MDEV-5257: MIN/MAX Optimization (Select tables optimized away) does not work for DateTime

- MIN/MAX optimizer does a check whether a "field CMP const" comparison uses a constant 
  that's longer than the field it is compared to.  Make this check only for string columns,
  also compare character lengths, not byte lengths.

mysql-test/r/func_group.result

@@ -2186,3 +2186,47 @@ AVG(DISTINCT outr.col_int_nokey)
 7.5000
 DROP TABLE t1;
 # End of the bug#57932
+#
+# MDEV-5257: MIN/MAX Optimization (Select tables optimized away) does not work for DateTime
+# MDEV-3855: MIN/MAX optimization doesnt work for int_col > INET_ATON
+#  (correct the fix for Bug #884175)
+#
+CREATE TABLE `t1` (
+`a` int(11) NOT NULL AUTO_INCREMENT,
+`b` datetime DEFAULT NULL,
+PRIMARY KEY (`a`),
+KEY `idx_b` (`b`)
+);
+INSERT INTO `t1` (b) VALUES ('2013-01-06 23:59:59');
+INSERT INTO `t1` (b) VALUES ('2013-02-06 23:59:59');
+INSERT INTO `t1` (b) VALUES ('2013-03-06 23:59:59');
+INSERT INTO `t1` (b) VALUES ('2013-04-06 23:59:59');
+INSERT INTO `t1` (b) VALUES ('2013-05-06 23:59:59');
+INSERT INTO `t1` (b) VALUES ('2013-06-06 23:59:59');
+INSERT INTO `t1` (b) VALUES ('2013-07-06 23:59:59');
+# The following should produce "Select tables optimized away"
+EXPLAIN SELECT MIN(b) FROM t1 WHERE b <= '2013-11-06 23:59:59';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+set names utf8;
+# Should be the same as above:
+EXPLAIN SELECT MIN(b) FROM t1 WHERE b <= '2013-11-06 23:59:59';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+DROP TABLE t1;
+CREATE TABLE `t1` (
+`a` int(11) NOT NULL AUTO_INCREMENT,
+`b` bigint(20) DEFAULT NULL,
+PRIMARY KEY (`a`),
+KEY `idx_b` (`b`)
+);
+insert into t1 (b) values (INET_ATON('192.168.0.1'));
+insert into t1 (b) values (INET_ATON('192.168.0.2'));
+insert into t1 (b) values (INET_ATON('192.168.0.3'));
+insert into t1 (b) values (INET_ATON('192.168.0.4'));
+insert into t1 (b) values (INET_ATON('192.168.200.200'));
+# should show "Select tables optimized away"
+explain select MIN(b) from t1 where b >= inet_aton('192.168.119.32');
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Select tables optimized away
+DROP TABLE t1;

mysql-test/t/func_group.test

@@ -1465,3 +1465,56 @@ SELECT AVG(DISTINCT outr.col_int_nokey) FROM t1 AS outr LEFT JOIN t1 AS outr2 ON
 outr.col_int_nokey = outr2.col_int_nokey;
 DROP TABLE t1;
 --echo # End of the bug#57932
+
+
+--echo #
+--echo # MDEV-5257: MIN/MAX Optimization (Select tables optimized away) does not work for DateTime
+--echo # MDEV-3855: MIN/MAX optimization doesnt work for int_col > INET_ATON
+--echo #  (correct the fix for Bug #884175)
+--echo #
+
+CREATE TABLE `t1` (
+  `a` int(11) NOT NULL AUTO_INCREMENT,
+  `b` datetime DEFAULT NULL,
+  PRIMARY KEY (`a`),
+  KEY `idx_b` (`b`)
+);
+
+INSERT INTO `t1` (b) VALUES ('2013-01-06 23:59:59');
+INSERT INTO `t1` (b) VALUES ('2013-02-06 23:59:59');
+INSERT INTO `t1` (b) VALUES ('2013-03-06 23:59:59');
+INSERT INTO `t1` (b) VALUES ('2013-04-06 23:59:59');
+INSERT INTO `t1` (b) VALUES ('2013-05-06 23:59:59');
+INSERT INTO `t1` (b) VALUES ('2013-06-06 23:59:59');
+INSERT INTO `t1` (b) VALUES ('2013-07-06 23:59:59');
+
+--echo # The following should produce "Select tables optimized away"
+EXPLAIN SELECT MIN(b) FROM t1 WHERE b <= '2013-11-06 23:59:59';
+
+-- connect (con1,localhost,root,,)
+-- connection con1
+set names utf8;
+-- echo # Should be the same as above:
+EXPLAIN SELECT MIN(b) FROM t1 WHERE b <= '2013-11-06 23:59:59';
+
+--connection default
+--disconnect con1
+
+DROP TABLE t1;
+
+CREATE TABLE `t1` (
+  `a` int(11) NOT NULL AUTO_INCREMENT,
+  `b` bigint(20) DEFAULT NULL,
+  PRIMARY KEY (`a`),
+  KEY `idx_b` (`b`)
+);
+
+insert into t1 (b) values (INET_ATON('192.168.0.1'));
+insert into t1 (b) values (INET_ATON('192.168.0.2'));
+insert into t1 (b) values (INET_ATON('192.168.0.3'));
+insert into t1 (b) values (INET_ATON('192.168.0.4'));
+insert into t1 (b) values (INET_ATON('192.168.200.200'));
+--echo # should show "Select tables optimized away"
+explain select MIN(b) from t1 where b >= inet_aton('192.168.119.32');
+DROP TABLE t1;
+

sql/opt_sum.cc

@@ -479,6 +479,24 @@ int opt_sum_query(THD *thd,
 }
 
 
+/*
+  Check if both item1 and item2 are strings, and item1 has fewer characters 
+  than item2.
+*/
+
+static bool check_item1_shorter_item2(Item *item1, Item *item2)
+{
+  if (item1->cmp_type() == STRING_RESULT &&
+      item2->cmp_type() == STRING_RESULT)
+  {
+    int len1= item1->max_length / item1->collation.collation->mbmaxlen;
+    int len2= item2->max_length / item2->collation.collation->mbmaxlen;
+    return len1 < len2;
+  }
+  return false;  /* When the check is not applicable, it means "not bigger" */
+}
+
+
 /**
   Test if the predicate compares a field with constants.
 
@@ -509,7 +527,7 @@ bool simple_pred(Item_func *func_item, Item **args, bool *inv_order)
       if (!(item= it++))
         return 0;
       args[0]= item->real_item();
-      if (args[0]->max_length < args[1]->max_length)
+      if (check_item1_shorter_item2(args[0], args[1]))
         return 0;
       if (it++)
         return 0;
@@ -544,7 +562,7 @@ bool simple_pred(Item_func *func_item, Item **args, bool *inv_order)
     }
     else
       return 0;
-    if (args[0]->max_length < args[1]->max_length)
+    if (check_item1_shorter_item2(args[0], args[1]))
       return 0;
     break;
   case 3:
@@ -559,7 +577,7 @@ bool simple_pred(Item_func *func_item, Item **args, bool *inv_order)
         if (!item->const_item())
           return 0;
         args[i]= item;
-        if (args[0]->max_length < args[i]->max_length)
+        if (check_item1_shorter_item2(args[0], args[1]))
           return 0;
       }
     }