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Further fixes for degenerate outer join clauses.

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Further testing revealed that commit f69b4b9495 was still a few
bricks shy of a load: minor tweaking of the previous test cases resulted
in the same wrong-outer-join-order problem coming back.  After study
I concluded that my previous changes in make_outerjoininfo() were just
accidentally masking the problem, and should be reverted in favor of
forcing syntactic join order whenever an upper outer join's predicate
doesn't mention a lower outer join's LHS.  This still allows the
chained-outer-joins style that is the normally optimizable case.

I also tightened things up some more in join_is_legal().  It seems to me
on review that what's really happening in the exception case where we
ignore a mismatched special join is that we're allowing the proposed join
to associate into the RHS of the outer join we're comparing it to.  As
such, we should *always* insist that the proposed join be a left join,
which eliminates a bunch of rather dubious argumentation.  The case where
we weren't enforcing that was the one that was already known buggy anyway
(it had a violatable Assert before the aforesaid commit) so it hardly
deserves a lot of deference.

Back-patch to all active branches, like the previous patch.  The added
regression test case failed in all branches back to 9.1, and I think it's
only an unrelated change in costing calculations that kept 9.0 from
choosing a broken plan.
This commit is contained in:
Tom Lane 2015-08-06 15:35:27 -04:00
parent df0a67f754
commit 8703059c6b
5 changed files with 153 additions and 68 deletions
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23
src/backend/optimizer/README
View File

@ -241,12 +241,23 @@ non-FULL joins can be freely associated into the lefthand side of an
OJ, but in some cases they can't be associated into the righthand side.
So the restriction enforced by join_is_legal is that a proposed join
can't join a rel within or partly within an RHS boundary to one outside
the boundary, unless the join validly implements some outer join.
(To support use of identity 3, we have to allow cases where an apparent
violation of a lower OJ's RHS is committed while forming an upper OJ.
If this wouldn't in fact be legal, the upper OJ's minimum LHS or RHS
set must be expanded to include the whole of the lower OJ, thereby
preventing it from being formed before the lower OJ is.)
the boundary, unless the proposed join is a LEFT join that can associate
into the SpecialJoinInfo's RHS using identity 3.
The use of minimum Relid sets has some pitfalls; consider a query like
A leftjoin (B leftjoin (C innerjoin D) on (Pbcd)) on Pa
where Pa doesn't mention B/C/D at all. In this case a naive computation
would give the upper leftjoin's min LHS as {A} and min RHS as {C,D} (since
we know that the innerjoin can't associate out of the leftjoin's RHS, and
enforce that by including its relids in the leftjoin's min RHS). And the
lower leftjoin has min LHS of {B} and min RHS of {C,D}. Given such
information, join_is_legal would think it's okay to associate the upper
join into the lower join's RHS, transforming the query to
B leftjoin (A leftjoin (C innerjoin D) on Pa) on (Pbcd)
which yields totally wrong answers. We prevent that by forcing the min LHS
for the upper join to include B. This is perhaps overly restrictive, but
such cases don't arise often so it's not clear that it's worth developing a
more complicated system.
Pulling Up Subqueries

