On 27.04.2025 22:58, Ilia Evdokimov wrote:
Therefore, I think it is better to report percentages directly. Since
non-text EXPLAIN formats do not display units, I propose to rename the
field to "hit_percent" in all formats, including the text format. This
way, the meaning of the value remains clear without needing additional
context. What do you think about this approach?
I attached updated v9 patch with the suggested changes. The updated line
in the EXPLAIN looks like this:
Text format:
Estimates: capacity=1 distinct keys=1 lookups=2 hit percent=50.00%
Non-text format:
Estimated Capacity: 1
Estimated Distinct Lookup Keys: 1
Estimated Lookups: 2
Estimated Hit Percent: 50.00
Any suggestions?
--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC.
From eff0cb420c922343af9e09e58cec7b5be19c35e4 Mon Sep 17 00:00:00 2001
From: Evdokimov Ilia <ilya.evdoki...@tantorlabs.com>
Date: Thu, 1 May 2025 15:20:07 +0300
Subject: [PATCH v9] Expose cache hit statistics in Memoize node EXPLAIN
output.
This patch adds additional information to the Memoize node's EXPLAIN output:
estimated cache capacity, number of distinct keys, total lookups, and
cache hit percentage.
Reviewed-by: David Rowley <dgrowle...@gmail.com>
Reviewed-by: Robert Haas <robertmh...@gmail.com>
Reviewed-by: Andrei Lepikhov <lepi...@gmail.com>
Reviewed-by: Tom Lane <t...@sss.pgh.pa.us>
---
src/backend/commands/explain.c | 20 ++++++++++++++++++++
src/backend/optimizer/path/costsize.c | 6 ++++++
src/backend/optimizer/plan/createplan.c | 15 ++++++++++++---
src/backend/optimizer/util/pathnode.c | 12 ++++++++++++
src/include/nodes/pathnodes.h | 3 +++
src/include/nodes/plannodes.h | 12 ++++++++++++
6 files changed, 65 insertions(+), 3 deletions(-)
diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index 786ee865f14..3a5b436b561 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -3628,6 +3628,26 @@ show_memoize_info(MemoizeState *mstate, List *ancestors, ExplainState *es)
ExplainPropertyText("Cache Key", keystr.data, es);
ExplainPropertyText("Cache Mode", mstate->binary_mode ? "binary" : "logical", es);
+ if (es->costs)
+ {
+ if (es->format == EXPLAIN_FORMAT_TEXT)
+ {
+ ExplainIndentText(es);
+ appendStringInfo(es->str, "Estimates: capacity=%u distinct keys=%.0f lookups=%.0f hit percent=%.2f%%\n",
+ ((Memoize *) plan)->est_entries,
+ ((Memoize *) plan)->est_unique_keys,
+ ((Memoize *) plan)->est_calls,
+ ((Memoize *) plan)->est_hit_ratio * 100.0);
+ }
+ else
+ {
+ ExplainPropertyUInteger("Estimated Capacity", NULL, ((Memoize *) plan)->est_entries, es);
+ ExplainPropertyFloat("Estimated Distinct Lookup Keys", NULL, ((Memoize *) plan)->est_unique_keys, 0, es);
+ ExplainPropertyFloat("Estimated Lookups", NULL, ((Memoize *) plan)->est_calls, 0, es);
+ ExplainPropertyFloat("Estimated Hit Percent", NULL, ((Memoize *) plan)->est_hit_ratio * 100.0, 2, es);
+ }
+ }
+
pfree(keystr.data);
if (!es->analyze)
diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index 60b0fcfb6be..226a7861543 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -2604,6 +2604,9 @@ cost_memoize_rescan(PlannerInfo *root, MemoizePath *mpath,
mpath->est_entries = Min(Min(ndistinct, est_cache_entries),
PG_UINT32_MAX);
+ /* Remember ndistinct for a potential EXPLAIN later */
+ mpath->est_unique_keys = ndistinct;
+
/*
* When the number of distinct parameter values is above the amount we can
* store in the cache, then we'll have to evict some entries from the
@@ -2621,6 +2624,9 @@ cost_memoize_rescan(PlannerInfo *root, MemoizePath *mpath,
hit_ratio = ((calls - ndistinct) / calls) *
(est_cache_entries / Max(ndistinct, est_cache_entries));
+ /* Remember cache hit ratio for a potential EXPLAIN later */
+ mpath->est_hit_ratio = hit_ratio;
+
Assert(hit_ratio >= 0 && hit_ratio <= 1.0);
/*
