WL#11322 SUPPORT LOOSE INDEX RANGE SCANS FOR LOW CARDINALITY.

Post push fix

sql/opt_range.cc:14196:22: runtime error: -256 is outside the range of representable values of type 'unsigned int'
    #0 0x4248a9d in cost_skip_scan(TABLE*, unsigned int, unsigned int, unsigned long long, Cost_estimate*, unsigned long long*, Item*, Opt_trace_object*) sql/opt_range.cc:14196:22
    #1 0x41c524c in get_best_skip_scan(PARAM*, SEL_TREE*, bool) sql/opt_range.cc:14086:5
    #2 0x41b7b65 in test_quick_select(THD*, Bitmap<64u>, unsigned long long, unsigned long long, bool, enum_order, QEP_shared_owner const*, Item*, Bitmap<64u>*, QUICK_SELECT_I**) sql/opt_range.cc:3352:23
    #3 0x458fc08 in get_quick_record_count(THD*, JOIN_TAB*, unsigned long long) sql/sql_optimizer.cc:5542:17
    #4 0x458a0cd in JOIN::estimate_rowcount() sql/sql_optimizer.cc:5290:25

The fix is to handle REC_PER_KEY_UNKNOWN explicitly, to avoid using
-1.0 in computations later.

Change-Id: Ie8a81bdf7323e4f66abcad0a9aca776de8acd945

Author: Tor Didriksen

sql/opt_range.cc

@@ -14186,10 +14186,11 @@ void cost_skip_scan(TABLE *table, uint key, uint distinct_key_parts,
     skip_scan_records = quick_prefix_records;
 
   /* Compute the number of keys in a group. */
-  if (index_info->has_records_per_key(distinct_key_parts))
+  if (index_info->has_records_per_key(distinct_key_parts)) {
     // Use index statistics
     keys_per_group = index_info->records_per_key(distinct_key_parts - 1);
-  else
+    if (keys_per_group == REC_PER_KEY_UNKNOWN) keys_per_group = 1.0;
+  } else
     /* If there is no statistics try to guess */
     keys_per_group = guess_rec_per_key(table, index_info, distinct_key_parts);