BEGIN WORK

</REFNAME>

<REFPURPOSE>

Begins a transaction

Begins a transaction in chained mode

</REFPURPOSE>

</refnamediv>

Description

</TITLE>

<para>

<command>BEGIN</command> initiates a user transaction

which <productname>Postgres</productname> will

guarantee is serializable with respect to all concurrently

executing transactions. <productname>Postgres</productname> uses two-phase

locking

to perform this task. If the transaction is committed,

<productname>Postgres</productname> will ensure either that all updates are

done or else

that none of

By default, <productname>Postgres</productname> executes transactions

in unchained mode (also known as autocommit feature in other DBMSes).

In other words, each user statement is executed in its own transaction

and commit is implicit (if execution was successfull).

<command>BEGIN</command> initiates a user transaction in chained mode,

i.e. all user statements after <command>BEGIN</command> command will

be executed in single transaction untill explicit COMMIT, ROLLBACK

or execution abort. Statements in chained mode are executed much faster,

because of transaction start/commit requires significant CPU and disk

activity. This mode is also required for consistency when changing

one of related tables.

</para>

<para>

Default transaction isolation level in <productname>Postgres</productname>

is READ COMMITTED one, when queries inside transaction see only changes

committed before query execution. So, you have to use

<command>SET TRANSACTION ISOLATION LEVEL SERIALIZABLE</command>

command just after BEGIN if you need in better transaction isolation.

In SERIALIZABLE mode queries will see only changes committed before entire

transaction began (actually, before execution of first DML statement

in serializable transaction).

</para>

<para>

If the transaction is committed, <productname>Postgres</productname>

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.

</para>

<PARA>

Refer to the <command>LOCK</command> statement for further information

about locking tables inside a transaction.

about locking tables inside a transaction.

</PARA>

<PARA>

Use <command>COMMIT</command> or <command>ROLLBACK</command>

to terminate a transaction.

to terminate a transaction.

</PARA>

</REFSECT2>

</refsect1>

</TITLE>

<PARA>

<command>BEGIN</command>

is a <productname>Postgres</productname> language extension.

is a <productname>Postgres</productname> language extension.

</para>

<REFSECT2 ID="R2-SQL-BEGINWORK-4">

<REFSECT2INFO>

</TITLE>

<PARA>

There is no explicit BEGIN WORK command in <acronym>SQL92</acronym>;

transaction initiation

is always implicit and it terminates either with a COMMIT or with

a ROLLBACK statement.

transaction initiation is always implicit and it terminates either

with a COMMIT or with a ROLLBACK statement.

</PARA>

<PARA>

<acronym>SQL92</acronym> also requires SERIALIZABLE to be default

transaction isolation level.

</PARA>

</refsect2>

</refsect1>