Make pg_statistic and related code account more honestly for collations.

When we first put in collations support, we basically punted on teaching
pg_statistic, ANALYZE, and the planner selectivity functions about that.
They've just used DEFAULT_COLLATION_OID independently of the actual
collation of the data.  It's time to improve that, so:

* Add columns to pg_statistic that record the specific collation associated
with each statistics slot.

* Teach ANALYZE to use the column's actual collation when comparing values
for statistical purposes, and record this in the appropriate slot.  (Note
that type-specific typanalyze functions are now expected to fill
stats->stacoll with the appropriate collation, too.)

* Teach assorted selectivity functions to use the actual collation of
the stats they are looking at, instead of just assuming it's
DEFAULT_COLLATION_OID.

This should give noticeably better results in selectivity estimates for
columns with nondefault collations, at least for query clauses that use
that same collation (which would be the default behavior in most cases).
It's still true that comparisons with explicit COLLATE clauses different
from the stored data's collation won't be well-estimated, but that's no
worse than before.  Also, this patch does make the first step towards
doing better with that, which is that it's now theoretically possible to
collect stats for a collation other than the column's own collation.

Patch by me; thanks to Peter Eisentraut for review.

Discussion: https://postgr.es/m/14706.1544630227@sss.pgh.pa.us

1 files changed, 30 insertions, 29 deletions

diff --git a/src/backend/utils/adt/array_selfuncs.c b/src/backend/utils/adt/array_selfuncs.c
index 339525b53b5..00b69bd1e3f 100644
--- a/src/backend/utils/adt/array_selfuncs.c
+++ b/src/backend/utils/adt/array_selfuncs.c
@@ -46,21 +46,21 @@ static Selectivity mcelem_array_selec(ArrayType *array,
 				   Datum *mcelem, int nmcelem,
 				   float4 *numbers, int nnumbers,
 				   float4 *hist, int nhist,
-				   Oid operator, FmgrInfo *cmpfunc);
+				   Oid operator);
 static Selectivity mcelem_array_contain_overlap_selec(Datum *mcelem, int nmcelem,
 								   float4 *numbers, int nnumbers,
 								   Datum *array_data, int nitems,
-								   Oid operator, FmgrInfo *cmpfunc);
+								   Oid operator, TypeCacheEntry *typentry);
 static Selectivity mcelem_array_contained_selec(Datum *mcelem, int nmcelem,
 							 float4 *numbers, int nnumbers,
 							 Datum *array_data, int nitems,
 							 float4 *hist, int nhist,
-							 Oid operator, FmgrInfo *cmpfunc);
+							 Oid operator, TypeCacheEntry *typentry);
 static float *calc_hist(const float4 *hist, int nhist, int n);
 static float *calc_distr(const float *p, int n, int m, float rest);
 static int	floor_log2(uint32 n);
 static bool find_next_mcelem(Datum *mcelem, int nmcelem, Datum value,
-				 int *index, FmgrInfo *cmpfunc);
+				 int *index, TypeCacheEntry *typentry);
 static int	element_compare(const void *key1, const void *key2, void *arg);
 static int	float_compare_desc(const void *key1, const void *key2);
 
@@ -166,7 +166,7 @@ scalararraysel_containment(PlannerInfo *root,
 														   sslot.nnumbers,
 														   &constval, 1,
 														   OID_ARRAY_CONTAINS_OP,
-														   cmpfunc);
+														   typentry);
 			else
 				selec = mcelem_array_contained_selec(sslot.values,
 													 sslot.nvalues,
@@ -176,7 +176,7 @@ scalararraysel_containment(PlannerInfo *root,
 													 hslot.numbers,
 													 hslot.nnumbers,
 													 OID_ARRAY_CONTAINED_OP,
-													 cmpfunc);
+													 typentry);
 
 			free_attstatsslot(&hslot);
 			free_attstatsslot(&sslot);
@@ -189,14 +189,14 @@ scalararraysel_containment(PlannerInfo *root,
 														   NULL, 0,
 														   &constval, 1,
 														   OID_ARRAY_CONTAINS_OP,
-														   cmpfunc);
+														   typentry);
 			else
 				selec = mcelem_array_contained_selec(NULL, 0,
 													 NULL, 0,
 													 &constval, 1,
 													 NULL, 0,
 													 OID_ARRAY_CONTAINED_OP,
-													 cmpfunc);
+													 typentry);
 		}
 
