begin(7) [linux man page]
BEGIN(7) SQL Commands BEGIN(7) NAME
BEGIN - start a transaction block SYNOPSIS
BEGIN [ WORK | TRANSACTION ] [ transaction_mode [, ...] ] where transaction_mode is one of: ISOLATION LEVEL { SERIALIZABLE | REPEATABLE READ | READ COMMITTED | READ UNCOMMITTED } READ WRITE | READ ONLY DESCRIPTION
BEGIN initiates a transaction block, that is, all statements after a BEGIN command will be executed in a single transaction until an explicit COMMIT [commit(7)] or ROLLBACK [rollback(7)] is given. By default (without BEGIN), PostgreSQL executes transactions in ``autocom- mit'' mode, that is, each statement is executed in its own transaction and a commit is implicitly performed at the end of the statement (if execution was successful, otherwise a rollback is done). Statements are executed more quickly in a transaction block, because transaction start/commit requires significant CPU and disk activity. Execution of multiple statements inside a transaction is also useful to ensure consistency when making several related changes: other ses- sions will be unable to see the intermediate states wherein not all the related updates have been done. If the isolation level or read/write mode is specified, the new transaction has those characteristics, as if SET TRANSACTION [set_transac- tion(7)] was executed. PARAMETERS
WORK TRANSACTION Optional key words. They have no effect. Refer to SET TRANSACTION [set_transaction(7)] for information on the meaning of the other parameters to this statement. NOTES
START TRANSACTION [start_transaction(7)] has the same functionality as BEGIN. Use COMMIT [commit(7)] or ROLLBACK [rollback(7)] to terminate a transaction block. Issuing BEGIN when already inside a transaction block will provoke a warning message. The state of the transaction is not affected. To nest transactions within a transaction block, use savepoints (see SAVEPOINT [savepoint(7)]). For reasons of backwards compatibility, the commas between successive transaction_modes can be omitted. EXAMPLES
To begin a transaction block: BEGIN; COMPATIBILITY
BEGIN is a PostgreSQL language extension. It is equivalent to the SQL-standard command START TRANSACTION [start_transaction(7)], whose ref- erence page contains additional compatibility information. Incidentally, the BEGIN key word is used for a different purpose in embedded SQL. You are advised to be careful about the transaction semantics when porting database applications. SEE ALSO
COMMIT [commit(7)], ROLLBACK [rollback(7)], START TRANSACTION [start_transaction(7)], SAVEPOINT [savepoint(7)] SQL - Language Statements 2010-05-14 BEGIN(7)
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BEGIN(7) SQL Commands BEGIN(7) NAME
BEGIN - start a transaction block SYNOPSIS
BEGIN [ WORK | TRANSACTION ] INPUTS WORK TRANSACTION Optional keywords. They have no effect. OUTPUTS BEGIN This signifies that a new transaction has been started. WARNING: BEGIN: already a transaction in progress This indicates that a transaction was already in progress. The current transaction is not affected. DESCRIPTION
By default, PostgreSQL executes transactions in unchained mode (also known as ``autocommit'' in other database systems). In other words, each user statement is executed in its own transaction and a commit is implicitly performed at the end of the statement (if execution was successful, otherwise a rollback is done). BEGIN initiates a user transaction in chained mode, i.e., all user statements after BEGIN com- mand will be executed in a single transaction until an explicit COMMIT [commit(7)] or ROLLBACK [rollback(7)]. Statements are executed more quickly in chained mode, because transaction start/commit requires significant CPU and disk activity. Execution of multiple statements inside a transaction is also useful to ensure consistency when changing several related tables: other clients will be unable to see the intermediate states wherein not all the related updates have been done. The default transaction isolation level in PostgreSQL is READ COMMITTED, wherein each query inside the transaction sees changes committed before that query begins execution. So, you have to use SET TRANSACTION ISOLATION LEVEL SERIALIZABLE just after BEGIN if you need more rig- orous transaction isolation. (Alternatively, you can change the default transaction isolation level; see the PostgreSQL Administrator's Guide for details.) In SERIALIZABLE mode queries will see only changes committed before the entire transaction began (actually, before execution of the first DML statement in the transaction). Transactions have the standard ACID (atomic, consistent, isolatable, and durable) properties. NOTES START TRANSACTION [start_transaction(7)] has the same functionality as BEGIN. Use COMMIT [commit(7)] or ROLLBACK [rollback(7)] to terminate a transaction. Refer to LOCK [lock(7)] for further information about locking tables inside a transaction. If you turn autocommit mode off, then BEGIN is not required: any SQL command automatically starts a transaction. USAGE
To begin a user transaction: BEGIN WORK; COMPATIBILITY
SQL92 BEGIN is a PostgreSQL language extension. There is no explicit BEGIN command in SQL92; transaction initiation is always implicit and it terminates either with a COMMIT or ROLLBACK statement. Note: Many relational database systems offer an autocommit feature as a convenience. Incidentally, the BEGIN keyword is used for a different purpose in embedded SQL. You are advised to be careful about the transaction seman- tics when porting database applications. SQL92 also requires SERIALIZABLE to be the default transaction isolation level. SQL - Language Statements 2002-11-22 BEGIN(7)