SET TRANSACTION

Sets the characteristics of the current transaction.

Synopsis

SET TRANSACTION <transaction_mode> [, ...]
SET TRANSACTION SNAPSHOT <snapshot_id>
SET SESSION CHARACTERISTICS AS TRANSACTION <transaction_mode> [, ...] 
     [READ ONLY | READ WRITE]
     [NOT] DEFERRABLE

where <transaction_mode> is one of:

    ISOLATION LEVEL {SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED}
    READ WRITE | READ ONLY
    [NOT] DEFERRABLE

and <snapshot_id> is the id of the existing transaction whose snapshot you want this transaction to run with.

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.

Note
Deferrable transactions require the transaction to be serializable. Greenplum Database does not support serializable transactions, so including the DEFERRABLE clause has no effect.

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 in the current transaction can only see rows committed before the first query or data-modification statement run in the transaction.

The SQL standard defines two additional levels, READ UNCOMMITTED and SERIALIZABLE. In Greenplum Database, READ UNCOMMITTED is treated as READ COMMITTED. If you specify SERIALIZABLE, Greenplum Database falls back to REPEATABLE READ.

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 run.

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 run 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 cancelled by a serialization failure. This mode is well suited for long-running reports or backups. Because Greenplum Database does not support serializable transactions, the DEFERRABLE transaction property has no effect in Greenplum Database.

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. 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 '000003A1-1'. 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, 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 run 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.

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 Greenplum Database extensions.

SERIALIZABLE is the default transaction isolation level in the standard. In Greenplum Database, the default is READ COMMITTED. Due to lack of predicate locking, Greenplum Database does not fully support the SERIALIZABLE level, so it falls back to the REPEATABLE READ level when SERIALIZABLE is specified. Essentially, a predicate-locking system prevents phantom reads by restricting what is written, whereas a multi-version concurrency control model (MVCC) as used in Greenplum Database prevents them by restricting what is read.

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 Greenplum Database server.

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