MDEV-6888: Query spends a long time in best_extension_by_limited_search with mrr enabled

- TABLE::create_key_part_by_field() should not set PART_KEY_FLAG in field->flags
  = The reason is that it is used by hash join code which calls it to create a hash
    table lookup structure. It doesn't create a real index.
  = Another caller of the function is TABLE::add_tmp_key(). Made it to set the flag itself.

- The differences in join_cache.result could also be observed before this patch: one
  could put "FLUSH TABLES" before the queries and get exactly the same difference.

1 files changed, 24 insertions, 0 deletions

diff --git a/mysql-test/t/join_outer.test b/mysql-test/t/join_outer.test
index 006df9f0359..184142361a2 100644
--- a/mysql-test/t/join_outer.test
+++ b/mysql-test/t/join_outer.test
@@ -1463,4 +1463,28 @@ SELECT * FROM t1 LEFT JOIN t2 LEFT JOIN t3 ON i2 = i3 ON i1 = i3
 
 DROP TABLE t1,t2,t3;
 
+--echo #
+--echo # Bug mdev-6705: wrong on expression after constant row substitution 
+--echo #                that triggers a simplification of WHERE condition
+--echo #
+
+CREATE TABLE t1 (a int, b int) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (10,8);
+
+CREATE TABLE t2 (c int) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (8),(9);
+
+CREATE TABLE t3 (d int) ENGINE=MyISAM;
+INSERT INTO t3 VALUES (3),(8);
+
+EXPLAIN EXTENDED
+SELECT * FROM t1 INNER JOIN t2 ON c = b LEFT JOIN t3 ON d = a
+  WHERE b IN (1,2,3) OR b = d;
+
+SELECT * FROM t1 INNER JOIN t2 ON c = b LEFT JOIN t3 ON d = a
+  WHERE b IN (1,2,3) OR b = d;
+
+DROP TABLE t1,t2,t3;
+
+
 SET optimizer_switch=@save_optimizer_switch;