SET TRANSACTION(7) PostgreSQL 9.1.9 Documentation SET TRANSACTION(7)NAMESET_TRANSACTION - set the characteristics of the current transaction
SYNOPSIS
SET TRANSACTION transaction_mode [, ...]
SET SESSION CHARACTERISTICS AS TRANSACTION transaction_mode [, ...]
where transaction_mode is one of:
ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED }
READ WRITE | READ ONLY
[ NOT ] DEFERRABLE
DESCRIPTION
The SET TRANSACTION command sets the characteristics of the current
transaction. It has no effect on any subsequent transactions. SET
SESSION CHARACTERISTICS sets the default transaction characteristics
for subsequent transactions of a session. These defaults can be
overridden by SET TRANSACTION for an individual transaction.
The available transaction characteristics are the transaction isolation
level, the transaction access mode (read/write or read-only), and the
deferrable mode.
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.
REPEATABLE READ
All statements of the current transaction can only see rows
committed before the first query or data-modification statement was
executed in this transaction.
SERIALIZABLE
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 serialization_failure SQLSTATE.
The SQL standard defines one additional level, READ UNCOMMITTED. In
PostgreSQL READ UNCOMMITTED is treated as READ COMMITTED.
The transaction isolation level cannot be changed after the first query
or data-modification statement (SELECT, INSERT, DELETE, UPDATE, FETCH,
or COPY) of a transaction has been executed. See Chapter 13,
Concurrency Control, in the documentation 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: INSERT,
UPDATE, DELETE, and COPY FROM if the table they would write to is not a
temporary table; all CREATE, ALTER, and DROP commands; COMMENT, GRANT,
REVOKE, TRUNCATE; and EXPLAIN ANALYZE and EXECUTE 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.
The DEFERRABLE transaction property has no effect unless the
transaction is also SERIALIZABLE and READ ONLY. 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 SERIALIZABLE 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.
NOTES
If SET TRANSACTION is executed without a prior START TRANSACTION or
BEGIN, it will appear to have no effect, since the transaction will
immediately end.
It is possible to dispense with SET TRANSACTION by instead specifying
the desired transaction_modes in BEGIN or START TRANSACTION.
The session default transaction modes can also be set by setting the
configuration parameters default_transaction_isolation,
default_transaction_read_only, and default_transaction_deferrable. (In
fact SET SESSION CHARACTERISTICS is just a verbose equivalent for
setting these variables with SET.) This means the defaults can be set
in the configuration file, via ALTER DATABASE, etc. Consult Chapter 18,
Server Configuration, in the documentation for more information.
COMPATIBILITY
Both commands are defined in the SQL standard. SERIALIZABLE is the
default transaction isolation level in the standard. In PostgreSQL the
default is ordinarily READ COMMITTED, but you can change it as
mentioned above.
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 PostgreSQL server.
The DEFERRABLE transaction_mode is a PostgreSQL language extension.
The SQL standard requires commas between successive transaction_modes,
but for historical reasons PostgreSQL allows the commas to be omitted.
PostgreSQL 9.1.9 2013-04-01 SET TRANSACTION(7)
