SET TRANSACTION(7) PostgreSQL 18.6 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 trans- action. It has no effect on any subsequent transactions. SET SESSION CHAR- ACTERISTICS sets the default transaction characteristics for subsequent transactions of a session. These defaults can be overridden by SET TRANSAC- TION for an individual transaction. The available transaction characteristics are the transaction isolation level, the transaction access mode (read/write or read-only), and the de- ferrable 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 se- rializable transactions would create a situation which could not have occurred for any serial (one-at-a-time) execution of those transac- tions, one of them will be rolled back with a serialization_failure er- ror. The SQL standard defines one additional level, READ UNCOMMITTED. In Post- greSQL 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 infor- mation 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 tem- porary 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 se- lected 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-run- ning 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 im- ported. 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-mod- ification statement (SELECT, INSERT, DELETE, UPDATE, MERGE, FETCH, or COPY) of the transaction. Furthermore, the transaction must already be set to SE- RIALIZABLE or REPEATABLE READ isolation level (otherwise, the snapshot would be discarded immediately, since READ COMMITTED mode takes a new snap- shot for each command). If the importing transaction uses SERIALIZABLE iso- lation 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_transac- tion_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 config- uration 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 Post- greSQL 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 Post- greSQL server. The SQL standard requires commas between successive transaction_modes, but for historical reasons PostgreSQL allows the commas to be omitted. PostgreSQL 18.6 2026 SET TRANSACTION(7)
NAME | SYNOPSIS | DESCRIPTION | NOTES | EXAMPLES | COMPATIBILITY
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