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Improve discussion of FOR UPDATE.

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This commit is contained in:
Tom Lane 2002-10-18 18:26:22 +00:00
parent f99e525f0f
commit fb9bc342ff
1 changed files with 21 additions and 7 deletions
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doc/src/sgml/ref/select.sgml
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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.61 2002/09/21 18:32:54 petere Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.62 2002/10/18 18:26:22 tgl Exp $
PostgreSQL documentation
-->
@ -368,13 +368,14 @@ where <replaceable class="PARAMETER">from_item</replaceable> can be:
</para>
<para>
The FOR UPDATE clause allows the SELECT statement to perform
exclusive locking of selected rows.
The FOR UPDATE clause causes the SELECT statement to lock the selected
rows against concurrent updates.
</para>
<para>
You must have SELECT privilege to a table to read its values
(See the <command>GRANT</command>/<command>REVOKE</command> statements).
Use of FOR UPDATE requires UPDATE privilege as well.
</para>
<refsect2 id="SQL-FROM">
@ -859,19 +860,32 @@ SELECT name FROM distributors ORDER BY code;
<para>
FOR UPDATE causes the rows retrieved by the query to be locked as though
for update. This prevents them from being modified or deleted by other
transactions until the current transaction ends.
transactions until the current transaction ends; that is, other
transactions that attempt UPDATE, DELETE, or SELECT FOR UPDATE of these
rows will be blocked until the current transaction ends. Also, if an
UPDATE, DELETE, or SELECT FOR UPDATE from another transaction has already
locked a selected row or rows, SELECT FOR UPDATE will wait for the other
transaction to complete, and will then lock and return the updated row
(or no row, if the row was deleted). For further discussion see the
concurrency chapter of the <citetitle>User's Guide</citetitle>.
</para>
<para>
If specific tables are named in FOR UPDATE, then only rows coming from
those tables are locked.
those tables are locked; any other tables used in the SELECT are simply
read as usual.
</para>
<para>
FOR UPDATE cannot be used in contexts where returned rows can't be clearly
identified with individual table rows; for example it can't be used with
aggregation. FOR UPDATE may also appear before LIMIT for portability with
pre-7.3 applications.
aggregation.
</para>
<para>
FOR UPDATE may appear before LIMIT for compatibility with
pre-7.3 applications. However, it effectively executes after LIMIT,
and so that is the recommended place to write it.
</para>
</refsect2>