Fix pg_mcv_list deserialization

The memcpy() was copying type OIDs in the wrong direction, so the
deserialized MCV list always had them as 0. This is mostly harmless
except when printing the data in pg_mcv_list_items(), in which case
it reported

    ERROR:  cache lookup failed for type 0

Also added a simple regression test for pg_mcv_list_items() function,
printing a single-item MCV list.

Reported-By: Dean Rasheed
Discussion: https://postgr.es/m/CAEZATCX6T0iDTTZrqyec4Cd6b4yuL7euu4=rQRXaVBAVrUi1Cg@mail.gmail.com

src/backend/statistics/mcv.c

@@ -905,7 +905,7 @@ statext_mcv_deserialize(bytea *data)
 			 VARSIZE_ANY(data), expected_size);
 
 	/* Now copy the array of type Oids. */
-	memcpy(ptr, mcvlist->types, sizeof(Oid) * ndims);
+	memcpy(mcvlist->types, ptr, sizeof(Oid) * ndims);
 	ptr += (sizeof(Oid) * ndims);
 
 	/* ensure alignment of the pointer (after the header fields) */

src/test/regress/expected/stats_ext.out

@@ -717,6 +717,17 @@ SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a IS NULL AND
         50 |     50
 (1 row)
 
+-- test pg_mcv_list_items with a very simple (single item) MCV list
+TRUNCATE mcv_lists;
+INSERT INTO mcv_lists (a, b, c) SELECT 1, 2, 3 FROM generate_series(1,1000) s(i);
+ANALYZE mcv_lists;
+SELECT m.* FROM pg_statistic_ext,
+              pg_mcv_list_items(stxmcv) m WHERE stxname = 'mcv_lists_stats';
+ index |  values   |  nulls  | frequency | base_frequency 
+-------+-----------+---------+-----------+----------------
+     0 | {1, 2, 3} | {f,f,f} |         1 |              1
+(1 row)
+
 RESET random_page_cost;
 -- mcv with arrays
 CREATE TABLE mcv_lists_arrays (

src/test/regress/sql/stats_ext.sql

@@ -411,6 +411,14 @@ SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a IS NULL AND
 
 SELECT * FROM check_estimated_rows('SELECT * FROM mcv_lists WHERE a IS NULL AND b IS NULL AND c IS NULL');
 
+-- test pg_mcv_list_items with a very simple (single item) MCV list
+TRUNCATE mcv_lists;
+INSERT INTO mcv_lists (a, b, c) SELECT 1, 2, 3 FROM generate_series(1,1000) s(i);
+ANALYZE mcv_lists;
+
+SELECT m.* FROM pg_statistic_ext,
+              pg_mcv_list_items(stxmcv) m WHERE stxname = 'mcv_lists_stats';
+
 RESET random_page_cost;
 
 -- mcv with arrays