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Commit43f0c20

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Fix overoptimistic assumptions in column width estimation for subqueries.
set_append_rel_pathlist supposed that, while computing per-column widthestimates for the appendrel, it could ignore child rels for which thetranslated reltargetlist entry wasn't a Var. This gave rise to completelysilly estimates in some common cases, such as constant outputs from some orall of the arms of a UNION ALL. Instead, fall back on get_typavgwidth toestimate from the value's datatype; which might be a poor estimate but atleast it's not completely wacko.That problem was exposed by an Assert in set_subquery_size_estimates, whichunfortunately was still overoptimistic even with that fix, since we don'tcompute attr_widths estimates for appendrels that are entirely excluded byconstraints. So remove the Assert; we'll just fall back on get_typavgwidthin such cases.Also, since set_subquery_size_estimates calls set_baserel_size_estimateswhich calls set_rel_width, there's no need for set_subquery_size_estimatesto call get_typavgwidth; set_rel_width will handle it for us if we justleave the estimate set to zero. Remove the unnecessary code.Per report from Erik Rijkers and subsequent investigation.
1 parent1af55e2 commit43f0c20

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2 files changed

+44
-22
lines changed

2 files changed

+44
-22
lines changed

‎src/backend/optimizer/path/allpaths.c‎

Lines changed: 33 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -403,7 +403,15 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
403403
continue;
404404
}
405405

406-
/* CE failed, so finish copying targetlist and join quals */
406+
/*
407+
* CE failed, so finish copying/modifying targetlist and join quals.
408+
*
409+
* Note: the resulting childrel->reltargetlist may contain arbitrary
410+
* expressions, which normally would not occur in a reltargetlist.
411+
* That is okay because nothing outside of this routine will look at
412+
* the child rel's reltargetlist. We do have to cope with the case
413+
* while constructing attr_widths estimates below, though.
414+
*/
407415
childrel->joininfo= (List*)
408416
adjust_appendrel_attrs((Node*)rel->joininfo,
409417
appinfo);
@@ -486,23 +494,36 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
486494
parent_rows+=childrel->rows;
487495
parent_size+=childrel->width*childrel->rows;
488496

497+
/*
498+
* Accumulate per-column estimates too. We need not do anything
499+
* for PlaceHolderVars in the parent list. If child expression
500+
* isn't a Var, or we didn't record a width estimate for it, we
501+
* have to fall back on a datatype-based estimate.
502+
*
503+
* By construction, child's reltargetlist is 1-to-1 with parent's.
504+
*/
489505
forboth(parentvars,rel->reltargetlist,
490506
childvars,childrel->reltargetlist)
491507
{
492508
Var*parentvar= (Var*)lfirst(parentvars);
493-
Var*childvar= (Var*)lfirst(childvars);
494-
495-
/*
496-
* Accumulate per-column estimates too. Whole-row Vars and
497-
* PlaceHolderVars can be ignored here.
498-
*/
499-
if (IsA(parentvar,Var)&&
500-
IsA(childvar,Var))
509+
Node*childvar= (Node*)lfirst(childvars);
510+
511+
if (IsA(parentvar,Var))
501512
{
502513
intpndx=parentvar->varattno-rel->min_attr;
503-
intcndx=childvar->varattno-childrel->min_attr;
504-
505-
parent_attrsizes[pndx]+=childrel->attr_widths[cndx]*childrel->rows;
514+
int32child_width=0;
515+
516+
if (IsA(childvar,Var))
517+
{
518+
intcndx= ((Var*)childvar)->varattno-childrel->min_attr;
519+
520+
child_width=childrel->attr_widths[cndx];
521+
}
522+
if (child_width <=0)
523+
child_width=get_typavgwidth(exprType(childvar),
524+
exprTypmod(childvar));
525+
Assert(child_width>0);
526+
parent_attrsizes[pndx]+=child_width*childrel->rows;
506527
}
507528
}
508529
}

‎src/backend/optimizer/path/costsize.c‎

Lines changed: 11 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -3238,14 +3238,14 @@ set_subquery_size_estimates(PlannerInfo *root, RelOptInfo *rel,
32383238
/*
32393239
* Compute per-output-column width estimates by examining the subquery's
32403240
* targetlist.For any output that is a plain Var, get the width estimate
3241-
* that was made while planning the subquery. Otherwise,fall back on a
3242-
* datatype-based estimate.
3241+
* that was made while planning the subquery. Otherwise,we leave it to
3242+
*set_rel_width to fill in adatatype-based default estimate.
32433243
*/
32443244
foreach(lc,subroot->parse->targetList)
32453245
{
32463246
TargetEntry*te= (TargetEntry*)lfirst(lc);
32473247
Node*texpr= (Node*)te->expr;
3248-
int32item_width;
3248+
int32item_width=0;
32493249

32503250
Assert(IsA(te,TargetEntry));
32513251
/* junk columns aren't visible to upper query */
@@ -3256,8 +3256,14 @@ set_subquery_size_estimates(PlannerInfo *root, RelOptInfo *rel,
32563256
* XXX This currently doesn't work for subqueries containing set
32573257
* operations, because the Vars in their tlists are bogus references
32583258
* to the first leaf subquery, which wouldn't give the right answer
3259-
* even if we could still get to its PlannerInfo. So fall back on
3260-
* datatype in that case.
3259+
* even if we could still get to its PlannerInfo.
3260+
*
3261+
* Also, the subquery could be an appendrel for which all branches are
3262+
* known empty due to constraint exclusion, in which case
3263+
* set_append_rel_pathlist will have left the attr_widths set to zero.
3264+
*
3265+
* In either case, we just leave the width estimate zero until
3266+
* set_rel_width fixes it.
32613267
*/
32623268
if (IsA(texpr,Var)&&
32633269
subroot->parse->setOperations==NULL)
@@ -3267,11 +3273,6 @@ set_subquery_size_estimates(PlannerInfo *root, RelOptInfo *rel,
32673273

32683274
item_width=subrel->attr_widths[var->varattno-subrel->min_attr];
32693275
}
3270-
else
3271-
{
3272-
item_width=get_typavgwidth(exprType(texpr),exprTypmod(texpr));
3273-
}
3274-
Assert(item_width>0);
32753276
Assert(te->resno >=rel->min_attr&&te->resno <=rel->max_attr);
32763277
rel->attr_widths[te->resno-rel->min_attr]=item_width;
32773278
}

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