Re: SELECT ... FOR UPDATE OF clause documentation implies use of table_names rather than aliases

[Bruce Momjian]

On Fri, Apr 27, 2018 at 01:47:49PM +0000, PG Doc comments form wrote:
> The following documentation comment has been logged on the website:
> 
> Page: https://www.postgresql.org/docs/9.5/static/sql-select.html
> Description:
> 
> In the SELECT statement page the argument type of the (FOR SHARE/UPDATE) OF
> clause is listed to be a table_name. This is not *quite* accurate - it
> should reference the *alias* assigned to the table if one was given. The
> distinction is subtly important, as without this information the
> documentation implies that the choice of rows to lock can only be done
> per-table (i.e. that in a query mentioning the same table twice, *any*
> tuples being pulled from that table would be given the same treatment).
> 
> But in fact postgres supports specifying the locking behaviour per-alias,
> which is a really powerful ability. And actually, trying to specify it by
> actual "table name" where an alias has been assigned won't work either.

I can confirm this report from 2018:

        CREATE TABLE test ( x INT );
        
        SELECT * FROM test AS t1 JOIN test AS t2 ON (TRUE) FOR UPDATE OF t1;
         x | x
        ---+---

        
        SELECT * FROM test AS t1 JOIN test AS t2 ON (TRUE) FOR UPDATE OF t2;
         x | x
        ---+---

The attached patch documents this.

-- 
  Bruce Momjian  <br...@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Only you can decide what is important to you.

diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml
index 227ba1993b..ab3760057c 100644
--- a/doc/src/sgml/ref/select.sgml
+++ b/doc/src/sgml/ref/select.sgml
@@ -45,7 +45,7 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="parameter">expression</replac
     [ LIMIT { <replaceable class="parameter">count</replaceable> | ALL } ]
     [ OFFSET <replaceable class="parameter">start</replaceable> [ ROW | ROWS ] ]
     [ FETCH { FIRST | NEXT } [ <replaceable class="parameter">count</replaceable> ] { ROW | ROWS } { ONLY | WITH TIES } ]
-    [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
+    [ FOR { UPDATE | NO KEY UPDATE | SHARE | KEY SHARE } [ OF { <replaceable class="parameter">table_name</replaceable> | <replaceable class="parameter">alias</replaceable> } [, ...] ] [ NOWAIT | SKIP LOCKED ] [...] ]
 
 <phrase>where <replaceable class="parameter">from_item</replaceable> can be one of:</phrase>