Don't read MCV stats needlessly in eqjoinsel().

eqjoinsel() currently makes use of MCV stats only when we have such
stats for both sides of the clause.  As coded, though, it would
fetch those stats even when they're present for just one side.
This can be a bit expensive with high statistics targets, leading
to wasted effort in common cases such as joining a unique column
to a non-unique column.  So it seems worth the trouble to do a quick
pre-check to confirm that both sides have MCVs before fetching either.

Also, tweak the API spec for get_attstatsslot() to document the
method we're using here.

David Geier, Tomas Vondra, Tom Lane

Discussion: https://postgr.es/m/b9846ca0-5f1c-9b26-5881-aad3f42b07f0@gmail.com

src/backend/utils/adt/selfuncs.c

@@ -2261,6 +2261,7 @@ eqjoinsel(PG_FUNCTION_ARGS)
 	Form_pg_statistic stats2 = NULL;
 	bool		have_mcvs1 = false;
 	bool		have_mcvs2 = false;
+	bool		get_mcv_stats;
 	bool		join_is_reversed;
 	RelOptInfo *inner_rel;
 
@@ -2275,11 +2276,25 @@ eqjoinsel(PG_FUNCTION_ARGS)
 	memset(&sslot1, 0, sizeof(sslot1));
 	memset(&sslot2, 0, sizeof(sslot2));
 
+	/*
+	 * There is no use in fetching one side's MCVs if we lack MCVs for the
+	 * other side, so do a quick check to verify that both stats exist.
+	 */
+	get_mcv_stats = (HeapTupleIsValid(vardata1.statsTuple) &&
+					 HeapTupleIsValid(vardata2.statsTuple) &&
+					 get_attstatsslot(&sslot1, vardata1.statsTuple,
+									  STATISTIC_KIND_MCV, InvalidOid,
+									  0) &&
+					 get_attstatsslot(&sslot2, vardata2.statsTuple,
+									  STATISTIC_KIND_MCV, InvalidOid,
+									  0));
+
 	if (HeapTupleIsValid(vardata1.statsTuple))
 	{
 		/* note we allow use of nullfrac regardless of security check */
 		stats1 = (Form_pg_statistic) GETSTRUCT(vardata1.statsTuple);
-		if (statistic_proc_security_check(&vardata1, opfuncoid))
+		if (get_mcv_stats &&
+			statistic_proc_security_check(&vardata1, opfuncoid))
 			have_mcvs1 = get_attstatsslot(&sslot1, vardata1.statsTuple,
 										  STATISTIC_KIND_MCV, InvalidOid,
 										  ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS);
@@ -2289,7 +2304,8 @@ eqjoinsel(PG_FUNCTION_ARGS)
 	{
 		/* note we allow use of nullfrac regardless of security check */
 		stats2 = (Form_pg_statistic) GETSTRUCT(vardata2.statsTuple);
-		if (statistic_proc_security_check(&vardata2, opfuncoid))
+		if (get_mcv_stats &&
+			statistic_proc_security_check(&vardata2, opfuncoid))
 			have_mcvs2 = get_attstatsslot(&sslot2, vardata2.statsTuple,
 										  STATISTIC_KIND_MCV, InvalidOid,
 										  ATTSTATSSLOT_VALUES | ATTSTATSSLOT_NUMBERS);

src/backend/utils/cache/lsyscache.c

@@ -3183,6 +3183,10 @@ get_attavgwidth(Oid relid, AttrNumber attnum)
  *
  * If it's desirable to call free_attstatsslot when get_attstatsslot might
  * not have been called, memset'ing sslot to zeroes will allow that.
+ *
+ * Passing flags=0 can be useful to quickly check if the requested slot type
+ * exists.  In this case no arrays are extracted, so free_attstatsslot need
+ * not be called.
  */
 bool
 get_attstatsslot(AttStatsSlot *sslot, HeapTuple statstuple,