73
src/backend/optimizer/path/joinrels.c
View File

@ -331,7 +331,7 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
SpecialJoinInfo *match_sjinfo;
bool reversed;
bool unique_ified;
bool is_valid_inner;
bool must_be_leftjoin;
bool lateral_fwd;
bool lateral_rev;
ListCell *l;
@ -346,12 +346,12 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
/*
* If we have any special joins, the proposed join might be illegal; and
* in any case we have to determine its join type. Scan the join info
* list for conflicts.
* list for matches and conflicts.
*/
match_sjinfo = NULL;
reversed = false;
unique_ified = false;
is_valid_inner = true;
must_be_leftjoin = false;
foreach(l, root->join_info_list)
{
@ -402,7 +402,8 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
* If one input contains min_lefthand and the other contains
* min_righthand, then we can perform the SJ at this join.
*
* Barf if we get matches to more than one SJ (is that possible?)
* Reject if we get matches to more than one SJ; that implies we're
* considering something that's not really valid.
*/
if (bms_is_subset(sjinfo->min_lefthand, rel1->relids) &&
bms_is_subset(sjinfo->min_righthand, rel2->relids))
@ -470,58 +471,38 @@ join_is_legal(PlannerInfo *root, RelOptInfo *rel1, RelOptInfo *rel2,
/*
* Otherwise, the proposed join overlaps the RHS but isn't a valid
* implementation of this SJ. It might still be a legal join,
* however, if it does not overlap the LHS. But we never allow
* violations of the RHS of SEMI or ANTI joins. (In practice,
* therefore, only LEFT joins ever allow RHS violation.)
* however, if we're allowed to associate it into the RHS of this
* SJ. That means this SJ must be a LEFT join (not SEMI or ANTI,
* and certainly not FULL) and the proposed join must not overlap
* the LHS.
*/
if (sjinfo->jointype == JOIN_SEMI ||
sjinfo->jointype == JOIN_ANTI ||
if (sjinfo->jointype != JOIN_LEFT ||
bms_overlap(joinrelids, sjinfo->min_lefthand))
return false; /* invalid join path */
/*----------
* If both inputs overlap the RHS, assume that it's OK. Since the
* inputs presumably got past this function's checks previously,
* their violations of the RHS boundary must represent SJs that
* have been determined to commute with this one.
* We have to allow this to work correctly in cases like
* (a LEFT JOIN (b JOIN (c LEFT JOIN d)))
* when the c/d join has been determined to commute with the join
* to a, and hence d is not part of min_righthand for the upper
* join. It should be legal to join b to c/d but this will appear
* as a violation of the upper join's RHS.
*
* Furthermore, if one input overlaps the RHS and the other does
* not, we should still allow the join if it is a valid
* implementation of some other SJ. We have to allow this to
* support the associative identity
* (a LJ b on Pab) LJ c ON Pbc = a LJ (b LJ c ON Pbc) on Pab
* since joining B directly to C violates the lower SJ's RHS.
* We assume that make_outerjoininfo() set things up correctly
* so that we'll only match to some SJ if the join is valid.
* Set flag here to check at bottom of loop.
*----------
/*
* To be valid, the proposed join must be a LEFT join; otherwise
* it can't associate into this SJ's RHS. But we may not yet have
* found the SpecialJoinInfo matching the proposed join, so we
* can't test that yet. Remember the requirement for later.
*/
if (bms_overlap(rel1->relids, sjinfo->min_righthand) &&
bms_overlap(rel2->relids, sjinfo->min_righthand))
{
/* both overlap; assume OK */
}
else
{
/* one overlaps, the other doesn't */
is_valid_inner = false;
}
must_be_leftjoin = true;
}
}
/*
* Fail if violated some SJ's RHS and didn't match to another SJ. However,
* "matching" to a semijoin we are implementing by unique-ification
* doesn't count (think: it's really an inner join).
* Fail if violated any SJ's RHS and didn't match to a LEFT SJ: the
* proposed join can't associate into an SJ's RHS.
*
* Also, fail if the proposed join's predicate isn't strict; we're
* essentially checking to see if we can apply outer-join identity 3, and
* that's a requirement. (This check may be redundant with checks in
* make_outerjoininfo, but I'm not quite sure, and it's cheap to test.)
*/
if (!is_valid_inner &&
(match_sjinfo == NULL || unique_ified))
if (must_be_leftjoin &&
(match_sjinfo == NULL ||
match_sjinfo->jointype != JOIN_LEFT ||
!match_sjinfo->lhs_strict))
return false; /* invalid join path */
/*

28
src/backend/optimizer/plan/initsplan.c
View File

@ -1128,17 +1128,6 @@ make_outerjoininfo(PlannerInfo *root,
min_righthand = bms_int_members(bms_union(clause_relids, inner_join_rels),
right_rels);
/*
* If we have a degenerate join clause that doesn't mention any RHS rels,
* force the min RHS to be the syntactic RHS; otherwise we can end up
* making serious errors, like putting the LHS on the wrong side of an
* outer join. It seems to be safe to not do this when we have a
* contribution from inner_join_rels, though; that's enough to pin the SJ
* to occur at a reasonable place in the tree.
*/
if (bms_is_empty(min_righthand))
min_righthand = bms_copy(right_rels);
/*
* Now check previous outer joins for ordering restrictions.
*/
@ -1181,9 +1170,15 @@ make_outerjoininfo(PlannerInfo *root,
* For a lower OJ in our RHS, if our join condition does not use the
* lower join's RHS and the lower OJ's join condition is strict, we
* can interchange the ordering of the two OJs; otherwise we must add
* the lower OJ's full syntactic relset to min_righthand. Also, we
* must preserve ordering anyway if either the current join or the
* lower OJ is either a semijoin or an antijoin.
* the lower OJ's full syntactic relset to min_righthand.
*
* Also, if our join condition does not use the lower join's LHS
* either, force the ordering to be preserved. Otherwise we can end
* up with SpecialJoinInfos with identical min_righthands, which can
* confuse join_is_legal (see discussion in backend/optimizer/README).
*
* Also, we must preserve ordering anyway if either the current join
* or the lower OJ is either a semijoin or an antijoin.
*
* Here, we have to consider that "our join condition" includes any
* clauses that syntactically appeared above the lower OJ and below
@ -1199,6 +1194,7 @@ make_outerjoininfo(PlannerInfo *root,
if (bms_overlap(right_rels, otherinfo->syn_righthand))
{
if (bms_overlap(clause_relids, otherinfo->syn_righthand) ||
!bms_overlap(clause_relids, otherinfo->min_lefthand) ||
jointype == JOIN_SEMI ||
jointype == JOIN_ANTI ||
otherinfo->jointype == JOIN_SEMI ||
@ -1238,10 +1234,12 @@ make_outerjoininfo(PlannerInfo *root,
* If we found nothing to put in min_lefthand, punt and make it the full
* LHS, to avoid having an empty min_lefthand which will confuse later
* processing. (We don't try to be smart about such cases, just correct.)
* We already forced min_righthand nonempty, so nothing to do for that.
* Likewise for min_righthand.
*/
if (bms_is_empty(min_lefthand))
min_lefthand = bms_copy(left_rels);
if (bms_is_empty(min_righthand))
min_righthand = bms_copy(right_rels);
/* Now they'd better be nonempty */
Assert(!bms_is_empty(min_lefthand));

