Minor improvements for the multivariate MCV lists

The MCV build should always call get_mincount_for_mcv_list(), as the
there is no other logic to decide whether the MCV list represents all
the data. So just remove the (ngroups > nitems) condition.

Also, when building MCV lists, the number of items was limited by the
statistics target (i.e. up to 10000). But when deserializing the MCV
list, a different value (8192) was used to check the input, causing
an error.  Simply ensure that the same value is used in both places.

This should have been included in 7300a699, but I forgot to include it
in that commit.

src/backend/statistics/mcv.c

@@ -155,15 +155,17 @@ statext_mcv_build(int numrows, HeapTuple *rows, Bitmapset *attrs,
 				numattrs,
 				ngroups,
 				nitems;
-
-	AttrNumber   *attnums = build_attnums_array(attrs, &numattrs);
-
+	AttrNumber *attnums;
+	double		mincount;
 	SortItem   *items;
 	SortItem   *groups;
 	MCVList    *mcvlist = NULL;
+	MultiSortSupport mss;
+
+	attnums = build_attnums_array(attrs, &numattrs);
 
 	/* comparator for all the columns */
-	MultiSortSupport mss = build_mss(stats, numattrs);
+	mss = build_mss(stats, numattrs);
 
 	/* sort the rows */
 	items = build_sorted_items(numrows, &nitems, rows, stats[0]->tupDesc,
@@ -196,19 +198,15 @@ statext_mcv_build(int numrows, HeapTuple *rows, Bitmapset *attrs,
 	 * per-column frequencies, as if the columns were independent).
 	 *
 	 * Using the same algorithm might exclude items that are close to the
-	 * "average" frequency. But it does not say whether the frequency is
-	 * close to base frequency or not. We also need to consider unexpectedly
-	 * uncommon items (compared to base frequency), and the single-column
-	 * algorithm ignores that entirely.
+	 * "average" frequency of the sample. But that does not say whether the
+	 * observed frequency is close to the base frequency or not. We also
+	 * need to consider unexpectedly uncommon items (again, compared to the
+	 * base frequency), and the single-column algorithm does not have to.
 	 *
-	 * If we can fit all the items onto the MCV list, do that. Otherwise
-	 * use get_mincount_for_mcv_list to decide which items to keep in the
-	 * MCV list, based on the number of occurrences in the sample.
+	 * We simply decide how many items to keep by computing minimum count
+	 * using get_mincount_for_mcv_list() and then keep all items that seem
+	 * to be more common than that.
 	 */
-	if (ngroups > nitems)
-	{
-		double mincount;
-
 	mincount = get_mincount_for_mcv_list(numrows, totalrows);
 
 	/*
@@ -224,7 +222,6 @@ statext_mcv_build(int numrows, HeapTuple *rows, Bitmapset *attrs,
 			break;
 		}
 	}
-	}
 
 	/*
 	 * At this point we know the number of items for the MCV list. There might
@@ -469,11 +466,12 @@ statext_mcv_load(Oid mvoid)
  * Each attribute has to be processed separately, as we may be mixing different
  * datatypes, with different sort operators, etc.
  *
- * We use uint16 values for the indexes in step (3), as we currently don't allow
- * more than 8k MCV items anyway, although that's mostly arbitrary limit. We might
- * increase this to 65k and still fit into uint16. Furthermore, this limit is on
- * the number of distinct values per column, and we usually have few of those
- * (and various combinations of them for the those MCV list). So uint16 seems fine.
+ * We use uint16 values for the indexes in step (3), as the number of MCV items
+ * is limited by the statistics target (which is capped to 10k at the moment).
+ * We might increase this to 65k and still fit into uint16, so there's a bit of
+ * slack. Furthermore, this limit is on the number of distinct values per column,
+ * and we usually have few of those (and various combinations of them for the
+ * those MCV list). So uint16 seems fine for now.
  *
  * We don't really expect the serialization to save as much space as for
  * histograms, as we are not doing any bucket splits (which is the source
@@ -1322,7 +1320,7 @@ pg_mcv_list_send(PG_FUNCTION_ARGS)
  * somewhat wasteful as we could do with just a single bit, thus reducing
  * the size to ~1/8. It would also allow us to combine bitmaps simply using
  * & and |, which should be faster than min/max. The bitmaps are fairly
- * small, though (as we cap the MCV list size to 8k items).
+ * small, though (thanks to the cap on the MCV list size).
  */
 static bool *
 mcv_get_match_bitmap(PlannerInfo *root, List *clauses,

src/include/statistics/statistics.h

@@ -82,8 +82,8 @@ typedef struct MVDependencies
 #define STATS_MCV_MAGIC			0xE1A651C2	/* marks serialized bytea */
 #define STATS_MCV_TYPE_BASIC	1	/* basic MCV list type */
 
-/* max items in MCV list (mostly arbitrary number) */
-#define STATS_MCVLIST_MAX_ITEMS        8192
+/* max items in MCV list (should be equal to max default_statistics_target) */
+#define STATS_MCVLIST_MAX_ITEMS        10000
 
 /*
  * Multivariate MCV (most-common value) lists