgreSQL</productname> language extension.
</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>
|
