By default, Postgres 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 command will
be executed in a single transaction until an explicit
commit(l),
rollback(l),
or execution abort. Statements in chained mode are executed much faster,
because transaction start/commit requires significant CPU and disk
activity. Execution of multiple statements inside a transaction
is also required for consistency when changing several
related tables.
The default transaction isolation level in
Postgres
is READ COMMITTED, where queries inside the transaction see only changes
committed before query execution. So, you have to use
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
just after BEGIN if you need more rigorous transaction isolation.
In SERIALIZABLE mode queries will see only changes committed before
the entire
transaction began (actually, before execution of the first DML statement
in a serializable transaction).
If the transaction is committed, Postgres
will ensure either that all updates are done or else that none of
them are done. Transactions have the standard ACID
(atomic, consistent, isolatable, and durable) property.
The keyword TRANSACTION is just a cosmetic alternative to WORK.
Neither keyword need be specified.
Refer to lock(l)
for further information
about locking tables inside a transaction.
Use commit(l)
or
rollback(l)
to terminate a transaction.