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Reduce semijoins with unique inner relations to plain inner joins.

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If the inner relation can be proven unique, that is it can have no more
than one matching row for any row of the outer query, then we might as
well implement the semijoin as a plain inner join, allowing substantially
more freedom to the planner.  This is a form of outer join strength
reduction, but it can't be implemented in reduce_outer_joins() because
we don't have enough info about the individual relations at that stage.
Instead do it much like remove_useless_joins(): once we've built base
relations, we can make another pass over the SpecialJoinInfo list and
get rid of any entries representing reducible semijoins.

This is essentially a followon to the inner-unique patch (commit 9c7f5229a)
and makes use of the proof machinery that that patch created.  We need only
minor refactoring of innerrel_is_unique's API to support this usage.

Per performance complaint from Teodor Sigaev.

Discussion: https://postgr.es/m/f994fc98-389f-4a46-d1bc-c42e05cb43ed@sigaev.ru
This commit is contained in:
Tom Lane
2017-05-01 14:53:42 -04:00
parent 2057a58d16
commit 92a43e4857
7 changed files with 181 additions and 27 deletions
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32
src/backend/optimizer/path/joinpath.c
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@ -126,13 +126,15 @@ add_paths_to_joinrel(PlannerInfo *root,
*
* We have some special cases: for JOIN_SEMI and JOIN_ANTI, it doesn't
* matter since the executor can make the equivalent optimization anyway;
* we need not expend planner cycles on proofs. For JOIN_UNIQUE_INNER, if
* the LHS covers all of the associated semijoin's min_lefthand, then it's
* appropriate to set inner_unique because the path produced by
* create_unique_path will be unique relative to the LHS. (If we have an
* LHS that's only part of the min_lefthand, that is *not* true.) For
* JOIN_UNIQUE_OUTER, pass JOIN_INNER to avoid letting that value escape
* this module.
* we need not expend planner cycles on proofs. For JOIN_UNIQUE_INNER, we
* must be considering a semijoin whose inner side is not provably unique
* (else reduce_unique_semijoins would've simplified it), so there's no
* point in calling innerrel_is_unique. However, if the LHS covers all of
* the semijoin's min_lefthand, then it's appropriate to set inner_unique
* because the path produced by create_unique_path will be unique relative
* to the LHS. (If we have an LHS that's only part of the min_lefthand,
* that is *not* true.) For JOIN_UNIQUE_OUTER, pass JOIN_INNER to avoid
* letting that value escape this module.
*/
switch (jointype)
{
@ -145,12 +147,20 @@ add_paths_to_joinrel(PlannerInfo *root,
outerrel->relids);
break;
case JOIN_UNIQUE_OUTER:
extra.inner_unique = innerrel_is_unique(root, outerrel, innerrel,
JOIN_INNER, restrictlist);
extra.inner_unique = innerrel_is_unique(root,
outerrel->relids,
innerrel,
JOIN_INNER,
restrictlist,
false);
break;
default:
extra.inner_unique = innerrel_is_unique(root, outerrel, innerrel,
jointype, restrictlist);
extra.inner_unique = innerrel_is_unique(root,
outerrel->relids,
innerrel,
jointype,
restrictlist,
false);
break;
}