SET TRANSACTION

SET TRANSACTION

Sets the characteristics of the current transaction.

Synopsis

SET TRANSACTION [transaction_mode] [READ ONLY | READ WRITE]

SET SESSION CHARACTERISTICS AS TRANSACTION transaction_mode 
     [READ ONLY | READ WRITE]

where transaction_mode is one of:

   ISOLATION LEVEL {SERIALIZABLE | READ COMMITTED | READ UNCOMMITTED}

Description

The SET TRANSACTION command sets the characteristics of the current transaction. It has no effect on any subsequent transactions.

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 Greenplum Database READ UNCOMMITTED is treated as READ COMMITTED. REPEATABLE READ is not supported; use SERIALIZABLE if REPEATABLE READ behavior is required.

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.

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; 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.

Parameters

SESSION CHARACTERISTICS
Sets the default transaction characteristics for subsequent transactions of a session.
SERIALIZABLE
READ COMMITTED
READ UNCOMMITTED
The SQL standard defines four transaction isolation levels: READ COMMITTED, READ UNCOMMITTED, SERIALIZABLE, and REPEATABLE READ. The default behavior is that a statement can only see rows committed before it began (READ COMMITTED). In Greenplum Database READ UNCOMMITTED is treated the same as READ COMMITTED. REPEATABLE READ is not supported; use SERIALIZABLE instead. SERIALIZABLE is the strictest transaction isolation. This level emulates serial transaction execution, as if transactions had been executed one after another, serially, rather than concurrently. Applications using this level must be prepared to retry transactions due to serialization failures.
READ WRITE
READ ONLY
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; GRANT, REVOKE, TRUNCATE; and EXPLAIN ANALYZE and EXECUTE if the command they would execute is among those listed.

Notes

If SET TRANSACTION is executed without a prior START TRANSACTION or BEGIN, it will appear to have no effect.

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.

Examples

Set the transaction isolation level for the current transaction:

BEGIN;
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;

Compatibility

Both commands are defined in the SQL standard. SERIALIZABLE is the default transaction isolation level in the standard. In Greenplum Database the default is READ COMMITTED. Because of lack of predicate locking, the SERIALIZABLE level is not truly serializable. 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.

See Also

BEGIN, LOCK