SET TRANSACTION - Set the characteristics of the current
SQL-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 cur
rent SQL-transaction. It has no effect on any subsequent
transactions. This command cannot be used after the first
DML 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 run
ning 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 DML statement was executed in this
transaction.
Tip: Intuitively, serializable means that two con
current transactions will leave the database in the
same state as if the two has been executed strictly
after one another in either order.
COMPATIBILITY
SQL92, SQL99
SERIALIZABLE is the default level in SQL. Postgres does
not provide the isolation levels READ UNCOMMITTED and
REPEATABLE READ. Because of multi-version concurrency con
trol, 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 transac
tion is read-only and the size of the diagnostics area.
Neither of these concepts are supported in Postgres.
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