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Make PlaceHolderInfo lookup O(1).

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Up to now we've just searched the placeholder_list when we want to
find the PlaceHolderInfo with a given ID.  While there's no evidence
of that being a problem in the field, an upcoming patch will add
find_placeholder_info() calls in build_joinrel_tlist(), which seems
likely to make it more of an issue: a joinrel emitting lots of
PlaceHolderVars would incur O(N^2) cost, and we might be building
a lot of joinrels in complex queries.  Hence, add an array that
can be indexed directly by phid to make the lookups constant-time.

Discussion: https://postgr.es/m/1405792.1660677844@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2022-08-17 15:35:51 -04:00
parent efd0c16bec
commit 6569ca4397
5 changed files with 50 additions and 14 deletions
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43
src/backend/optimizer/util/placeholder.c
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@@ -71,16 +71,19 @@ find_placeholder_info(PlannerInfo *root, PlaceHolderVar *phv,
{
PlaceHolderInfo *phinfo;
Relids rels_used;
ListCell *lc;
/* if this ever isn't true, we'd need to be able to look in parent lists */
Assert(phv->phlevelsup == 0);
foreach(lc, root->placeholder_list)
/* Use placeholder_array to look up existing PlaceHolderInfo quickly */
if (phv->phid < root->placeholder_array_size)
phinfo = root->placeholder_array[phv->phid];
else
phinfo = NULL;
if (phinfo != NULL)
{
phinfo = (PlaceHolderInfo *) lfirst(lc);
if (phinfo->phid == phv->phid)
return phinfo;
Assert(phinfo->phid == phv->phid);
return phinfo;
}
/* Not found, so create it */
@@ -115,8 +118,38 @@ find_placeholder_info(PlannerInfo *root, PlaceHolderVar *phv,
phinfo->ph_width = get_typavgwidth(exprType((Node *) phv->phexpr),
exprTypmod((Node *) phv->phexpr));
/*
* Add to both placeholder_list and placeholder_array. Note: because we
* store pointers to the PlaceHolderInfos in two data structures, it'd be
* unsafe to pass the whole placeholder_list structure through
* expression_tree_mutator or the like --- or at least, you'd have to
* rebuild the placeholder_array afterwards.
*/
root->placeholder_list = lappend(root->placeholder_list, phinfo);
if (phinfo->phid >= root->placeholder_array_size)
{
/* Must allocate or enlarge placeholder_array */
int new_size;
new_size = root->placeholder_array_size ? root->placeholder_array_size * 2 : 8;
while (phinfo->phid >= new_size)
new_size *= 2;
if (root->placeholder_array)
{
root->placeholder_array = (PlaceHolderInfo **)
repalloc(root->placeholder_array,
sizeof(PlaceHolderInfo *) * new_size);
MemSet(root->placeholder_array + root->placeholder_array_size, 0,
sizeof(PlaceHolderInfo *) * (new_size - root->placeholder_array_size));
}
else
root->placeholder_array = (PlaceHolderInfo **)
palloc0(new_size * sizeof(PlaceHolderInfo *));
root->placeholder_array_size = new_size;
}
root->placeholder_array[phinfo->phid] = phinfo;
/*
* The PHV's contained expression may contain other, lower-level PHVs. We
* now know we need to get those into the PlaceHolderInfo list, too, so we