blob: b8b94537aa24dec256b7b29e65e142cf851dd562 [file] [log] [blame]
<!-- $PostgreSQL: pgsql/doc/src/sgml/ref/set_transaction.sgml,v 1.25 2006/09/16 00:30:20 momjian Exp $ -->
<refentry id="SQL-SET-TRANSACTION">
<refentrytitle id="SQL-SET-TRANSACTION-TITLE">SET TRANSACTION</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
<refname>SET TRANSACTION</refname>
<refpurpose>set the characteristics of the current transaction</refpurpose>
<indexterm zone="sql-set-transaction">
<primary>SET TRANSACTION</primary>
SET TRANSACTION <replaceable class="parameter">transaction_mode</replaceable> [, ...]
SET SESSION CHARACTERISTICS AS TRANSACTION <replaceable class="parameter">transaction_mode</replaceable> [, ...]
where <replaceable class="parameter">transaction_mode</replaceable> is one of:
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.
The available transaction characteristics are the transaction
isolation level and the transaction access mode (read/write or
The isolation level of a transaction determines what data the
transaction can see when other transactions are running concurrently:
<term><literal>READ COMMITTED</literal></term>
A statement can only see rows committed before it began. This
is the default.
All statements of the current transaction can only see rows committed
before the first query or data-modification statement was executed in
this transaction.
The SQL standard defines two additional levels, <literal>READ
UNCOMMITTED</literal> and <literal>REPEATABLE READ</literal>.
In <productname>PostgreSQL</productname> <literal>READ
UNCOMMITTED</literal> is treated as
<literal>READ COMMITTED</literal>, while <literal>REPEATABLE
READ</literal> is treated as <literal>SERIALIZABLE</literal>.
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.
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.
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
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>.
The session default transaction modes can also be set by setting the
configuration parameters <xref linkend="guc-default-transaction-isolation">
and <xref linkend="guc-default-transaction-read-only">.
(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.
<refsect1 id="R1-SQL-SET-TRANSACTION-3">
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. Because of lack of predicate locking, the
<literal>SERIALIZABLE</literal> level is not truly
serializable. See <xref linkend="mvcc"> for details.
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.
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