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Fix overoptimistic assumptions in column width estimation for subqueries. 

set_append_rel_pathlist supposed that, while computing per-column width
estimates for the appendrel, it could ignore child rels for which the
translated reltargetlist entry wasn't a Var.  This gave rise to completely
silly estimates in some common cases, such as constant outputs from some or
all of the arms of a UNION ALL.  Instead, fall back on get_typavgwidth to
estimate from the value's datatype; which might be a poor estimate but at
least it's not completely wacko.

That problem was exposed by an Assert in set_subquery_size_estimates, which
unfortunately was still overoptimistic even with that fix, since we don't
compute attr_widths estimates for appendrels that are entirely excluded by
constraints.  So remove the Assert; we'll just fall back on get_typavgwidth
in such cases.

Also, since set_subquery_size_estimates calls set_baserel_size_estimates
which calls set_rel_width, there's no need for set_subquery_size_estimates
to call get_typavgwidth; set_rel_width will handle it for us if we just
leave the estimate set to zero.  Remove the unnecessary code.

Per report from Erik Rijkers and subsequent investigation.
This commit is contained in:
Tom Lane 2011-08-23 17:11:41 -04:00
parent 1af55e2751
commit 43f0c20839
2 changed files with 42 additions and 20 deletions
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41
src/backend/optimizer/path/allpaths.c
View File

@ -403,7 +403,15 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
continue;
}
/* CE failed, so finish copying targetlist and join quals */
/*
* CE failed, so finish copying/modifying targetlist and join quals.
*
* Note: the resulting childrel->reltargetlist may contain arbitrary
* expressions, which normally would not occur in a reltargetlist.
* That is okay because nothing outside of this routine will look at
* the child rel's reltargetlist. We do have to cope with the case
* while constructing attr_widths estimates below, though.
*/
childrel->joininfo = (List *)
adjust_appendrel_attrs((Node *) rel->joininfo,
appinfo);
@ -486,23 +494,36 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
parent_rows += childrel->rows;
parent_size += childrel->width * childrel->rows;
/*
* Accumulate per-column estimates too. We need not do anything
* for PlaceHolderVars in the parent list. If child expression
* isn't a Var, or we didn't record a width estimate for it, we
* have to fall back on a datatype-based estimate.
*
* By construction, child's reltargetlist is 1-to-1 with parent's.
*/
forboth(parentvars, rel->reltargetlist,
childvars, childrel->reltargetlist)
{
Var *parentvar = (Var *) lfirst(parentvars);
Var *childvar = (Var *) lfirst(childvars);
Node *childvar = (Node *) lfirst(childvars);
/*
* Accumulate per-column estimates too. Whole-row Vars and
* PlaceHolderVars can be ignored here.
*/
if (IsA(parentvar, Var) &&
IsA(childvar, Var))
if (IsA(parentvar, Var))
{
int pndx = parentvar->varattno - rel->min_attr;
int cndx = childvar->varattno - childrel->min_attr;
int32 child_width = 0;
parent_attrsizes[pndx] += childrel->attr_widths[cndx] * childrel->rows;
if (IsA(childvar, Var))
{
int cndx = ((Var *) childvar)->varattno - childrel->min_attr;
child_width = childrel->attr_widths[cndx];
}
if (child_width <= 0)
child_width = get_typavgwidth(exprType(childvar),
exprTypmod(childvar));
Assert(child_width > 0);
parent_attrsizes[pndx] += child_width * childrel->rows;
}
}
}

21
src/backend/optimizer/path/costsize.c
View File

@ -3238,14 +3238,14 @@ set_subquery_size_estimates(PlannerInfo *root, RelOptInfo *rel,
/*
* Compute per-output-column width estimates by examining the subquery's
* targetlist. For any output that is a plain Var, get the width estimate
* that was made while planning the subquery. Otherwise, fall back on a
* datatype-based estimate.
* that was made while planning the subquery. Otherwise, we leave it to
* set_rel_width to fill in a datatype-based default estimate.
*/
foreach(lc, subroot->parse->targetList)
{
TargetEntry *te = (TargetEntry *) lfirst(lc);
Node *texpr = (Node *) te->expr;
int32 item_width;
int32 item_width = 0;
Assert(IsA(te, TargetEntry));
/* junk columns aren't visible to upper query */
@ -3256,8 +3256,14 @@ set_subquery_size_estimates(PlannerInfo *root, RelOptInfo *rel,
* XXX This currently doesn't work for subqueries containing set
* operations, because the Vars in their tlists are bogus references
* to the first leaf subquery, which wouldn't give the right answer
* even if we could still get to its PlannerInfo. So fall back on
* datatype in that case.
* even if we could still get to its PlannerInfo.
*
* Also, the subquery could be an appendrel for which all branches are
* known empty due to constraint exclusion, in which case
* set_append_rel_pathlist will have left the attr_widths set to zero.
*
* In either case, we just leave the width estimate zero until
* set_rel_width fixes it.
*/
if (IsA(texpr, Var) &&
subroot->parse->setOperations == NULL)
@ -3267,11 +3273,6 @@ set_subquery_size_estimates(PlannerInfo *root, RelOptInfo *rel,
item_width = subrel->attr_widths[var->varattno - subrel->min_attr];
}
else
{
item_width = get_typavgwidth(exprType(texpr), exprTypmod(texpr));
}
Assert(item_width > 0);
Assert(te->resno >= rel->min_attr && te->resno <= rel->max_attr);
rel->attr_widths[te->resno - rel->min_attr] = item_width;
}