MDEV-20501: Assertion `maybe_null || !null_value' failed in Item_func_round::date_opbb-10.4-mdev20501-v2

When the optimizer finds a constant (or system) table $TBL which is empty
or has no matching row, it would set table->null_row=true. This is done
even for tables with table->maybe_null==0.

Then, it would proceed to perform query optimization phases (what for?)
which will attempt to evaluate Item expressions referring to $TBL.

Eventually some Item expression will get the value of $TBL.not_null_field,
get SQL NULL and fail an assertion.

Fixed by not performing any query optimization steps after we've got
constant/empty tables with no matching rows.

Test result changes contain a lot of changes like
- ... Impossible WHERE noticed after reading const tables
+ ... no matching row in const table

as well as other changes caused by slightly-different processing of
the special case of empty constant tables.

1 files changed, 20 insertions, 0 deletions

diff --git a/mysql-test/main/explain.test b/mysql-test/main/explain.test
index 0c31f94f8f9..245b10e183c 100644
--- a/mysql-test/main/explain.test
+++ b/mysql-test/main/explain.test
@@ -370,3 +370,23 @@ drop table t1;
 
 explain
 VALUES ( (VALUES (2))) UNION VALUES ( (SELECT 3));
+
+--echo #
+--echo # MDEV-20501: Assertion `maybe_null || !null_value' failed in Item_func_round::date_op
+--echo #
+
+CREATE TABLE t1 (t TIMESTAMP NOT NULL) ENGINE=MyISAM;
+SELECT * FROM t1 WHERE TRUNCATE( t, 1 );
+DROP TABLE t1;
+
+--echo #
+--echo # MDEV-26536: Assertion `maybe_null() || !null_value' failed in Item_func_ceiling::time_op
+--echo #
+CREATE TABLE t1 (a INT,b TIME NOT NULL DEFAULT 1) ENGINE=MyISAM;
+SELECT b FROM t1 GROUP BY b HAVING CEILING (b)>0;
+drop table t1;
+
+create table t1 (b time not null ) engine=myisam;
+select floor(b) as f from t1 group by b having f=1;
+drop table t1;
+