Re: Deleting older versions in unique indexes to avoid page splits

And some more comments after another round of reading the patch.

1. Looks like UNIQUE_CHECK_NO_WITH_UNCHANGED is used for HOT updates,
   should we use UNIQUE_CHECK_NO_HOT here? It is better understood like this.

2. You're modifying the table_tuple_update() function on 1311 line of include/access/tableam.h,
   adding modified_attrs_hint. There's a large comment right before it describing parameters,
   I think there should be a note about modified_attrs_hint parameter in that comment, 'cos
   it is referenced from other places in tableam.h and also from backend/access/heap/heapam.c

3. Can you elaborate on the scoring model you're using?
   Why do we expect a score of 25, what's the rationale behind this number?
   And should it be #define-d ?

4. heap_compute_xid_horizon_for_tuples contains duplicate logic. Is it possible to avoid this?

5. In this comment

+ * heap_index_batch_check() helper function.  Sorts deltids array in the
+ * order needed for useful processing.

   perhaps it is better to replace "useful" with more details? Or point to the place
   where "useful processing" is described.

6. In this snippet in _bt_dedup_delete_pass()

+       else if (_bt_keep_natts_fast(rel, state->base, itup) > nkeyatts &&
+                _bt_dedup_save_htid(state, itup))
+       {
+
+       }

   I would rather add a comment, explaining that the empty body of the clause is actually expected.

7. In the _bt_dedup_delete_finish_pending() you're setting ispromising to false for both
   posting and non-posting tuples. This contradicts comments before function.