From 84c7cef5eb0f0d08d40dbfcf396d4c830c1f1030 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Sat, 18 Sep 2004 19:39:50 +0000
Subject: [PATCH] Fix estimate_num_groups to be able to use expression-index
statistics when there is an expressional index matching a GROUP BY item.
---
src/backend/utils/adt/selfuncs.c | 184 +++++++++++++++++++------------
1 file changed, 112 insertions(+), 72 deletions(-)
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 412a0a32310..24759bf5c09 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -15,7 +15,7 @@
*
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.165 2004/08/30 02:54:39 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/utils/adt/selfuncs.c,v 1.166 2004/09/18 19:39:50 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@@ -1869,6 +1869,71 @@ fail:
ReleaseVariableStats(rightvar);
}
+
+/*
+ * Helper routine for estimate_num_groups: add an item to a list of
+ * GroupVarInfos, but only if it's not known equal to any of the existing
+ * entries.
+ */
+typedef struct
+{
+ Node *var; /* might be an expression, not just a Var */
+ RelOptInfo *rel; /* relation it belongs to */
+ double ndistinct; /* # distinct values */
+} GroupVarInfo;
+
+static List *
+add_unique_group_var(Query *root, List *varinfos,
+ Node *var, VariableStatData *vardata)
+{
+ GroupVarInfo *varinfo;
+ double ndistinct;
+ ListCell *lc;
+
+ ndistinct = get_variable_numdistinct(vardata);
+
+ /* cannot use foreach here because of possible list_delete */
+ lc = list_head(varinfos);
+ while (lc)
+ {
+ varinfo = (GroupVarInfo *) lfirst(lc);
+
+ /* must advance lc before list_delete possibly pfree's it */
+ lc = lnext(lc);
+
+ /* Drop exact duplicates */
+ if (equal(var, varinfo->var))
+ return varinfos;
+
+ /*
+ * Drop known-equal vars, but only if they belong to different
+ * relations (see comments for estimate_num_groups)
+ */
+ if (vardata->rel != varinfo->rel &&
+ exprs_known_equal(root, var, varinfo->var))
+ {
+ if (varinfo->ndistinct <= ndistinct)
+ {
+ /* Keep older item, forget new one */
+ return varinfos;
+ }
+ else
+ {
+ /* Delete the older item */
+ varinfos = list_delete_ptr(varinfos, varinfo);
+ }
+ }
+ }
+
+ varinfo = (GroupVarInfo *) palloc(sizeof(GroupVarInfo));
+
+ varinfo->var = var;
+ varinfo->rel = vardata->rel;
+ varinfo->ndistinct = ndistinct;
+ varinfos = lappend(varinfos, varinfo);
+ return varinfos;
+}
+
/*
* estimate_num_groups - Estimate number of groups in a grouped query
*
@@ -1900,6 +1965,9 @@ fail:
* increase the number of distinct values (unless it is volatile,
* which we consider unlikely for grouping), but it probably won't
* reduce the number of distinct values much either.
+ * As a special case, if a GROUP BY expression can be matched to an
+ * expressional index for which we have statistics, then we treat the
+ * whole expression as though it were just a Var.
* 2. If the list contains Vars of different relations that are known equal
* due to equijoin clauses, then drop all but one of the Vars from each
* known-equal set, keeping the one with smallest estimated # of values
@@ -1926,25 +1994,44 @@ fail:
double
estimate_num_groups(Query *root, List *groupExprs, double input_rows)
{
- List *allvars = NIL;
List *varinfos = NIL;
double numdistinct;
ListCell *l;
- typedef struct
- { /* varinfos is a List of these */
- Var *var;
- double ndistinct;
- } MyVarInfo;
/* We should not be called unless query has GROUP BY (or DISTINCT) */
Assert(groupExprs != NIL);
- /* Step 1: get the unique Vars used */
+ /*
+ * Steps 1/2: find the unique Vars used, treating an expression as a Var
+ * if we can find stats for it. For each one, record the statistical
+ * estimate of number of distinct values (total in its table, without
+ * regard for filtering).
+ */
foreach(l, groupExprs)
{
Node *groupexpr = (Node *) lfirst(l);
+ VariableStatData vardata;
List *varshere;
+ ListCell *l2;
+
+ /*
+ * If examine_variable is able to deduce anything about the GROUP BY
+ * expression, treat it as a single variable even if it's really more
+ * complicated.
