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Rethink nullingrel marking rules in build_joinrel_tlist().

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The logic for when to add the current outer join's own relid
to the nullingrels sets of output Vars and PHVs was overly
complicated and underly correct.  Not sure why I didn't think
of this before, but since what we want is marking per the
syntactic structure, we can just consult our records about
the syntactic structure, ie syn_righthand/syn_lefthand.

Also, tighten the rule about when to add the commute_above_r
bits, in hopes of eliminating some squishy reasoning.  I do not
know of a reason to think that that's broken as-is, but this way
seems better.

Per bug #17781 from Robins Tharakan.

Discussion: https://postgr.es/m/17781-c0405c8b3cd5e072@postgresql.org
This commit is contained in:
Tom Lane
2023-02-07 18:26:16 -05:00
parent e2c78e7ab4
commit fee7b77b90
3 changed files with 96 additions and 19 deletions
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43
src/backend/optimizer/util/relnode.c
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@ -1043,23 +1043,23 @@ min_join_parameterization(PlannerInfo *root,
* two joins had been done in syntactic order; else they won't match Vars
* appearing higher in the query tree. We need to do two things:
*
* First, sjinfo->commute_above_r is added to the nulling bitmaps of RHS Vars.
* This takes care of the case where we implement
* First, we add the outer join's relid to the nulling bitmap only if the Var
* or PHV actually comes from within the syntactically nullable side(s) of the
* outer join. This takes care of the possibility that we have transformed
* (A leftjoin B on (Pab)) leftjoin C on (Pbc)
* to
* A leftjoin (B leftjoin C on (Pbc)) on (Pab)
* Here the now-upper A/B join must not mark C columns as nulled by itself.
*
* Second, if sjinfo->commute_above_r is already part of the joinrel then
* it is added to the nulling bitmaps of nullable Vars. This takes care of
* the reverse case where we implement
* A leftjoin (B leftjoin C on (Pbc)) on (Pab)
* as
* (A leftjoin B on (Pab)) leftjoin C on (Pbc)
* The C columns emitted by the B/C join need to be shown as nulled by both
* the B/C and A/B joins, even though they've not traversed the A/B join.
* (If the joins haven't been commuted, we are adding the nullingrel bits
* prematurely; but that's okay because the C columns can't be referenced
* between here and the upper join.)
*
* Second, if a RHS Var has any of the relids in sjinfo->commute_above_l
* already set in its nulling bitmap, then we *don't* add sjinfo->ojrelid
* to its nulling bitmap (but we do still add commute_above_r). This takes
* care of the reverse transformation: if the original syntax was
* (A leftjoin B on (Pab)) leftjoin C on (Pbc)
* then the now-upper A/B join must not mark C columns as nulled by itself.
* the B/C and A/B joins, even though they've not physically traversed the
* A/B join.
*/
static void
build_joinrel_tlist(PlannerInfo *root, RelOptInfo *joinrel,
@ -1095,10 +1095,12 @@ build_joinrel_tlist(PlannerInfo *root, RelOptInfo *joinrel,
phv = copyObject(phv);
/* See comments above to understand this logic */
if (sjinfo->ojrelid != 0 &&
!bms_overlap(phv->phnullingrels, sjinfo->commute_above_l))
(bms_is_subset(phv->phrels, sjinfo->syn_righthand) ||
(sjinfo->jointype == JOIN_FULL &&
bms_is_subset(phv->phrels, sjinfo->syn_lefthand))))
phv->phnullingrels = bms_add_member(phv->phnullingrels,
sjinfo->ojrelid);
if (sjinfo->commute_above_r)
if (bms_overlap(sjinfo->commute_above_r, joinrel->relids))
phv->phnullingrels = bms_add_members(phv->phnullingrels,
sjinfo->commute_above_r);
}
@ -1149,17 +1151,20 @@ build_joinrel_tlist(PlannerInfo *root, RelOptInfo *joinrel,
/*
* Add the Var to the output. If this join potentially nulls this
* input, we have to update the Var's varnullingrels, which means
* making a copy.
* making a copy. But note that we don't ever add nullingrel bits to
* row identity Vars (cf. comments in setrefs.c).
*/
if (can_null)
if (can_null && var->varno != ROWID_VAR)
{
var = copyObject(var);
/* See comments above to understand this logic */
if (sjinfo->ojrelid != 0 &&
!bms_overlap(var->varnullingrels, sjinfo->commute_above_l))
(bms_is_member(var->varno, sjinfo->syn_righthand) ||
(sjinfo->jointype == JOIN_FULL &&
bms_is_member(var->varno, sjinfo->syn_lefthand))))
var->varnullingrels = bms_add_member(var->varnullingrels,
sjinfo->ojrelid);
if (sjinfo->commute_above_r)
if (bms_overlap(sjinfo->commute_above_r, joinrel->relids))
var->varnullingrels = bms_add_members(var->varnullingrels,
sjinfo->commute_above_r);
}