Allow SET STATISTICS on expression indexes

Index columns are referenced by ordinal number rather than name, e.g.
CREATE INDEX coord_idx ON measured (x, y, (z + t));
ALTER INDEX coord_idx ALTER COLUMN 3 SET STATISTICS 1000;

Incompatibility note for release notes:
\d+ for indexes now also displays Stats Target

Authors: Alexander Korotkov, with contribution by Adrien NAYRAT
Review: Adrien NAYRAT, Simon Riggs
Wordsmith: Simon Riggs

src/backend/utils/cache/syscache.c

@@ -1256,6 +1256,52 @@ SearchSysCacheExistsAttName(Oid relid, const char *attname)
 }
 
 
+/*
+ * SearchSysCacheAttNum
+ *
+ * This routine is equivalent to SearchSysCache on the ATTNUM cache,
+ * except that it will return NULL if the found attribute is marked
+ * attisdropped.  This is convenient for callers that want to act as
+ * though dropped attributes don't exist.
+ */
+HeapTuple
+SearchSysCacheAttNum(Oid relid, int16 attnum)
+{
+	HeapTuple	tuple;
+
+	tuple = SearchSysCache2(ATTNUM,
+							ObjectIdGetDatum(relid),
+							Int16GetDatum(attnum));
+	if (!HeapTupleIsValid(tuple))
+		return NULL;
+	if (((Form_pg_attribute) GETSTRUCT(tuple))->attisdropped)
+	{
+		ReleaseSysCache(tuple);
+		return NULL;
+	}
+	return tuple;
+}
+
+/*
+ * SearchSysCacheCopyAttNum
+ *
+ * As above, an attisdropped-aware version of SearchSysCacheCopy.
+ */
+HeapTuple
+SearchSysCacheCopyAttNum(Oid relid, int16 attnum)
+{
+	HeapTuple	tuple,
+				newtuple;
+
+	tuple = SearchSysCacheAttNum(relid, attnum);
+	if (!HeapTupleIsValid(tuple))
+		return NULL;
+	newtuple = heap_copytuple(tuple);
+	ReleaseSysCache(tuple);
+	return newtuple;
+}
+
+
 /*
  * SysCacheGetAttr
  *