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

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

be executed in a single transaction until an explicit

<xref linkend="sql-commit" endterm="sql-commit-title">,

<xref linkend="sql-rollback" endterm="sql-rollback-title">,

or execution abort. Statementsin chained mode are executed much faster,

<xref linkend="sql-commit" endterm="sql-commit-title"> or

<xref linkend="sql-rollback" endterm="sql-rollback-title">.

Statements are executed more quicklyin chained mode,

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.

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.

</para>

<para>

The default transaction isolation level in

<productname>PostgreSQL</productname>

is READ COMMITTED,where queriesinside the transactionsee only changes

committed before query execution. So, you have to use

is READ COMMITTED,wherein each queryinside the transactionsees changes

committed beforethatquery begins execution. So, you have to use

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

just after <command>BEGIN</command> if you need more rigorous transaction isolation.

just after <command>BEGIN</command> if you need more rigorous transaction

isolation. (Alternatively, you can change the default transaction

isolation level; see the <citetitle>PostgreSQL Administrator's

Guide</citetitle> for details.)

In SERIALIZABLE mode queries will see only changes committed before

the entire

transaction began (actually, before execution of the first DML statement

ina serializable transaction).

inthe transaction).

</para>

<para>

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

will ensure either that all updates are done or else that none of

them are done. Transactions have the standard <acronym>ACID</acronym>

(atomic, consistent, isolatable, and durable) property.

Transactions have the standard <acronym>ACID</acronym>

(atomic, consistent, isolatable, and durable) properties.

</para>

<refsect2 id="R2-SQL-BEGIN-3">

Notes

</title>

<para>

Refer to<xref linkend="sql-lock" endterm="sql-lock-title">

about locking tables inside a transaction.

<xref linkend="sql-start-transaction"

as <command>BEGIN</>.

</para>

<para>

<xref linkend="SQL-ROLLBACK" endterm="SQL-ROLLBACK-TITLE">

to terminate a transaction.

</para>

<para>

Refer to <xref linkend="sql-lock" endterm="sql-lock-title">

for further information

about locking tables inside a transaction.

</para>

<para>

If you turn <literal>autocommit</> mode off, then <command>BEGIN</>

is not required: any SQL command automatically starts a transaction.

</para>

</refsect2>

</refsect1>

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<refentry id="SQL-START-TRANSACTION">

<docinfo>

<date>2002-07-26</date>

This command begins a new transaction. If the isolation level is

specified, the new transaction has that isolation level. In all other

respects, the behavior of this command is identical to the

<command>BEGIN</command> command.

<xref linkend="sql-begin" endterm="sql-begin-title"> command.

</para>

</refsect1>

<para>

The isolation level of a transaction can also be set with the <xref

linkend="sql-set-transaction" endterm="sql-set-transaction-title">

command. If no isolation level is specified, thelevel defaults to

<option>READ COMMITTED</option>.

command. If no isolation level is specified, thedefault isolation

level is used.

</para>

</refsect1>

<title>SQL99</title>

<para>

<option>SERIALIZABLE</option> is the default level in

<acronym>SQL</acronym>. <productname>PostgreSQL</productname>

<option>SERIALIZABLE</option> is the default isolation level in

<acronym>SQL99</acronym>, but it is not the usual default in

<productname>PostgreSQL</productname>: the factory default setting

is READ COMMITTED.

<productname>PostgreSQL</productname>

does not provide the isolation levels <option>READ UNCOMMITTED</option>

and <option>REPEATABLE READ</option>. Because ofmultiversion

concurrency control, the <option>SERIALIZABLE</option> level is

and <option>REPEATABLE READ</option>. Because oflack of predicate

locking, the <option>SERIALIZABLE</option> level is

not truly serializable. See the <citetitle>User's Guide</citetitle>

for details.

</para>