72
src/test/regress/expected/join.out
View File

@ -3425,7 +3425,7 @@ select t1.* from
Output: t1.f1
Hash Cond: (i8.q2 = i4.f1)
-> Nested Loop Left Join
Output: i8.q2, t1.f1
Output: t1.f1, i8.q2
Join Filter: (t1.f1 = '***'::text)
-> Seq Scan on public.text_tbl t1
Output: t1.f1
@ -3473,6 +3473,76 @@ select t1.* from
hi de ho neighbor
(2 rows)
explain (verbose, costs off)
select t1.* from
text_tbl t1
left join (select *, '***'::text as d1 from int8_tbl i8b1) b1
left join int8_tbl i8
left join (select *, null::int as d2 from int8_tbl i8b2, int4_tbl i4b2
where q1 = f1) b2
on (i8.q1 = b2.q1)
on (b2.d2 = b1.q2)
on (t1.f1 = b1.d1)
left join int4_tbl i4
on (i8.q2 = i4.f1);
QUERY PLAN
----------------------------------------------------------------------------
Hash Left Join
Output: t1.f1
Hash Cond: (i8.q2 = i4.f1)
-> Nested Loop Left Join
Output: t1.f1, i8.q2
Join Filter: (t1.f1 = '***'::text)
-> Seq Scan on public.text_tbl t1
Output: t1.f1
-> Materialize
Output: i8.q2
-> Hash Right Join
Output: i8.q2
Hash Cond: ((NULL::integer) = i8b1.q2)
-> Hash Right Join
Output: i8.q2, (NULL::integer)
Hash Cond: (i8b2.q1 = i8.q1)
-> Hash Join
Output: i8b2.q1, NULL::integer
Hash Cond: (i8b2.q1 = i4b2.f1)
-> Seq Scan on public.int8_tbl i8b2
Output: i8b2.q1, i8b2.q2
-> Hash
Output: i4b2.f1
-> Seq Scan on public.int4_tbl i4b2
Output: i4b2.f1
-> Hash
Output: i8.q1, i8.q2
-> Seq Scan on public.int8_tbl i8
Output: i8.q1, i8.q2
-> Hash
Output: i8b1.q2
-> Seq Scan on public.int8_tbl i8b1
Output: i8b1.q2
-> Hash
Output: i4.f1
-> Seq Scan on public.int4_tbl i4
Output: i4.f1
(37 rows)
select t1.* from
text_tbl t1
left join (select *, '***'::text as d1 from int8_tbl i8b1) b1
left join int8_tbl i8
left join (select *, null::int as d2 from int8_tbl i8b2, int4_tbl i4b2
where q1 = f1) b2
on (i8.q1 = b2.q1)
on (b2.d2 = b1.q2)
on (t1.f1 = b1.d1)
left join int4_tbl i4
on (i8.q2 = i4.f1);
f1
-------------------
doh!
hi de ho neighbor
(2 rows)
--
-- test ability to push constants through outer join clauses
--

25
src/test/regress/sql/join.sql
View File

@ -1065,6 +1065,31 @@ select t1.* from
left join int4_tbl i4
on (i8.q2 = i4.f1);
explain (verbose, costs off)
select t1.* from
text_tbl t1
left join (select *, '***'::text as d1 from int8_tbl i8b1) b1
left join int8_tbl i8
left join (select *, null::int as d2 from int8_tbl i8b2, int4_tbl i4b2
where q1 = f1) b2
on (i8.q1 = b2.q1)
on (b2.d2 = b1.q2)
on (t1.f1 = b1.d1)
left join int4_tbl i4
on (i8.q2 = i4.f1);
select t1.* from
text_tbl t1
left join (select *, '***'::text as d1 from int8_tbl i8b1) b1
left join int8_tbl i8
left join (select *, null::int as d2 from int8_tbl i8b2, int4_tbl i4b2
where q1 = f1) b2
on (i8.q1 = b2.q1)
on (b2.d2 = b1.q2)
on (t1.f1 = b1.d1)
left join int4_tbl i4
on (i8.q2 = i4.f1);
--
-- test ability to push constants through outer join clauses
--