 		/*
@@ -212,14 +212,14 @@ scalararraysel_containment(PlannerInfo *root,
 													   NULL, 0,
 													   &constval, 1,
 													   OID_ARRAY_CONTAINS_OP,
-													   cmpfunc);
+													   typentry);
 		else
 			selec = mcelem_array_contained_selec(NULL, 0,
 												 NULL, 0,
 												 &constval, 1,
 												 NULL, 0,
 												 OID_ARRAY_CONTAINED_OP,
-												 cmpfunc);
+												 typentry);
 		/* we assume no nulls here, so no stanullfrac correction */
 	}
 
@@ -385,7 +385,7 @@ calc_arraycontsel(VariableStatData *vardata, Datum constval,
 									   sslot.values, sslot.nvalues,
 									   sslot.numbers, sslot.nnumbers,
 									   hslot.numbers, hslot.nnumbers,
-									   operator, cmpfunc);
+									   operator);
 
 			free_attstatsslot(&hslot);
 			free_attstatsslot(&sslot);
@@ -395,7 +395,7 @@ calc_arraycontsel(VariableStatData *vardata, Datum constval,
 			/* No most-common-elements info, so do without */
 			selec = mcelem_array_selec(array, typentry,
 									   NULL, 0, NULL, 0, NULL, 0,
-									   operator, cmpfunc);
+									   operator);
 		}
 
 		/*
@@ -408,7 +408,7 @@ calc_arraycontsel(VariableStatData *vardata, Datum constval,
 		/* No stats at all, so do without */
 		selec = mcelem_array_selec(array, typentry,
 								   NULL, 0, NULL, 0, NULL, 0,
-								   operator, cmpfunc);
+								   operator);
 		/* we assume no nulls here, so no stanullfrac correction */
 	}
 
@@ -431,7 +431,7 @@ mcelem_array_selec(ArrayType *array, TypeCacheEntry *typentry,
 				   Datum *mcelem, int nmcelem,
 				   float4 *numbers, int nnumbers,
 				   float4 *hist, int nhist,
-				   Oid operator, FmgrInfo *cmpfunc)
+				   Oid operator)
 {
 	Selectivity selec;
 	int			num_elems;
@@ -476,20 +476,20 @@ mcelem_array_selec(ArrayType *array, TypeCacheEntry *typentry,
 
 	/* Sort extracted elements using their default comparison function. */
 	qsort_arg(elem_values, nonnull_nitems, sizeof(Datum),
-			  element_compare, cmpfunc);
+			  element_compare, typentry);
 
 	/* Separate cases according to operator */
 	if (operator == OID_ARRAY_CONTAINS_OP || operator == OID_ARRAY_OVERLAP_OP)
 		selec = mcelem_array_contain_overlap_selec(mcelem, nmcelem,
 												   numbers, nnumbers,
 												   elem_values, nonnull_nitems,
-												   operator, cmpfunc);
+												   operator, typentry);
 	else if (operator == OID_ARRAY_CONTAINED_OP)
 		selec = mcelem_array_contained_selec(mcelem, nmcelem,
 											 numbers, nnumbers,
 											 elem_values, nonnull_nitems,
 											 hist, nhist,
-											 operator, cmpfunc);
+											 operator, typentry);
 	else
 	{
 		elog(ERROR, "arraycontsel called for unrecognized operator %u",
@@ -523,7 +523,7 @@ static Selectivity
 mcelem_array_contain_overlap_selec(Datum *mcelem, int nmcelem,
 								   float4 *numbers, int nnumbers,
 								   Datum *array_data, int nitems,
-								   Oid operator, FmgrInfo *cmpfunc)
+								   Oid operator, TypeCacheEntry *typentry)
 {
 	Selectivity selec,
 				elem_selec;
@@ -586,14 +586,14 @@ mcelem_array_contain_overlap_selec(Datum *mcelem, int nmcelem,
 
