Starts a transaction block.
START TRANSACTION [SERIALIZABLE | READ COMMITTED | READ UNCOMMITTED] [READ WRITE | READ ONLY]
START TRANSACTION begins a new transaction block. If the isolation level or read/write mode is specified, the new transaction has those characteristics, as if SET TRANSACTION was executed. This is the same as the BEGIN command.
- 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 if this behavior is required. SERIALIZABLE, wherein all statements of the current transaction can only see rows committed before the first statement was executed in the transaction, 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.
To begin a transaction block:
In the standard, it is not necessary to issue START TRANSACTION to start a transaction block: any SQL command implicitly begins a block. Greenplum Database behavior can be seen as implicitly issuing a COMMIT after each command that does not follow START TRANSACTION (or BEGIN), and it is therefore often called 'autocommit'. Other relational database systems may offer an autocommit feature as a convenience.
The SQL standard requires commas between successive transaction_modes, but for historical reasons Greenplum Database allows the commas to be omitted.
See also the compatibility section of SET TRANSACTION.