begin(7) [centos man page]
BEGIN(7) PostgreSQL 9.2.7 Documentation 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 [ NOT ] DEFERRABLE 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(7) or ROLLBACK(7) is given. By default (without BEGIN), PostgreSQL executes transactions in "autocommit" 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 sessions will be unable to see the intermediate states wherein not all the related updates have been done. If the isolation level, read/write mode, or deferrable mode is specified, the new transaction has those characteristics, as if SET TRANSACTION (SET_TRANSACTION(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(7) or 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(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 reference page contains additional compatibility information. The DEFERRABLE transaction_mode is a PostgreSQL language extension. 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(7), ROLLBACK(7), START TRANSACTION (START_TRANSACTION(7)), SAVEPOINT(7) PostgreSQL 9.2.7 2014-02-17 BEGIN(7)
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SET
TRANSACTION(7) SQL Commands SET TRANSACTION(7) NAME
SET TRANSACTION - set the characteristics of the current transaction SYNOPSIS
SET TRANSACTION transaction_mode [, ...] 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 DESCRIPTION
The SET TRANSACTION command sets the characteristics of the current transaction. It has no effect on any subsequent transactions. SET SES- SION CHARACTERISTICS sets the default transaction characteristics for subsequent transactions of a session. These defaults can be overrid- den by SET TRANSACTION for an individual transaction. 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 exe- cuted in this transaction. The SQL standard defines two additional levels, READ UNCOMMITTED and REPEATABLE READ. In PostgreSQL READ UNCOMMITTED is treated as READ COMMITTED, while REPEATABLE READ is treated as SERIALIZABLE. 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. See in the documentation for more information about transaction isolation and concur- rency 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, 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. NOTES
If SET TRANSACTION is executed without a prior START TRANSACTION or BEGIN, it will appear to have no effect, since the transaction will immediately end. 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. (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 in the documentation for more information. COMPATIBILITY
Both commands are defined in the SQL standard. 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. Because of lack of predicate locking, the SERIALIZABLE level is not truly serializable. See in the documentation for details. 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 PostgreSQL server. The SQL standard requires commas between successive transaction_modes, but for historical reasons PostgreSQL allows the commas to be omit- ted. SQL - Language Statements 2010-05-14 SET TRANSACTION(7)