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Invent recursive_worktable_factor GUC to replace hard-wired constant.

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Up to now, the planner estimated the size of a recursive query's
worktable as 10 times the size of the non-recursive term.  It's hard
to see how to do significantly better than that automatically, but
we can give users control over the multiplier to allow tuning for
specific use-cases.  The default behavior remains the same.

Simon Riggs

Discussion: https://postgr.es/m/CANbhV-EuaLm4H3g0+BSTYHEGxJj3Kht0R+rJ8vT57Dejnh=_nA@mail.gmail.com
This commit is contained in:
Tom Lane
2022-03-24 11:47:41 -04:00
parent a47651447f
commit 0bd7af082a
6 changed files with 44 additions and 3 deletions
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8
src/backend/optimizer/path/costsize.c
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@@ -123,6 +123,7 @@ double cpu_index_tuple_cost = DEFAULT_CPU_INDEX_TUPLE_COST;
double cpu_operator_cost = DEFAULT_CPU_OPERATOR_COST;
double parallel_tuple_cost = DEFAULT_PARALLEL_TUPLE_COST;
double parallel_setup_cost = DEFAULT_PARALLEL_SETUP_COST;
double recursive_worktable_factor = DEFAULT_RECURSIVE_WORKTABLE_FACTOR;
int effective_cache_size = DEFAULT_EFFECTIVE_CACHE_SIZE;
@@ -5665,10 +5666,11 @@ set_cte_size_estimates(PlannerInfo *root, RelOptInfo *rel, double cte_rows)
if (rte->self_reference)
{
/*
* In a self-reference, arbitrarily assume the average worktable size
* is about 10 times the nonrecursive term's size.
* In a self-reference, we assume the average worktable size is a
* multiple of the nonrecursive term's size. The best multiplier will
* vary depending on query "fan-out", so make its value adjustable.
*/
rel->tuples = 10 * cte_rows;
rel->tuples = clamp_row_est(recursive_worktable_factor * cte_rows);
}
else
{