Fix BRIN cost estimation

The original code was overly optimistic about the cost of scanning a
BRIN index, leading to BRIN indexes being selected when they'd be a
worse choice than some other index.  This complete rewrite should be
more accurate.

Author: David Rowley, based on an earlier patch by Emre Hasegeli
Reviewed-by: Emre Hasegeli
Discussion: https://postgr.es/m/CAKJS1f9n-Wapop5Xz1dtGdpdqmzeGqQK4sV2MK-zZugfC14Xtw@mail.gmail.com

src/backend/access/brin/brin.c

@@ -1049,6 +1049,27 @@ brin_free_desc(BrinDesc *bdesc)
 	MemoryContextDelete(bdesc->bd_context);
 }
 
+/*
+ * Fetch index's statistical data into *stats
+ */
+void
+brinGetStats(Relation index, BrinStatsData *stats)
+{
+	Buffer		metabuffer;
+	Page		metapage;
+	BrinMetaPageData *metadata;
+
+	metabuffer = ReadBuffer(index, BRIN_METAPAGE_BLKNO);
+	LockBuffer(metabuffer, BUFFER_LOCK_SHARE);
+	metapage = BufferGetPage(metabuffer);
+	metadata = (BrinMetaPageData *) PageGetContents(metapage);
+
+	stats->pagesPerRange = metadata->pagesPerRange;
+	stats->revmapNumPages = metadata->lastRevmapPage - 1;
+
+	UnlockReleaseBuffer(metabuffer);
+}
+
 /*
  * Initialize a BrinBuildState appropriate to create tuples on the given index.
  */

src/backend/utils/adt/selfuncs.c

@@ -101,6 +101,7 @@
 #include <float.h>
 #include <math.h>
 
+#include "access/brin.h"
 #include "access/gin.h"
 #include "access/htup_details.h"
 #include "access/sysattr.h"
@@ -7721,56 +7722,200 @@ brincostestimate(PlannerInfo *root, IndexPath *path, double loop_count,
 {
 	IndexOptInfo *index = path->indexinfo;
 	List	   *indexQuals = path->indexquals;
-	List	   *indexOrderBys = path->indexorderbys;
 	double		numPages = index->pages;
-	double		numTuples = index->tuples;
+	RelOptInfo *baserel = index->rel;
+	RangeTblEntry *rte = planner_rt_fetch(baserel->relid, root);
 	List	   *qinfos;
 	Cost		spc_seq_page_cost;
 	Cost		spc_random_page_cost;
-	double		qual_op_cost;
 	double		qual_arg_cost;
+	double		qualSelectivity;
+	BrinStatsData statsData;
+	double		indexRanges;
+	double		minimalRanges;
+	double		estimatedRanges;
+	double		selec;
+	Relation	indexRel;
+	ListCell   *l;
+	VariableStatData vardata;
 
-	/* Do preliminary analysis of indexquals */
+	Assert(rte->rtekind == RTE_RELATION);
-	qinfos = deconstruct_indexquals(path);
 
-	/* fetch estimated page cost for tablespace containing index */
+	/* fetch estimated page cost for the tablespace containing the index */
 	get_tablespace_page_costs(index->reltablespace,
 							  &spc_random_page_cost,
 							  &spc_seq_page_cost);
 