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index a8f22a8c154..b3d3fac187f 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -284,7 +284,9 @@ static Material *make_material(Plan *lefttree);
static Memoize *make_memoize(Plan *lefttree, Oid *hashoperators,
Oid *collations, List *param_exprs,
bool singlerow, bool binary_mode,
- uint32 est_entries, Bitmapset *keyparamids);
+ uint32 est_entries, Bitmapset *keyparamids,
+ double est_unique_keys, double est_hit_ratio,
+ double est_calls);
static WindowAgg *make_windowagg(List *tlist, WindowClause *wc,
int partNumCols, AttrNumber *partColIdx, Oid *partOperators, Oid *partCollations,
int ordNumCols, AttrNumber *ordColIdx, Oid *ordOperators, Oid *ordCollations,
@@ -1703,7 +1705,9 @@ create_memoize_plan(PlannerInfo *root, MemoizePath *best_path, int flags)
plan = make_memoize(subplan, operators, collations, param_exprs,
best_path->singlerow, best_path->binary_mode,
- best_path->est_entries, keyparamids);
+ best_path->est_entries, keyparamids,
+ best_path->est_unique_keys, best_path->est_hit_ratio,
+ best_path->calls);
copy_generic_path_info(&plan->plan, (Path *) best_path);
@@ -6636,7 +6640,9 @@ materialize_finished_plan(Plan *subplan)
static Memoize *
make_memoize(Plan *lefttree, Oid *hashoperators, Oid *collations,
List *param_exprs, bool singlerow, bool binary_mode,
- uint32 est_entries, Bitmapset *keyparamids)
+ uint32 est_entries, Bitmapset *keyparamids,
+ double est_unique_keys, double est_hit_ratio,
+ double est_calls)
{
Memoize *node = makeNode(Memoize);
Plan *plan = &node->plan;
@@ -6654,6 +6660,9 @@ make_memoize(Plan *lefttree, Oid *hashoperators, Oid *collations,
node->binary_mode = binary_mode;
node->est_entries = est_entries;
node->keyparamids = keyparamids;
+ node->est_unique_keys = est_unique_keys;
+ node->est_hit_ratio = est_hit_ratio;
+ node->est_calls = est_calls;
return node;
}
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 93e73cb44db..7e35421f410 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -1701,6 +1701,18 @@ create_memoize_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
Assert(enable_memoize);
pathnode->path.disabled_nodes = subpath->disabled_nodes;
+ /*
+ * Estimated number of distinct memoization keys,
+ * computed using estimate_num_groups()
+ */
+ pathnode->est_unique_keys = 0;
+
+ /*
+ * The estimated cache hit ratio will calculated later
+ * by cost_memoize_rescan().
+ */
+ pathnode->est_hit_ratio = 0;
+
/*
* Add a small additional charge for caching the first entry. All the
* harder calculations for rescans are performed in cost_memoize_rescan().
diff --git a/src/include/nodes/pathnodes.h b/src/include/nodes/pathnodes.h
index 011e5a811c3..938e4c43a81 100644
--- a/src/include/nodes/pathnodes.h
+++ b/src/include/nodes/pathnodes.h
@@ -2138,6 +2138,9 @@ typedef struct MemoizePath
uint32 est_entries; /* The maximum number of entries that the
* planner expects will fit in the cache, or 0
* if unknown */
+ double est_unique_keys; /* Estimated number of distinct memoization keys,
+ * used for cache size evaluation. Kept for EXPLAIN */
+ double est_hit_ratio; /* Estimated cache hit ratio. Kept for EXPLAIN */
} MemoizePath;
/*
diff --git a/src/include/nodes/plannodes.h b/src/include/nodes/plannodes.h
index 658d76225e4..26a4c78afe8 100644
--- a/src/include/nodes/plannodes.h
+++ b/src/include/nodes/plannodes.h
@@ -1063,6 +1063,18 @@ typedef struct Memoize
/* paramids from param_exprs */
Bitmapset *keyparamids;
+
+ /*
+ * Estimated number of distinct memoization keys,
+ * used for cache size evaluation. Kept for EXPLAIN
+ */
+ double est_unique_keys;
+
+ /* Estimated cache hit ratio. Kept for EXPLAIN */
+ double est_hit_ratio;
+
+ /* Estimated number of rescans. Kept for EXPLAIN */
+ double est_calls;
} Memoize;
/* ----------------
--
2.34.1
