Thread: Re: could not devise a query plan

Dear Gurus,

Sorry for upping a 13-month-old thread; please tell if I should've opened 
another one.

Here I come again, with another silly join. Please forgive me, but our 
queries are built from blocks :)

VERSION: 7.4.6, 7.4.8, 8.0.0rc4 (sorry, no newer installed right now)

ABSTRACT: The following query fails.
SELECT * FROM  (SELECT a,b FROM cdqp WHERE a=1 AND b=2) AS aa  NATURAL FULL JOIN  (SELECT a,b FROM cdqp WHERE a=1 and
b=2)AS bbWHERE a+b = 3;
 

DETAILS: It seems it's something about the redundant WHERE clauses. If I 
comment (replace with "WHERE true") any of the three WHERE clauses, it works.

Session log: See below.

TIA,

--
G.


serv1:tir=> \d cdqp   Table "pg_temp_20.cdqp" Column |  Type   | Modifiers
--------+---------+----------- a      | integer | b      | integer |

serv1:tir=> SELECT * FROM
serv1:tir->   (SELECT a,b FROM cdqp WHERE a=1 AND b=2) AS aa
serv1:tir->   NATURAL FULL JOIN
serv1:tir->   (SELECT a,b FROM cdqp WHERE a=1 and b=2) AS bb
serv1:tir-> WHERE a+b = 3;
ERROR:  could not devise a query plan for the given query
serv1:tir=> SELECT * FROM
serv1:tir->   (SELECT a,b FROM cdqp WHERE a=1 AND b=2) AS aa
serv1:tir->   NATURAL FULL JOIN
serv1:tir->   (SELECT a,b FROM cdqp WHERE a=1 and b=2) AS bb
serv1:tir-> WHERE true; a | b
---+--- 1 | 2
(1 row)

Szűcs Gábor <surrano@gmail.com> writes:
> ABSTRACT: The following query fails.

>     SELECT * FROM
>       (SELECT a,b FROM cdqp WHERE a=1 AND b=2) AS aa
>       NATURAL FULL JOIN
>       (SELECT a,b FROM cdqp WHERE a=1 and b=2) AS bb
>     WHERE a+b = 3;

Thanks for the report!  Seems I overlooked a case when fixing the
original report last year.  Patch for 8.0 attached (it's the same in
7.4 too).
        regards, tom lane

Index: joinpath.c
===================================================================
RCS file: /cvsroot/pgsql/src/backend/optimizer/path/joinpath.c,v
retrieving revision 1.91.4.1
diff -c -r1.91.4.1 joinpath.c
*** joinpath.c    23 Jan 2005 02:22:27 -0000    1.91.4.1
--- joinpath.c    24 May 2005 17:54:15 -0000
***************
*** 498,512 ****          * nestloop path, but since mergejoin is our only join type that          * supports FULL
JOIN,it's necessary to generate a clauseless          * mergejoin path instead.
 
-          *
-          * Unfortunately this can't easily be extended to handle the case
-          * where there are joinclauses but none of them use mergejoinable
-          * operators; nodeMergejoin.c can only do a full join correctly if
-          * all the joinclauses are mergeclauses.          */         if (mergeclauses == NIL)         {
!             if (jointype == JOIN_FULL && restrictlist == NIL)                  /* okay to try for mergejoin */ ;
      else                 continue;
 
--- 498,507 ----          * nestloop path, but since mergejoin is our only join type that          * supports FULL
JOIN,it's necessary to generate a clauseless          * mergejoin path instead.          */         if (mergeclauses ==
NIL)        {
 
!             if (jointype == JOIN_FULL)                  /* okay to try for mergejoin */ ;             else
    continue;