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Display Memoize planner estimates in EXPLAIN

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There've been a few complaints that it can be overly difficult to figure
out why the planner picked a Memoize plan.  To help address that, here we
adjust the EXPLAIN output to display the following additional details:

1) The estimated number of cache entries that can be stored at once
2) The estimated number of unique lookup keys that we expect to see
3) The number of lookups we expect
4) The estimated hit ratio

Technically #4 can be calculated using #1, #2 and #3, but it's not a
particularly obvious calculation, so we opt to display it explicitly.
The original patch by Lukas Fittl only displayed the hit ratio, but
there was a fear that might lead to more questions about how that was
calculated.  The idea with displaying all 4 is to be transparent which
may allow queries to be tuned more easily.  For example, if #2 isn't
correct then maybe extended statistics or a manual n_distinct estimate can
be used to help fix poor plan choices.

Author: Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>
Author: Lukas Fittl <lukas@fittl.com>
Reviewed-by: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://postgr.es/m/CAP53Pky29GWAVVk3oBgKBDqhND0BRBN6yTPeguV_qSivFL5N_g%40mail.gmail.com
This commit is contained in:
David Rowley
2025-07-29 15:18:01 +12:00
parent 71c0921b64
commit 4bc62b8684
7 changed files with 66 additions and 15 deletions
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11
src/backend/optimizer/util/pathnode.c
View File

@@ -1689,7 +1689,7 @@ create_material_path(RelOptInfo *rel, Path *subpath)
MemoizePath *
create_memoize_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
List *param_exprs, List *hash_operators,
bool singlerow, bool binary_mode, double calls)
bool singlerow, bool binary_mode, Cardinality est_calls)
{
MemoizePath *pathnode = makeNode(MemoizePath);
@@ -1710,7 +1710,6 @@ create_memoize_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
pathnode->param_exprs = param_exprs;
pathnode->singlerow = singlerow;
pathnode->binary_mode = binary_mode;
pathnode->calls = clamp_row_est(calls);
/*
* For now we set est_entries to 0. cost_memoize_rescan() does all the
@@ -1720,6 +1719,12 @@ create_memoize_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
*/
pathnode->est_entries = 0;
pathnode->est_calls = clamp_row_est(est_calls);
/* These will also be set later in cost_memoize_rescan() */
pathnode->est_unique_keys = 0.0;
pathnode->est_hit_ratio = 0.0;
/* we should not generate this path type when enable_memoize=false */
Assert(enable_memoize);
pathnode->path.disabled_nodes = subpath->disabled_nodes;
@@ -4259,7 +4264,7 @@ reparameterize_path(PlannerInfo *root, Path *path,
mpath->hash_operators,
mpath->singlerow,
mpath->binary_mode,
mpath->calls);
mpath->est_calls);
}
default:
break;