[BACKPORT 2.14][#12327] YSQL: Workaround for FK constraint violation …

…in case of dynamic table split

Summary:
Original commit: D17142 / 17f7b80

In context of the change D16751 / 4e11189 redundant sorting of ybctids (before calling of the `PgDocOp::PopulateDmlByYbctidOps` method) was removed. But this change highlighted the problem that in case ybctids is not sorted read result might be incomplete in case of dynamic table split. The issue #12648 is created to solve the problem and create unit tests.
Current change restores code with ybctids sorting as a workaround to unblock TPC-C benchmark.

Test Plan:
Jenkins

Unit test will be added withing the context of fix for #12648

Reviewers: dmitry, hbhanawat, rthallam

Reviewed By: rthallam

Subscribers: yql, smishra

Differential Revision: https://phabricator.dev.yugabyte.com/D17251

src/yb/yql/pggate/pggate.cc

@@ -251,7 +251,13 @@ Status FetchExistingYbctids(PgSession::ScopedRefPtr session,
                             uint64_t read_time) {
   // Group the items by the table ID.
   std::sort(ybctids->begin(), ybctids->end(), [](const auto& a, const auto& b) {
+    // TODO(dmitry): By design it is only necessary to group ybctids by table, sorting of ybctids
+    // itself is not required. But due to problem described in #12648 unsorted ybctids may produce
+    // incomplete result. Remove ybctid comparision once #12648 is fixed.
+    if (a.table_id != b.table_id) {
       return a.table_id < b.table_id;
+    }
+    return a.ybctid < b.ybctid;
   });
 
   auto arena = std::make_shared<Arena>();