brin-correlation.patch

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 37fad86..5506f67 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -7531,6 +7531,7 @@ brincostestimate(PG_FUNCTION_ARGS)
 	Cost		spc_random_page_cost;
 	double		qual_op_cost;
 	double		qual_arg_cost;
+	VariableStatData vardata;
 
 	/* Do preliminary analysis of indexquals */
 	qinfos = deconstruct_indexquals(path);
@@ -7544,6 +7545,8 @@ brincostestimate(PG_FUNCTION_ARGS)
 	 * BRIN indexes are always read in full; use that as startup cost.
 	 *
 	 * XXX maybe only include revmap pages here?
+	 * XXX we should consider the revmap at seqpage cost, and regular pages
+	 * at random page cost.
 	 */
 	*indexStartupCost = spc_seq_page_cost * numPages * loop_count;
 
@@ -7558,7 +7561,88 @@ brincostestimate(PG_FUNCTION_ARGS)
 		clauselist_selectivity(root, indexQuals,
 							   path->indexinfo->rel->relid,
 							   JOIN_INNER, NULL);
-	*indexCorrelation = 1;
+
+	/*
+	 * 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.
+	 */
+	*indexCorrelation = 0;
+
+	{
+		RangeTblEntry *rte = planner_rt_fetch(index->rel->relid, root);
+		Oid			relid;
+		ListCell   *cell;
+
+		Assert(rte->rtekind == RTE_RELATION);
+		relid = rte->relid;
+		Assert(relid != InvalidOid);
+
+		foreach (cell, qinfos)
+		{
+			IndexQualInfo  *qinfo = (IndexQualInfo *) lfirst(cell);
+			AttrNumber		colnum = index->indexkeys[qinfo->indexcol];
+
+			if (colnum != 0)
+			{
+				/* Simple variable -- look to stats for the underlying table */
+				if (get_relation_stats_hook &&
+					(*get_relation_stats_hook) (root, rte, colnum, &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(relid),
+														 Int16GetDatum(colnum),
+														 /* XXX no inh */
+														 BoolGetDatum(false));
+					vardata.freefunc = ReleaseSysCache;
+				}
+
+				if (HeapTupleIsValid(vardata.statsTuple))
+				{
+					float4	   *numbers;
+					int			nnumbers;
+
+					/* XXX is InvalidOID reqop fine?? */
+					if (get_attstatsslot(vardata.statsTuple, InvalidOid, 0,
+										 STATISTIC_KIND_CORRELATION,
+										 InvalidOid,
+										 NULL,
+										 NULL, NULL,
+										 &numbers, &nnumbers))
+					{
+						double	varCorrelation;
+
+						Assert(nnumbers == 1);
+						varCorrelation = numbers[0];
+
+						if (Abs(varCorrelation) > Abs(*indexCorrelation))
+							*indexCorrelation = varCorrelation;
+
+						free_attstatsslot(InvalidOid, NULL, 0, numbers, nnumbers);
+					}
+				}
+			}
+			else
+			{
+				/* Expression --- maybe there are stats for the index itself */
+				Assert(false);
+			}
+
+			ReleaseVariableStats(vardata);
+		}
+	}
 
 	/*
 	 * Add on index qual eval costs, much as in genericcostestimate.