SET TRANSACTION — set the characteristics of the current transaction
SET TRANSACTIONtransaction_mode
[, ...] SET SESSION CHARACTERISTICS AS TRANSACTIONtransaction_mode
[, ...] wheretransaction_mode
is one of: ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED } READ WRITE | READ ONLY
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 and the transaction access mode (read/write or read-only).
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
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, READ
UNCOMMITTED
and REPEATABLE READ
.
In PostgreSQL READ
UNCOMMITTED
is treated as
READ COMMITTED
, while REPEATABLE
READ
is treated as SERIALIZABLE
.
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 12, Concurrency Control 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.
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
and default_transaction_read_only.
(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 17, Server Configuration
for more information.
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. Because of lack of predicate locking, the
SERIALIZABLE
level is not truly
serializable. See Chapter 12, Concurrency Control 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 PostgreSQL server.
The SQL standard requires commas between successive transaction_modes
, but for historical
reasons PostgreSQL allows the commas to be
omitted.