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Speedup ScalarArrayOpExpr evaluation

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ScalarArrayOpExprs with "useOr=true" and a set of Consts on the righthand
side have traditionally been evaluated by using a linear search over the
array.  When these arrays contain large numbers of elements then this
linear search could become a significant part of execution time.

Here we add a new method of evaluating ScalarArrayOpExpr expressions to
allow them to be evaluated by first building a hash table containing each
element, then on subsequent evaluations, we just probe that hash table to
determine if there is a match.

The planner is in charge of determining when this optimization is possible
and it enables it by setting hashfuncid in the ScalarArrayOpExpr.  The
executor will only perform the hash table evaluation when the hashfuncid
is set.

This means that not all cases are optimized. For example CHECK constraints
containing an IN clause won't go through the planner, so won't get the
hashfuncid set.  We could maybe do something about that at some later
date.  The reason we're not doing it now is from fear that we may slow
down cases where the expression is evaluated only once.  Those cases can
be common, for example, a single row INSERT to a table with a CHECK
constraint containing an IN clause.

In the planner, we enable this when there are suitable hash functions for
the ScalarArrayOpExpr's operator and only when there is at least
MIN_ARRAY_SIZE_FOR_HASHED_SAOP elements in the array.  The threshold is
currently set to 9.

Author: James Coleman, David Rowley
Reviewed-by: David Rowley, Tomas Vondra, Heikki Linnakangas
Discussion: https://postgr.es/m/CAAaqYe8x62+=wn0zvNKCj55tPpg-JBHzhZFFc6ANovdqFw7-dA@mail.gmail.com
This commit is contained in:
David Rowley
2021-04-08 23:51:22 +12:00
parent 1d257577e0
commit 50e17ad281
21 changed files with 711 additions and 29 deletions
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43
src/backend/optimizer/path/costsize.c
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@@ -4436,21 +4436,50 @@ cost_qual_eval_walker(Node *node, cost_qual_eval_context *context)
}
else if (IsA(node, ScalarArrayOpExpr))
{
/*
* Estimate that the operator will be applied to about half of the
* array elements before the answer is determined.
*/
ScalarArrayOpExpr *saop = (ScalarArrayOpExpr *) node;
Node *arraynode = (Node *) lsecond(saop->args);
QualCost sacosts;
QualCost hcosts;
int estarraylen = estimate_array_length(arraynode);
set_sa_opfuncid(saop);
sacosts.startup = sacosts.per_tuple = 0;
add_function_cost(context->root, saop->opfuncid, NULL,
&sacosts);
context->total.startup += sacosts.startup;
context->total.per_tuple += sacosts.per_tuple *
estimate_array_length(arraynode) * 0.5;
if (OidIsValid(saop->hashfuncid))
{
/* Handle costs for hashed ScalarArrayOpExpr */
hcosts.startup = hcosts.per_tuple = 0;
add_function_cost(context->root, saop->hashfuncid, NULL, &hcosts);
context->total.startup += sacosts.startup + hcosts.startup;
/* Estimate the cost of building the hashtable. */
context->total.startup += estarraylen * hcosts.per_tuple;
/*
* XXX should we charge a little bit for sacosts.per_tuple when
* building the table, or is it ok to assume there will be zero
* hash collision?
*/
/*
* Charge for hashtable lookups. Charge a single hash and a
* single comparison.
*/
context->total.per_tuple += hcosts.per_tuple + sacosts.per_tuple;
}
else
{
/*
* Estimate that the operator will be applied to about half of the
* array elements before the answer is determined.
*/
context->total.startup += sacosts.startup;
context->total.per_tuple += sacosts.per_tuple *
estimate_array_length(arraynode) * 0.5;
}
}
else if (IsA(node, Aggref) ||
IsA(node, WindowFunc))