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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
3.0.116416
(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
GCC3.0.1
(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..21.82rows=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..4.05rows=2 width=4) (actual time=0.00..0.00 rows=0 loops=1) Filter: (ctid = $0) InitPlan -> Limit
(cost=0.00..0.04rows=1 width=6) (never executed) -> Seq Scan on pg_class (cost=0.00..9.30 rows=230 width=6)
(neverexecuted)
--
Greg Sabino Mullane greg@turnstep.com
PGP Key: 0x14964AC8 200309251929
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