 	/*
-	 * BRIN indexes are always read in full; use that as startup cost.
+	 * Obtain some data from the index itself.
-	 *
-	 * XXX maybe only include revmap pages here?
 	 */
-	*indexStartupCost = spc_seq_page_cost * numPages * loop_count;
+	indexRel = index_open(index->indexoid, AccessShareLock);
+	brinGetStats(indexRel, &statsData);
+	index_close(indexRel, AccessShareLock);
+
+	/*
+	 * Compute index correlation
+	 *
+	 * Because we can use all index quals equally when scanning, we can use
+	 * the largest correlation (in absolute value) among columns used by the
+	 * query.  Start at zero, the worst possible case.  If we cannot find
+	 * any correlation statistics, we will keep it as 0.
+	 */
+	*indexCorrelation = 0;
+
+	qinfos = deconstruct_indexquals(path);
+	foreach(l, qinfos)
+	{
+		IndexQualInfo *qinfo = (IndexQualInfo *) lfirst(l);
+		AttrNumber	attnum = index->indexkeys[qinfo->indexcol];
+
+		/* attempt to lookup stats in relation for this index column */
+		if (attnum != 0)
+		{
+			/* Simple variable -- look to stats for the underlying table */
+			if (get_relation_stats_hook &&
+				(*get_relation_stats_hook) (root, rte, attnum, &vardata))
+			{
+				/*
+				 * The hook took control of acquiring a stats tuple.  If it
+				 * did supply a tuple, it'd better have supplied a freefunc.
+				 */
+				if (HeapTupleIsValid(vardata.statsTuple) && !vardata.freefunc)
+					elog(ERROR,
+						 "no function provided to release variable stats with");
+			}
+			else
+			{
+				vardata.statsTuple =
+					SearchSysCache3(STATRELATTINH,
+									ObjectIdGetDatum(rte->relid),
+									Int16GetDatum(attnum),
+									BoolGetDatum(false));
+				vardata.freefunc = ReleaseSysCache;
+			}
+		}
+		else
+		{
+			/*
+			 * Looks like we've found an expression column in the index. Let's
+			 * see if there's any stats for it.
+			 */
+
+			/* get the attnum from the 0-based index. */
+			attnum = qinfo->indexcol + 1;
+
+			if (get_index_stats_hook &&
+				(*get_index_stats_hook) (root, index->indexoid, attnum, &vardata))
+			{
+				/*
+				 * The hook took control of acquiring a stats tuple.  If it did
+				 * supply a tuple, it'd better have supplied a freefunc.
+				 */
+				if (HeapTupleIsValid(vardata.statsTuple) &&
+					!vardata.freefunc)
+					elog(ERROR, "no function provided to release variable stats with");
+			}
+			else
+			{
+				vardata.statsTuple = SearchSysCache3(STATRELATTINH,
+													 ObjectIdGetDatum(index->indexoid),
+													 Int16GetDatum(attnum),
+													 BoolGetDatum(false));
+				vardata.freefunc = ReleaseSysCache;
+			}
+		}
+
+		if (HeapTupleIsValid(vardata.statsTuple))
+		{
+			float4	   *numbers;
+			int			nnumbers;
+
+			if (get_attstatsslot(vardata.statsTuple, InvalidOid, 0,
+								 STATISTIC_KIND_CORRELATION,
+								 InvalidOid,
+								 NULL,
+								 NULL, NULL,
+								 &numbers, &nnumbers))
+			{
+				double		varCorrelation = 0.0;
+
+				if (nnumbers > 0)
+					varCorrelation = Abs(numbers[0]);
+
+				if (varCorrelation > *indexCorrelation)
+					*indexCorrelation = varCorrelation;
+
+				free_attstatsslot(InvalidOid, NULL, 0, numbers, nnumbers);
+			}
+		}
+
+		ReleaseVariableStats(vardata);
+	}
+
+	qualSelectivity = clauselist_selectivity(root, indexQuals,
+											 baserel->relid,
+											 JOIN_INNER, NULL,
+											 baserel);
+
+	/* work out the actual number of ranges in the index */
+	indexRanges = Max(ceil((double) baserel->pages / statsData.pagesPerRange),
+					  1.0);
+
+	/*
+	 * Now calculate the minimum possible ranges we could match with if all of
+	 * the rows were in the perfect order in the table's heap.
+	 */
+	minimalRanges = ceil(indexRanges * qualSelectivity);
+
+	/*
+	 * Now estimate the number of ranges that we'll touch by using the
+	 * indexCorrelation from the stats. Careful not to divide by zero
+	 * (note we're using the absolute value of the correlation).
+	 */
+	if (*indexCorrelation < 1.0e-10)
+		estimatedRanges = indexRanges;
+	else
+		estimatedRanges = Min(minimalRanges / *indexCorrelation, indexRanges);
+
+	/* we expect to visit this portion of the table */
+	selec = estimatedRanges / indexRanges;
+
+	CLAMP_PROBABILITY(selec);
+
+	*indexSelectivity = selec;
+
+	/*
+	 * Compute the index qual costs, much as in genericcostestimate, to add
+	 * to the index costs.
+	 */
+	qual_arg_cost = other_operands_eval_cost(root, qinfos) +
+		orderby_operands_eval_cost(root, path);
+
+	/*
+	 * Compute the startup cost as the cost to read the whole revmap
+	 * sequentially, including the cost to execute the index quals.
+	 */
+	*indexStartupCost =
+		spc_seq_page_cost * statsData.revmapNumPages * loop_count;
+	*indexStartupCost += qual_arg_cost;
 
 	/*
 	 * To read a BRIN index there might be a bit of back and forth over
 	 * regular pages, as revmap might point to them out of sequential order;
-	 * calculate this as reading the whole index in random order.
+	 * calculate the total cost as reading the whole index in random order.
 	 */
-	*indexTotalCost = spc_random_page_cost * numPages * loop_count;
+	*indexTotalCost = *indexStartupCost +
-
+		spc_random_page_cost * (numPages - statsData.revmapNumPages) * loop_count;
-	*indexSelectivity =
-		clauselist_selectivity(root, indexQuals,
-							   path->indexinfo->rel->relid,
-							   JOIN_INNER, NULL,
-							   path->indexinfo->rel);
-	*indexCorrelation = 1;
 
