diff options
| -rw-r--r-- | doc/src/sgml/ref/select.sgml | 28 |
1 files changed, 21 insertions, 7 deletions
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml index cad427da3c6..0ffc9cecb73 100644 --- a/doc/src/sgml/ref/select.sgml +++ b/doc/src/sgml/ref/select.sgml @@ -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> |