Subject: [PATCH 6/8] Ignore correlation for new BRIN opclasses

From: Tomas Vondra <[email-masked]>
Date: Sat, 12 Sep 2020 15:07:59 +0200

The new BRIN opclasses (bloom and minmax-multi) are less sensitive to
poorly correlated data, so just assume the data is perfectly correlated
during costing.

Author: Tomas Vondra <[email-masked]>
Discussion: https://postgr.es/m/[email-masked]
---
 src/backend/access/brin/brin_bloom.c        |  1 +
 src/backend/access/brin/brin_minmax_multi.c |  1 +
 src/backend/utils/adt/selfuncs.c            | 19 ++++++++++++++++++-
 src/include/access/brin_internal.h          |  3 +++
 4 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/src/backend/access/brin/brin_bloom.c b/src/backend/access/brin/brin_bloom.c
index 000c2387ee..0137095660 100644
--- a/src/backend/access/brin/brin_bloom.c
+++ b/src/backend/access/brin/brin_bloom.c
@@ -385,6 +385,7 @@ brin_bloom_opcinfo(PG_FUNCTION_ARGS)
 
 	result = palloc0(MAXALIGN(SizeofBrinOpcInfo(1)) +
 					 sizeof(BloomOpaque));
+	result->oi_ignore_correlation = true;
 	result->oi_nstored = 1;
 	result->oi_regular_nulls = true;
 	result->oi_opaque = (BloomOpaque *)
diff --git a/src/backend/access/brin/brin_minmax_multi.c b/src/backend/access/brin/brin_minmax_multi.c
index bd17da8cd5..6ef5c5f2bf 100644
--- a/src/backend/access/brin/brin_minmax_multi.c
+++ b/src/backend/access/brin/brin_minmax_multi.c
@@ -1309,6 +1309,7 @@ brin_minmax_multi_opcinfo(PG_FUNCTION_ARGS)
 
 	result = palloc0(MAXALIGN(SizeofBrinOpcInfo(1)) +
 					 sizeof(MinmaxMultiOpaque));
+	result->oi_ignore_correlation = true;
 	result->oi_nstored = 1;
 	result->oi_regular_nulls = true;
 	result->oi_opaque = (MinmaxMultiOpaque *)
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index d5e61664bc..d0cc145938 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -98,6 +98,7 @@
 #include <math.h>
 
 #include "access/brin.h"
+#include "access/brin_internal.h"
 #include "access/brin_page.h"
 #include "access/gin.h"
 #include "access/table.h"
@@ -7352,7 +7353,8 @@ brincostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 	double		minimalRanges;
 	double		estimatedRanges;
 	double		selec;
-	Relation	indexRel;
+	Relation	indexRel = NULL;
+	TupleDesc	tupdesc = NULL;
 	ListCell   *l;
 	VariableStatData vardata;
 
@@ -7374,6 +7376,7 @@ brincostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 		 */
 		indexRel = index_open(index->indexoid, NoLock);
 		brinGetStats(indexRel, &statsData);
+		tupdesc = RelationGetDescr(indexRel);
 		index_close(indexRel, NoLock);
 
 		/* work out the actual number of ranges in the index */
@@ -7407,6 +7410,17 @@ brincostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 	{
 		IndexClause *iclause = lfirst_node(IndexClause, l);
 		AttrNumber	attnum = index->indexkeys[iclause->indexcol];
+		FmgrInfo   *opcInfoFn;
+		BrinOpcInfo *opcInfo;
+		Form_pg_attribute attr = TupleDescAttr(tupdesc, iclause->indexcol);
+		bool		ignore_correlation;
+
+		opcInfoFn = index_getprocinfo(indexRel, iclause->indexcol + 1, BRIN_PROCNUM_OPCINFO);
+
+		opcInfo = (BrinOpcInfo *)
+			DatumGetPointer(FunctionCall1(opcInfoFn, attr->atttypid));
+
+		ignore_correlation = opcInfo->oi_ignore_correlation;
 
 		/* attempt to lookup stats in relation for this index column */
 		if (attnum != 0)
@@ -7477,6 +7491,9 @@ brincostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 				if (sslot.nnumbers > 0)
 					varCorrelation = Abs(sslot.numbers[0]);
 
+				if (ignore_correlation)
+					varCorrelation = 1.0;
+
 				if (varCorrelation > *indexCorrelation)
 					*indexCorrelation = varCorrelation;
 
diff --git a/src/include/access/brin_internal.h b/src/include/access/brin_internal.h
index fdaff42722..5fbf8cf9c7 100644
--- a/src/include/access/brin_internal.h
+++ b/src/include/access/brin_internal.h
@@ -30,6 +30,9 @@ typedef struct BrinOpcInfo
 	/* Regular processing of NULLs in BrinValues? */
 	bool		oi_regular_nulls;
 
+	/* Ignore correlation during cost estimation */
+	bool		oi_ignore_correlation;
+
 	/* Opaque pointer for the opclass' private use */
 	void	   *oi_opaque;
 
-- 
2.26.2

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