 		/* Ignore any duplicates in the array data. */
 		if (i > 0 &&
-			element_compare(&array_data[i - 1], &array_data[i], cmpfunc) == 0)
+			element_compare(&array_data[i - 1], &array_data[i], typentry) == 0)
 			continue;
 
 		/* Find the smallest MCELEM >= this array item. */
 		if (use_bsearch)
 		{
 			match = find_next_mcelem(mcelem, nmcelem, array_data[i],
-									 &mcelem_index, cmpfunc);
+									 &mcelem_index, typentry);
 		}
 		else
 		{
@@ -601,7 +601,7 @@ mcelem_array_contain_overlap_selec(Datum *mcelem, int nmcelem,
 			{
 				int			cmp = element_compare(&mcelem[mcelem_index],
 												  &array_data[i],
-												  cmpfunc);
+												  typentry);
 
 				if (cmp < 0)
 					mcelem_index++;
@@ -699,7 +699,7 @@ mcelem_array_contained_selec(Datum *mcelem, int nmcelem,
 							 float4 *numbers, int nnumbers,
 							 Datum *array_data, int nitems,
 							 float4 *hist, int nhist,
-							 Oid operator, FmgrInfo *cmpfunc)
+							 Oid operator, TypeCacheEntry *typentry)
 {
 	int			mcelem_index,
 				i,
@@ -765,7 +765,7 @@ mcelem_array_contained_selec(Datum *mcelem, int nmcelem,
 
 		/* Ignore any duplicates in the array data. */
 		if (i > 0 &&
-			element_compare(&array_data[i - 1], &array_data[i], cmpfunc) == 0)
+			element_compare(&array_data[i - 1], &array_data[i], typentry) == 0)
 			continue;
 
 		/*
@@ -777,7 +777,7 @@ mcelem_array_contained_selec(Datum *mcelem, int nmcelem,
 		{
 			int			cmp = element_compare(&mcelem[mcelem_index],
 											  &array_data[i],
-											  cmpfunc);
+											  typentry);
 
 			if (cmp < 0)
 			{
@@ -1130,7 +1130,7 @@ floor_log2(uint32 n)
  */
 static bool
 find_next_mcelem(Datum *mcelem, int nmcelem, Datum value, int *index,
-				 FmgrInfo *cmpfunc)
+				 TypeCacheEntry *typentry)
 {
 	int			l = *index,
 				r = nmcelem - 1,
@@ -1140,7 +1140,7 @@ find_next_mcelem(Datum *mcelem, int nmcelem, Datum value, int *index,
 	while (l <= r)
 	{
 		i = (l + r) / 2;
-		res = element_compare(&mcelem[i], &value, cmpfunc);
+		res = element_compare(&mcelem[i], &value, typentry);
 		if (res == 0)
 		{
 			*index = i;
@@ -1158,7 +1158,7 @@ find_next_mcelem(Datum *mcelem, int nmcelem, Datum value, int *index,
 /*
  * Comparison function for elements.
  *
- * We use the element type's default btree opclass, and the default collation
+ * We use the element type's default btree opclass, and its default collation
  * if the type is collation-sensitive.
  *
  * XXX consider using SortSupport infrastructure
@@ -1168,10 +1168,11 @@ element_compare(const void *key1, const void *key2, void *arg)
 {
 	Datum		d1 = *((const Datum *) key1);
 	Datum		d2 = *((const Datum *) key2);
-	FmgrInfo   *cmpfunc = (FmgrInfo *) arg;
+	TypeCacheEntry *typentry = (TypeCacheEntry *) arg;
+	FmgrInfo   *cmpfunc = &typentry->cmp_proc_finfo;
 	Datum		c;
 
-	c = FunctionCall2Coll(cmpfunc, DEFAULT_COLLATION_OID, d1, d2);
+	c = FunctionCall2Coll(cmpfunc, typentry->typcollation, d1, d2);
 	return DatumGetInt32(c);
 }