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Fix an oversight in checking whether a join with LATERAL refs is legal.

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In many cases, we can implement a semijoin as a plain innerjoin by first
passing the righthand-side relation through a unique-ification step.
However, one of the cases where this does NOT work is where the RHS has
a LATERAL reference to the LHS; that makes the RHS dependent on the LHS
so that unique-ification is meaningless.  joinpath.c understood this,
and so would not generate any join paths of this kind ... but join_is_legal
neglected to check for the case, so it would think that we could do it.
The upshot would be a "could not devise a query plan for the given query"
failure once we had failed to generate any join paths at all for the bogus
join pair.

Back-patch to 9.3 where LATERAL was added.
This commit is contained in:
Tom Lane
2015-07-31 19:26:33 -04:00
parent 16c4e6d8dc
commit a6492ff897
3 changed files with 54 additions and 2 deletions
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8
src/backend/optimizer/path/joinrels.c
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@ -536,7 +536,9 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
if (!bms_is_subset(ljinfo->lateral_lhs, rel1->relids))
return false; /* rel1 can't compute the required parameter */
if (match_sjinfo &&
(reversed || match_sjinfo->jointype == JOIN_FULL))
(reversed ||
unique_ified ||
match_sjinfo->jointype == JOIN_FULL))
return false; /* not implementable as nestloop */
}
if (bms_is_subset(ljinfo->lateral_rhs, rel1->relids) &&
@ -549,7 +551,9 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
if (!bms_is_subset(ljinfo->lateral_lhs, rel2->relids))
return false; /* rel2 can't compute the required parameter */
if (match_sjinfo &&
(!reversed || match_sjinfo->jointype == JOIN_FULL))
(!reversed ||
unique_ified ||
match_sjinfo->jointype == JOIN_FULL))
return false; /* not implementable as nestloop */
}
}