From: David Rowley Date: Tue, 29 Jul 2025 03:18:01 +0000 (+1200) Subject: Display Memoize planner estimates in EXPLAIN X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=4bc62b86849065939a6b85273fece6b92d6e97bf;p=thirdparty%2Fpostgresql.git Display Memoize planner estimates in EXPLAIN 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 Author: Lukas Fittl Reviewed-by: David Rowley Reviewed-by: Andrei Lepikhov Reviewed-by: Robert Haas Discussion: https://postgr.es/m/CAP53Pky29GWAVVk3oBgKBDqhND0BRBN6yTPeguV_qSivFL5N_g%40mail.gmail.com --- diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c index 7e2792ead71..8345bc0264b 100644 --- a/src/backend/commands/explain.c +++ b/src/backend/commands/explain.c @@ -3582,6 +3582,7 @@ static void show_memoize_info(MemoizeState *mstate, List *ancestors, ExplainState *es) { Plan *plan = ((PlanState *) mstate)->plan; + Memoize *mplan = (Memoize *) plan; ListCell *lc; List *context; StringInfoData keystr; @@ -3602,7 +3603,7 @@ show_memoize_info(MemoizeState *mstate, List *ancestors, ExplainState *es) plan, ancestors); - foreach(lc, ((Memoize *) plan)->param_exprs) + foreach(lc, mplan->param_exprs) { Node *expr = (Node *) lfirst(lc); @@ -3618,6 +3619,24 @@ show_memoize_info(MemoizeState *mstate, List *ancestors, ExplainState *es) pfree(keystr.data); + 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", + mplan->est_entries, mplan->est_unique_keys, + mplan->est_calls, mplan->est_hit_ratio * 100.0); + } + else + { + ExplainPropertyUInteger("Estimated Capacity", NULL, mplan->est_entries, es); + ExplainPropertyFloat("Estimated Distinct Lookup Keys", NULL, mplan->est_unique_keys, 0, es); + ExplainPropertyFloat("Estimated Lookups", NULL, mplan->est_calls, 0, es); + ExplainPropertyFloat("Estimated Hit Percent", NULL, mplan->est_hit_ratio * 100.0, 2, es); + } + } + if (!es->analyze) return; diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c index 1f04a2c182c..344a3188317 100644 --- a/src/backend/optimizer/path/costsize.c +++ b/src/backend/optimizer/path/costsize.c @@ -2572,13 +2572,13 @@ cost_memoize_rescan(PlannerInfo *root, MemoizePath *mpath, Cost input_startup_cost = mpath->subpath->startup_cost; Cost input_total_cost = mpath->subpath->total_cost; double tuples = mpath->subpath->rows; - double calls = mpath->calls; + Cardinality est_calls = mpath->est_calls; int width = mpath->subpath->pathtarget->width; double hash_mem_bytes; double est_entry_bytes; - double est_cache_entries; - double ndistinct; + Cardinality est_cache_entries; + Cardinality ndistinct; double evict_ratio; double hit_ratio; Cost startup_cost; @@ -2604,7 +2604,7 @@ cost_memoize_rescan(PlannerInfo *root, MemoizePath *mpath, est_cache_entries = floor(hash_mem_bytes / est_entry_bytes); /* estimate on the distinct number of parameter values */ - ndistinct = estimate_num_groups(root, mpath->param_exprs, calls, NULL, + ndistinct = estimate_num_groups(root, mpath->param_exprs, est_calls, NULL, &estinfo); /* @@ -2616,7 +2616,10 @@ cost_memoize_rescan(PlannerInfo *root, MemoizePath *mpath, * certainly mean a MemoizePath will never survive add_path(). */ if ((estinfo.flags & SELFLAG_USED_DEFAULT) != 0) - ndistinct = calls; + ndistinct = est_calls; + + /* Remember the ndistinct estimate for EXPLAIN */ + mpath->est_unique_keys = ndistinct; /* * Since we've already estimated the maximum number of entries we can @@ -2644,9 +2647,12 @@ cost_memoize_rescan(PlannerInfo *root, MemoizePath *mpath, * must look at how many scans are estimated in total for this node and * how many of those scans we expect to get a cache hit. */ - hit_ratio = ((calls - ndistinct) / calls) * + hit_ratio = ((est_calls - ndistinct) / est_calls) * (est_cache_entries / Max(ndistinct, est_cache_entries)); + /* Remember the hit ratio estimate for EXPLAIN */ + 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 8a9f1d7a943..bfefc7dbea1 100644 --- a/src/backend/optimizer/plan/createplan.c +++ b/src/backend/optimizer/plan/createplan.c @@ -284,7 +284,10 @@ 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, + Cardinality est_calls, + Cardinality est_unique_keys, + double est_hit_ratio); static WindowAgg *make_windowagg(List *tlist, WindowClause *wc, int partNumCols, AttrNumber *partColIdx, Oid *partOperators, Oid *partCollations, int ordNumCols, AttrNumber *ordColIdx, Oid *ordOperators, Oid *ordCollations, @@ -1753,7 +1756,8 @@ 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_calls, + best_path->est_unique_keys, best_path->est_hit_ratio); copy_generic_path_info(&plan->plan, (Path *) best_path); @@ -6749,7 +6753,9 @@ 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, + Cardinality est_calls, Cardinality est_unique_keys, + double est_hit_ratio) { Memoize *node = makeNode(Memoize); Plan *plan = &node->plan; @@ -6767,6 +6773,9 @@ make_memoize(Plan *lefttree, Oid *hashoperators, Oid *collations, node->binary_mode = binary_mode; node->est_entries = est_entries; node->keyparamids = keyparamids; + node->est_calls = est_calls; + node->est_unique_keys = est_unique_keys; + node->est_hit_ratio = est_hit_ratio; return node; } diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c index 9cc602788ea..a4c5867cdcb 100644 --- a/src/backend/optimizer/util/pathnode.c +++ b/src/backend/optimizer/util/pathnode.c @@ -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; diff --git a/src/include/nodes/pathnodes.h b/src/include/nodes/pathnodes.h index e5dd15098f6..ad2726f026f 100644 --- a/src/include/nodes/pathnodes.h +++ b/src/include/nodes/pathnodes.h @@ -2133,10 +2133,12 @@ typedef struct MemoizePath * complete after caching the first record. */ bool binary_mode; /* true when cache key should be compared bit * by bit, false when using hash equality ops */ - Cardinality calls; /* expected number of rescans */ uint32 est_entries; /* The maximum number of entries that the * planner expects will fit in the cache, or 0 * if unknown */ + Cardinality est_calls; /* expected number of rescans */ + Cardinality est_unique_keys; /* estimated unique keys, for EXPLAIN */ + double est_hit_ratio; /* estimated cache hit ratio, for EXPLAIN */ } MemoizePath; /* diff --git a/src/include/nodes/plannodes.h b/src/include/nodes/plannodes.h index 46e2e09ea35..6d8e1e99db3 100644 --- a/src/include/nodes/plannodes.h +++ b/src/include/nodes/plannodes.h @@ -1073,6 +1073,16 @@ typedef struct Memoize /* paramids from param_exprs */ Bitmapset *keyparamids; + + /* Estimated number of rescans, for EXPLAIN */ + Cardinality est_calls; + + /* Estimated number of distinct lookup keys, for EXPLAIN */ + Cardinality est_unique_keys; + + /* Estimated cache hit ratio, for EXPLAIN */ + double est_hit_ratio; + } Memoize; /* ---------------- diff --git a/src/include/optimizer/pathnode.h b/src/include/optimizer/pathnode.h index 60dcdb77e41..58936e963cb 100644 --- a/src/include/optimizer/pathnode.h +++ b/src/include/optimizer/pathnode.h @@ -90,7 +90,7 @@ extern MemoizePath *create_memoize_path(PlannerInfo *root, List *hash_operators, bool singlerow, bool binary_mode, - double calls); + Cardinality est_calls); extern UniquePath *create_unique_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath, SpecialJoinInfo *sjinfo); extern GatherPath *create_gather_path(PlannerInfo *root,