by <command>SELECT</command> it doesn't mean that this row really

exists at the time it is returned (i.e. sometime after the

statement or transaction began) nor

that the row is protected from deletion orupdate by concurrent

that the row is protected from deletion orupdation by concurrent

transactions before the current transaction does a commit or rollback.

</para>

is required for those wishing to migrate data from any

previous release of <productname>Postgres</productname>.

</para>

</sect2>

<para>

Because readers in 6.5 don't lock data, regardless of transaction

isolation level, data read by one transaction can be overwritten by

another. In the other words, if a row is returned by

<command>SELECT</command> it doesn't mean that this row really exists

at the time it is returned (i.e. sometime after the statement or

transaction began) nor that the row is protected from deletion or

updation by concurrent transactions before the current transaction does

a commit or rollback.

</para>

<para>

To ensure the actual existance of a row and protect it against

concurrent updates one must use <command>SELECT FOR UPDATE</command> or

an appropriate <command>LOCK TABLE</command> statement. This should be

taken into account when porting applications from previous releases of

<productname>Postgres</productname> and other environments.

</para>

<para>

Keep above in mind if you are using contrib/refint.* triggers for

referential integrity. Additional technics are required now. One way is

to use <command>LOCK parent_table IN SHARE ROW EXCLUSIVE MODE</command>

command if a transaction is going to update/delete a primary key and

use <command>LOCK parent_table IN SHARE MODE</command> command if a

transaction is going to update/insert a foreign key.

<note>

<para>

Note that if you run a transaction in SERIALIZABLE mode then you must

execute <command>LOCK</command> commands above before execution of any

DML statement

(<command>SELECT/INSERT/DELETE/UPDATE/FETCH/COPY_TO</command>) in the

transaction.

</para>

</note>

<para>

These inconveniences will disappear when the ability to read durty

(uncommitted) data, regardless of isolation level, and true referential

integrity will be implemented.

</para>

</para>

</sect2>

<sect2>

<title>Detailed Change List</title>

<para>

<programlisting>

Bug Fixes