+ */
+ examine_variable(root, groupexpr, 0, &vardata);
+ if (vardata.statsTuple != NULL || vardata.isunique)
+ {
+ varinfos = add_unique_group_var(root, varinfos,
+ groupexpr, &vardata);
+ ReleaseVariableStats(vardata);
+ continue;
+ }
+ ReleaseVariableStats(vardata);
+ /*
+ * Else pull out the component Vars
+ */
varshere = pull_var_clause(groupexpr, false);
/*
@@ -1959,70 +2046,24 @@ estimate_num_groups(Query *root, List *groupExprs, double input_rows)
return input_rows;
continue;
}
- allvars = list_concat(allvars, varshere);
- }
-
- /* If now no Vars, we must have an all-constant GROUP BY list. */
- if (allvars == NIL)
- return 1.0;
-
- /* Use list_union() to discard duplicates */
- allvars = list_union(NIL, allvars);
-
- /*
- * Step 2: acquire statistical estimate of number of distinct values
- * of each Var (total in its table, without regard for filtering).
- * Also, detect known-equal Vars and discard the ones we don't want.
- */
- foreach(l, allvars)
- {
- Var *var = (Var *) lfirst(l);
- VariableStatData vardata;
- double ndistinct;
- bool keep = true;
- ListCell *l2;
-
- examine_variable(root, (Node *) var, 0, &vardata);
- ndistinct = get_variable_numdistinct(&vardata);
- ReleaseVariableStats(vardata);
-
- /* cannot use foreach here because of possible list_delete */
- l2 = list_head(varinfos);
- while (l2)
- {
- MyVarInfo *varinfo = (MyVarInfo *) lfirst(l2);
-
- /* must advance l2 before list_delete possibly pfree's it */
- l2 = lnext(l2);
-
- if (var->varno != varinfo->var->varno &&
- exprs_known_equal(root, (Node *) var, (Node *) varinfo->var))
- {
- /* Found a match */
- if (varinfo->ndistinct <= ndistinct)
- {
- /* Keep older item, forget new one */
- keep = false;
- break;
- }
- else
- {
- /* Delete the older item */
- varinfos = list_delete_ptr(varinfos, varinfo);
- }
- }
- }
- if (keep)
+ /*
+ * Else add variables to varinfos list
+ */
+ foreach(l2, varshere)
{
- MyVarInfo *varinfo = (MyVarInfo *) palloc(sizeof(MyVarInfo));
+ Node *var = (Node *) lfirst(l2);
- varinfo->var = var;
- varinfo->ndistinct = ndistinct;
- varinfos = lcons(varinfo, varinfos);
+ examine_variable(root, var, 0, &vardata);
+ varinfos = add_unique_group_var(root, varinfos, var, &vardata);
+ ReleaseVariableStats(vardata);
}
}
+ /* If now no Vars, we must have an all-constant GROUP BY list. */
+ if (varinfos == NIL)
+ return 1.0;
+
/*
* Steps 3/4: group Vars by relation and estimate total numdistinct.
*
@@ -2031,25 +2072,24 @@ estimate_num_groups(Query *root, List *groupExprs, double input_rows)
* these Vars from the newvarinfos list for the next iteration. This
* is the easiest way to group Vars of same rel together.
*/
- Assert(varinfos != NIL);
numdistinct = 1.0;
do
{
- MyVarInfo *varinfo1 = (MyVarInfo *) linitial(varinfos);
- RelOptInfo *rel = find_base_rel(root, varinfo1->var->varno);
+ GroupVarInfo *varinfo1 = (GroupVarInfo *) linitial(varinfos);
+ RelOptInfo *rel = varinfo1->rel;
double reldistinct = varinfo1->ndistinct;
List *newvarinfos = NIL;
/*
- * Get the largest numdistinct estimate of the Vars for this rel.
+ * Get the product of numdistinct estimates of the Vars for this rel.
* Also, construct new varinfos list of remaining Vars.
*/
for_each_cell(l, lnext(list_head(varinfos)))
{
- MyVarInfo *varinfo2 = (MyVarInfo *) lfirst(l);
+ GroupVarInfo *varinfo2 = (GroupVarInfo *) lfirst(l);
- if (varinfo2->var->varno == varinfo1->var->varno)
+ if (varinfo2->rel == varinfo1->rel)
reldistinct *= varinfo2->ndistinct;
else
{
--
GitLab