 	/*
-	 * Add on index qual eval costs, much as in genericcostestimate.
+	 * Charge a small amount per range tuple which we expect to match to. This
+	 * is meant to reflect the costs of manipulating the bitmap. The BRIN scan
+	 * will set a bit for each page in the range when we find a matching
+	 * range, so we must multiply the charge by the number of pages in the
+	 * range.
 	 */
-	qual_arg_cost = other_operands_eval_cost(root, qinfos) +
+	*indexTotalCost += 0.1 * cpu_operator_cost * estimatedRanges *
-		orderby_operands_eval_cost(root, path);
+		statsData.pagesPerRange;
-	qual_op_cost = cpu_operator_cost *
-		(list_length(indexQuals) + list_length(indexOrderBys));
 
-	*indexStartupCost += qual_arg_cost;
-	*indexTotalCost += qual_arg_cost;
-	*indexTotalCost += (numTuples * *indexSelectivity) * (cpu_index_tuple_cost + qual_op_cost);
 	*indexPages = index->pages;
-
-	/* XXX what about pages_per_range? */
 }

src/include/access/brin.h

@@ -25,6 +25,17 @@ typedef struct BrinOptions
 	bool		autosummarize;
 } BrinOptions;
 
+
+/*
+ * BrinStatsData represents stats data for planner use
+ */
+typedef struct BrinStatsData
+{
+	BlockNumber	pagesPerRange;
+	BlockNumber	revmapNumPages;
+} BrinStatsData;
+
+
 #define BRIN_DEFAULT_PAGES_PER_RANGE	128
 #define BrinGetPagesPerRange(relation) \
 	((relation)->rd_options ? \
@@ -35,4 +46,7 @@ typedef struct BrinOptions
 	 ((BrinOptions *) (relation)->rd_options)->autosummarize : \
 	  false)
 
+
+extern void brinGetStats(Relation index, BrinStatsData *stats);
+
 #endif   /* BRIN_H */

src/test/regress/expected/brin.out

@@ -363,6 +363,8 @@ BEGIN
 	END LOOP;
 END;
 $x$;
+RESET enable_seqscan;
+RESET enable_bitmapscan;
 INSERT INTO brintest SELECT
 	repeat(stringu1, 42)::bytea,
 	substr(stringu1, 1, 1)::"char",
@@ -481,3 +483,27 @@ SELECT brin_summarize_range('brin_summarize_idx', -1);
 ERROR:  block number out of range: -1
 SELECT brin_summarize_range('brin_summarize_idx', 4294967296);
 ERROR:  block number out of range: 4294967296
+-- test brin cost estimates behave sanely based on correlation of values
+CREATE TABLE brin_test (a INT, b INT);
+INSERT INTO brin_test SELECT x/100,x%100 FROM generate_series(1,10000) x(x);
+CREATE INDEX brin_test_a_idx ON brin_test USING brin (a) WITH (pages_per_range = 2);
+CREATE INDEX brin_test_b_idx ON brin_test USING brin (b) WITH (pages_per_range = 2);
+VACUUM ANALYZE brin_test;
+-- Ensure brin index is used when columns are perfectly correlated
+EXPLAIN (COSTS OFF) SELECT * FROM brin_test WHERE a = 1;
+                 QUERY PLAN                 
+--------------------------------------------
+ Bitmap Heap Scan on brin_test
+   Recheck Cond: (a = 1)
+   ->  Bitmap Index Scan on brin_test_a_idx
+         Index Cond: (a = 1)
+(4 rows)
+
+-- Ensure brin index is not used when values are not correlated
+EXPLAIN (COSTS OFF) SELECT * FROM brin_test WHERE b = 1;
+      QUERY PLAN       
+-----------------------
+ Seq Scan on brin_test
+   Filter: (b = 1)
+(2 rows)
+

src/test/regress/sql/brin.sql

@@ -370,6 +370,9 @@ BEGIN
 END;
 $x$;
 
+RESET enable_seqscan;
+RESET enable_bitmapscan;
+
 INSERT INTO brintest SELECT
 	repeat(stringu1, 42)::bytea,
 	substr(stringu1, 1, 1)::"char",
@@ -444,3 +447,16 @@ SELECT brin_summarize_range('brin_summarize_idx', 4294967295);
 -- invalid block number values
 SELECT brin_summarize_range('brin_summarize_idx', -1);
 SELECT brin_summarize_range('brin_summarize_idx', 4294967296);
+
+
+-- test brin cost estimates behave sanely based on correlation of values
+CREATE TABLE brin_test (a INT, b INT);
+INSERT INTO brin_test SELECT x/100,x%100 FROM generate_series(1,10000) x(x);
+CREATE INDEX brin_test_a_idx ON brin_test USING brin (a) WITH (pages_per_range = 2);
+CREATE INDEX brin_test_b_idx ON brin_test USING brin (b) WITH (pages_per_range = 2);
+VACUUM ANALYZE brin_test;
+
+-- Ensure brin index is used when columns are perfectly correlated
+EXPLAIN (COSTS OFF) SELECT * FROM brin_test WHERE a = 1;
+-- Ensure brin index is not used when values are not correlated
+EXPLAIN (COSTS OFF) SELECT * FROM brin_test WHERE b = 1;