DB_KEY based join of two tables may be ineffective
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Key: CORE-4530
URL: http://tracker.firebirdsql.org/browse/CORE-4530
Project: Firebird Core
Issue Type: Bug
Components: Engine
Affects Versions: 2.5.3, 3.0 Alpha 2
Reporter: Dmitry Yemanov
Simplified test case:
set planonly;
recreate table t (id int primary key);
commit;
select count(*)
from (select id, rdb$db_key k from t a order by id) x
left join t z on x.k = z.rdb$db_key -- (*)
PLAN JOIN (X A ORDER RDB$PRIMARY4, Z NATURAL)
vs
select count(*)
from (select id, rdb$db_key k from t a order by id) x
left join t z on z.rdb$db_key = x.k -- (*)
PLAN JOIN (X A ORDER RDB$PRIMARY4, Z INDEX ())
i.e. order of expressions in the join condition affects the generated plan and
thus performance.
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