Add support for multivariate MCV lists

Introduce a third extended statistic type, supported by the CREATE
STATISTICS command - MCV lists, a generalization of the statistic
already built and used for individual columns.

Compared to the already supported types (n-distinct coefficients and
functional dependencies), MCV lists are more complex, include column
values and allow estimation of much wider range of common clauses
(equality and inequality conditions, IS NULL, IS NOT NULL etc.).
Similarly to the other types, a new pseudo-type (pg_mcv_list) is used.

Author: Tomas Vondra
Reviewed-by: Dean Rasheed, David Rowley, Mark Dilger, Alvaro Herrera
Discussion: https://postgr.es/m/dfdac334-9cf2-2597-fb27-f0fb3753f435@2ndquadrant.com

1 files changed, 21 insertions, 3 deletions

diff --git a/src/backend/utils/adt/ruleutils.c b/src/backend/utils/adt/ruleutils.c
index 85055bbb95a..88dc09cae68 100644
--- a/src/backend/utils/adt/ruleutils.c
+++ b/src/backend/utils/adt/ruleutils.c
@@ -1509,6 +1509,7 @@ pg_get_statisticsobj_worker(Oid statextid, bool missing_ok)
 	bool		isnull;
 	bool		ndistinct_enabled;
 	bool		dependencies_enabled;
+	bool		mcv_enabled;
 	int			i;
 
 	statexttup = SearchSysCache1(STATEXTOID, ObjectIdGetDatum(statextid));
@@ -1544,6 +1545,7 @@ pg_get_statisticsobj_worker(Oid statextid, bool missing_ok)
 
 	ndistinct_enabled = false;
 	dependencies_enabled = false;
+	mcv_enabled = false;
 
 	for (i = 0; i < ARR_DIMS(arr)[0]; i++)
 	{
@@ -1551,6 +1553,8 @@ pg_get_statisticsobj_worker(Oid statextid, bool missing_ok)
 			ndistinct_enabled = true;
 		if (enabled[i] == STATS_EXT_DEPENDENCIES)
 			dependencies_enabled = true;
+		if (enabled[i] == STATS_EXT_MCV)
+			mcv_enabled = true;
 	}
 
 	/*
@@ -1560,13 +1564,27 @@ pg_get_statisticsobj_worker(Oid statextid, bool missing_ok)
 	 * statistics types on a newer postgres version, if the statistics had all
 	 * options enabled on the original version.
 	 */
-	if (!ndistinct_enabled || !dependencies_enabled)
+	if (!ndistinct_enabled || !dependencies_enabled || !mcv_enabled)
 	{
+		bool	gotone = false;
+
 		appendStringInfoString(&buf, " (");
+
 		if (ndistinct_enabled)
+		{
 			appendStringInfoString(&buf, "ndistinct");
-		else if (dependencies_enabled)
-			appendStringInfoString(&buf, "dependencies");
+			gotone = true;
+		}
+
+		if (dependencies_enabled)
+		{
+			appendStringInfo(&buf, "%sdependencies", gotone ? ", " : "");
+			gotone = true;
+		}
+
+		if (mcv_enabled)
+			appendStringInfo(&buf, "%smcv", gotone ? ", " : "");
+
 		appendStringInfoChar(&buf, ')');
 	}