Testing for MDEV-4345 (sampling probably selective predicates)

+ Controls number of record samples to check condition

+ selectivity

+  ulong check_rows=

+    min(thd->variables.optimizer_selectivity_sampling_limit,

+        (ulong) (table_records * SELECTIVITY_SAMPLING_SHARE));

+  if (cond && check_rows > SELECTIVITY_SAMPLING_THRESHOLD &&

+      thd->variables.optimizer_use_condition_selectivity > 4)

+#define SELECTIVITY_SAMPLING_SHARE 0.10

+#define SELECTIVITY_SAMPLING_LIMIT 5000

+/* check this number of rows (default value) */

+#define SELECTIVITY_SAMPLING_LIMIT 5000

+/* but no more then this part of table (10%) */

+#define SELECTIVITY_SAMPLING_SHARE 0.10

+/* do not check if we are going check less then this number of records */

+#define SELECTIVITY_SAMPLING_THRESHOLD 10

+       "5 - additionally use selectivity of certain non-range predicates "

+       "calculated on record samples",