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Why is the subselect never executed for the inner select?


pgrocks=> SELECT VERSION() UNION ALL SELECT oid::text FROM pg_class WHERE ctid = (SELECT ctid FROM pg_class LIMIT 1);
                           version                            
--------------------------------------------------------------
 PostgreSQL 7.3.4 on i686-pc-linux-gnu, compiled by GCC 3.0.1
 16416
(2 rows)


pgrocks=> SELECT VERSION() UNION ALL SELECT oid::text FROM pg_class WHERE ctid = (SELECT ctid FROM pg_class LIMIT 1);
                             version                             
-----------------------------------------------------------------
 PostgreSQL 7.4beta3 on i686-pc-linux-gnu, compiled by GCC 3.0.1
(1 row)


7.3.4:
pgrocks=> EXPLAIN ANALYZE SELECT oid FROM pg_class WHERR ctid = (SELECT ctid FROM pg_class LIMIT 1);
                                                   QUERY PLAN                                                    
------------------------------------------------------------------------------------------------------
 Tid Scan on pg_class  (cost=0.00..4.01 rows=1 width=4) (actual time=0.01..0.01 rows=1 loops=1)
   Filter: (ctid = $0)
   InitPlan
     ->  Limit  (cost=0.00..0.03 rows=1 width=6) (actual time=0.02..0.03 rows=1 loops=1)
         ->  Seq Scan on pg_class  (cost=0.00..21.82 rows=682 width=6) (actual time=0.02..0.02 rows=2 loops=1)


7.4beta3:
pgrocks=> EXPLAIN ANALYZE SELECT oid FROM pg_class WHERR ctid = (SELECT ctid FROM pg_class LIMIT 1);
                                           QUERY PLAN                                           
------------------------------------------------------------------------------------------------
 Tid Scan on pg_class  (cost=0.04..4.05 rows=2 width=4) (actual time=0.00..0.00 rows=0 loops=1)
   Filter: (ctid = $0)
   InitPlan
     ->  Limit  (cost=0.00..0.04 rows=1 width=6) (never executed)
         ->  Seq Scan on pg_class  (cost=0.00..9.30 rows=230 width=6) (never executed)






--
Greg Sabino Mullane greg@turnstep.com
PGP Key: 0x14964AC8 200309251929
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Thu, 25 Sep 2003 23:31:43 -0000
"Greg Sabino Mullane" <greg@turnstep.com>
ctid matching behavior changed?
"Greg Sabino Mullane" <greg@turnstep.com> writes:
> Why is the subselect never executed for the inner select?

Fixed.  I'm kinda surprised that code ever worked; it was trying to run
a sub-plan before the rest of the executor was set up.

			regards, tom lane

Thu, 25 Sep 2003 21:18:11 -0400
Tom Lane <tgl@sss.pgh.pa.us>
Re: ctid matching behavior changed? 