MDEV-6480: Remove conditions for which range optimizer returned SEL_ARG::IMPOSSIBLE

Let range optimizer remove parts of OR-clauses for which range analysis
produced SEL_TREE(IMPOSSIBLE).
There is no need to remove parts of AND-clauses: either they are inside
of OR (and the whole AND-clause will be removed), or the AND-clause is
at the top level, in which case the whole WHERE (or ON) is always FALSE
and this is a degenerate case which receives special treatment.

The removal process takes care not to produce 1-way ORs (in that case
we substitute the OR for its remaining member).

1 files changed, 1 insertions, 1 deletions

diff --git a/mysql-test/r/index_merge_myisam.result b/mysql-test/r/index_merge_myisam.result
index c907997573a..9820170aa59 100644
--- a/mysql-test/r/index_merge_myisam.result
+++ b/mysql-test/r/index_merge_myisam.result
@@ -74,7 +74,7 @@ id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t0	index_merge	i1,i2	i1,i2	4,4	NULL	17	Using sort_union(i1,i2); Using where
 explain select * from t0 where key2 = 45 or key1 <=> null;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
-1	SIMPLE	t0	range	i1,i2	i2	4	NULL	1	Using where
+1	SIMPLE	t0	range	i1,i2	i2	4	NULL	1	Using index condition
 explain select * from t0 where key2 = 45 or key1 is not null;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	SIMPLE	t0	ALL	i1,i2	NULL	NULL	NULL	1024	Using where