Generated columns

This is an SQL-standard feature that allows creating columns that are
computed from expressions rather than assigned, similar to a view or
materialized view but on a column basis.

This implements one kind of generated column: stored (computed on
write).  Another kind, virtual (computed on read), is planned for the
future, and some room is left for it.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/b151f851-4019-bdb1-699e-ebab07d2f40a@2ndquadrant.com

src/backend/utils/cache/lsyscache.c

@@ -821,6 +821,39 @@ get_attnum(Oid relid, const char *attname)
 		return InvalidAttrNumber;
 }
 
+/*
+ * get_attgenerated
+ *
+ *		Given the relation id and the attribute name,
+ *		return the "attgenerated" field from the attribute relation.
+ *
+ *		Errors if not found.
+ *
+ *		Since not generated is represented by '\0', this can also be used as a
+ *		Boolean test.
+ */
+char
+get_attgenerated(Oid relid, AttrNumber attnum)
+{
+	HeapTuple	tp;
+
+	tp = SearchSysCache2(ATTNUM,
+						 ObjectIdGetDatum(relid),
+						 Int16GetDatum(attnum));
+	if (HeapTupleIsValid(tp))
+	{
+		Form_pg_attribute att_tup = (Form_pg_attribute) GETSTRUCT(tp);
+		char		result;
+
+		result = att_tup->attgenerated;
+		ReleaseSysCache(tp);
+		return result;
+	}
+	else
+		elog(ERROR, "cache lookup failed for attribute %d of relation %u",
+			 attnum, relid);
+}
+
 /*
  * get_atttype
  *