Source: firebird2.5
Version: 2.5.1
Severity: important
Tags: upstream fixed-upstream
Forwarded: http://tracker.firebirdsql.org/browse/CORE-3902
This is a performance regression compared to 2.1 series.
Quoting from upstream bug report:
select rdb$database.rdb$relation_id
from rdb$database
left outer join
(
select
rdb$relations.rdb$relation_id as tempid
from rdb$relations
) temp (tempid)
on temp.tempid = rdb$database.rdb$relation_id
n Firebird 2.1 the plan is good, i.e. an Index join to RDB$RELATIONS.
PLAN JOIN (RDB$DATABASE NATURAL, TEMP RDB$RELATIONS INDEX (RDB$INDEX_1))
In Firebird 2.5 the plan becomes an natural read join to RDB$RELATIONS.
PLAN JOIN (RDB$DATABASE NATURAL, TEMP RDB$RELATIONS NATURAL)
However, the interesting thing is if I remove either the 'as tempid' or
'(tempid)' alias, or don't use alias at all, the plan goes back to the 2.1
version.
Clearly, using an alias should not have caused a change in execution plan.
Upstream patches are at
http://firebird.svn.sourceforge.net/viewvc/firebird/firebird/branches/B2_5_Release/src/dsql/gen.cpp?pathrev=57052&view=diff&r1=57052&r2=57051&diff_format=h
and
http://firebird.svn.sourceforge.net/viewvc/firebird/firebird/branches/B2_5_Release/src/dsql/gen.cpp?pathrev=57244&view=diff&r1=57244&r2=57243&diff_format=h
-- System Information:
Debian Release: wheezy/sid
APT prefers unstable
APT policy: (500, 'unstable'), (500, 'testing'), (500, 'stable'), (1,
'experimental')
Architecture: amd64 (x86_64)
Foreign Architectures: i386
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