SET TRANSACTION — set the characteristics of the current transaction
SET TRANSACTIONtransaction_mode[, ...] SET SESSION CHARACTERISTICS AS TRANSACTIONtransaction_mode[, ...] wheretransaction_modeis 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 COMMITTEDA statement can only see rows committed before it began. This is the default.
SERIALIZABLEAll 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 TO 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.