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SET TRANSACTION(7)	 PostgreSQL 17.5 Documentation	    SET	TRANSACTION(7)

NAME
       SET_TRANSACTION - set the characteristics of the	current	transaction

SYNOPSIS
       SET TRANSACTION transaction_mode	[, ...]
       SET TRANSACTION SNAPSHOT	snapshot_id
       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.	In addition, a snapshot	can be selected, though	only
       for the current transaction, not	as a session default.

       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	error.
       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, MERGE,
       FETCH, or COPY) of a transaction	has been executed. See Chapter 13 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, MERGE, 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	three of these
       properties are selected for 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.

       The SET TRANSACTION SNAPSHOT command allows a new transaction to	run
       with the	same snapshot as an existing transaction. The pre-existing
       transaction must	have exported its snapshot with	the pg_export_snapshot
       function	(see Section 9.28.5). That function returns a snapshot
       identifier, which must be given to SET TRANSACTION SNAPSHOT to specify
       which snapshot is to be imported. The identifier	must be	written	as a
       string literal in this command, for example '00000003-0000001B-1'.  SET
       TRANSACTION SNAPSHOT can	only be	executed at the	start of a
       transaction, before the first query or data-modification	statement
       (SELECT,	INSERT,	DELETE,	UPDATE,	MERGE, FETCH, or COPY) of the
       transaction. Furthermore, the transaction must already be set to
       SERIALIZABLE or REPEATABLE READ isolation level (otherwise, the
       snapshot	would be discarded immediately,	since READ COMMITTED mode
       takes a new snapshot for	each command). If the importing	transaction
       uses SERIALIZABLE isolation level, then the transaction that exported
       the snapshot must also use that isolation level.	Also, a	non-read-only
       serializable transaction	cannot import a	snapshot from a	read-only
       transaction.

NOTES
       If SET TRANSACTION is executed without a	prior START TRANSACTION	or
       BEGIN, it emits a warning and otherwise has no effect.

       It is possible to dispense with SET TRANSACTION by instead specifying
       the desired transaction_modes in	BEGIN or START TRANSACTION. But	that
       option is not available for SET TRANSACTION SNAPSHOT.

       The session default transaction modes can also be set or	examined via
       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 19
       for more	information.

       The current transaction's modes can similarly be	set or examined	via
       the configuration parameters transaction_isolation,
       transaction_read_only, and transaction_deferrable. Setting one of these
       parameters acts the same	as the corresponding SET TRANSACTION option,
       with the	same restrictions on when it can be done. However, these
       parameters cannot be set	in the configuration file, or from any source
       other than live SQL.

EXAMPLES
       To begin	a new transaction with the same	snapshot as an already
       existing	transaction, first export the snapshot from the	existing
       transaction. That will return the snapshot identifier, for example:

	   BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE	READ;
	   SELECT pg_export_snapshot();
	    pg_export_snapshot
	   ---------------------
	    00000003-0000001B-1
	   (1 row)

       Then give the snapshot identifier in a SET TRANSACTION SNAPSHOT command
       at the beginning	of the newly opened transaction:

	   BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE	READ;
	   SET TRANSACTION SNAPSHOT '00000003-0000001B-1';

COMPATIBILITY
       These commands are defined in the SQL standard, except for the
       DEFERRABLE transaction mode and the SET TRANSACTION SNAPSHOT form,
       which are PostgreSQL extensions.

       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 SQL standard	requires commas	between	successive transaction_modes,
       but for historical reasons PostgreSQL allows the	commas to be omitted.

PostgreSQL 17.5			     2025		    SET	TRANSACTION(7)

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