SET TRANSACTIOSQL)- Language Statements (2000-1SET4TRANSACTION(l)
NAME
SET TRANSACTION - set the characteristics of the current
transaction
SYNOPSIS
SET TRANSACTION ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE }
SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL
{ READ COMMITTED | SERIALIZABLE }
DESCRIPTION
This command sets the transaction isolation level. The SET
TRANSACTION 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 (SELECT, INSERT,
DELETE, UPDATE, FETCH, COPY) of a transaction has been
executed. SET SESSION CHARACTERISTICS sets the default
transaction isolation level for each transaction for a
session. SET TRANSACTION can override it for an individual
transaction.
The isolation level of a transaction determines what data
the transaction can see when other transactions are running
concurrently.
READ COMMITTED
A statement can only see rows committed before it
began. This is the default.
SERIALIZABLE
The current transaction can only see rows committed
before first query or data-modification statement was
executed in this transaction.
Tip: 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.
NOTES
The session default transaction isolation level can also be
set with the command
SET default_transaction_isolation = 'value'
and in the configuration file. Consult the Administrator's
Guide for more information.
COMPATIBILITY
Page 1 (printed 3/24/03)
SET TRANSACTIOSQL)- Language Statements (2000-1SET4TRANSACTION(l)
SQL92, SQL99
SERIALIZABLE is the default level in SQL. PostgreSQL does
not provide the isolation levels READ UNCOMMITTED and
REPEATABLE READ. Because of multiversion concurrency
control, the SERIALIZABLE level is not truly serializable.
See the User's Guide for details.
In SQL 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 PostgreSQL.
Page 2 (printed 3/24/03)
