Optimization that transforms IN-predicate in IN-subselect made.

Trasformation goes that way:
a in (1,2) ->
a in (select * from (values ((1),(2))) as new_tvc)

Special variable that controlls optimization added.
Now optimization works only in several cases.
New tests added.

Old tests corrected. Now with TVC explain can be used. TVC also can be used in recursive CTEs.

mysql-test/t/opt_tvc.test

+create table t1 (a int, b int);
+
+insert into t1 
+values (1,2), (4,6), (9,7),
+       (1,1), (2,5), (7,8);
+
+create table t2 (a int, b int, c int);
+
+insert into t2 
+values (1,2,3), (5,1,2), (4,3,7), 
+       (8,9,0), (10,7,1), (5,5,1);
+
+--echo # optimization is not used
+
+let $query= select * from t1 where a in (1,2);
+eval $query;
+eval explain $query;
+eval explain format=json $query; 
+
+--echo # set minimum number of values in VALUEs list when optimization works to 2
+
+set @@in_subquery_conversion_threshold= 2;
+
+--echo # single IN-predicate in WHERE-part
+
+let $query= select * from t1 where a in (1,2);
+
+let $optimized_query= 
+select * from t1 
+where a in 
+      (
+        select * 
+        from (values (1),(2)) as new_tvc
+      );
+
+eval $query;
+eval $optimized_query;
+eval explain $query;
+eval explain $optimized_query;
+eval explain format=json $query; 
+
+--echo # AND-condition with IN-predicates in WHERE-part
+
+let $query= 
+select * from t1 
+where a in (1,2) and 
+      b in (1,5);
+
+let $optimized_query= 
+select * from t1 
+where a in 
+(
+  select * 
+  from (values (1),(2)) as new_tvc
+) 
+and b in
+(
+  select * 
+  from (values (1),(5)) as new_tvc
+);
+      
+eval $query;
+eval $optimized_query;
+eval explain $query;
+eval explain $optimized_query;
+eval explain format=json $query; 
+
+--echo # OR-condition with IN-predicates in WHERE-part
+
+let $query= 
+select * from t1 
+where a in (1,2) or 
+      b in (4,5);
+
+let $optimized_query= 
+select * from t1 
+where a in 
+(
+  select * 
+  from (values (1),(2)) as new_tvc
+) 
+or b in
+(
+  select * 
+  from (values (4),(5)) as new_tvc
+);
+      
+eval $query;
+eval $optimized_query;
+eval explain $query;
+eval explain $optimized_query;
+eval explain format=json $query; 
+
+--echo # subquery with IN-predicate
+
+let $query= 
+select * from t1 
+where a in 
+  (
+   select a 
+   from t2 where b in (3,4)
+  )
+;
+
+let $optimized_query= 
+select * from t1 
+where a in (
+  select a from t2 
+  where b in 
+  (
+    select * 
+    from (values (3),(4)) as new_tvc)
+  )
+;
+      
+eval $query;
+eval $optimized_query;
+eval explain $query;
+eval explain $optimized_query;
+eval explain format=json $query; 
+
+drop table t1;
+set @@in_subquery_conversion_threshold= default;
+