change test_if_equality_guarantees_uniqueness()

from an ad hoc set of limitations
to a correct rule

mysql-test/r/distinct.result

@@ -804,6 +804,32 @@ select distinct a from t1 where a = DATE('2010-10-10');
 a
 2010-10-10
 20101010
+explain select distinct a from t1 where a = DATE('2010-10-10');
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	Using where; Using temporary
+drop table t1;
+# date = string
+create table t1 (a date);
+insert t1 values ('2010-10-10'), ('20101010');
+explain select distinct a from t1 where a = '2010-10-10';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+drop table t1;
+# double = string
+create table t1 (a double);
+insert t1 values (2), (2);
+explain select distinct a from t1 where a = '2';
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+# double = int
+explain select distinct a from t1 where a = 2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+# string = double
+alter table t1 modify a varchar(100);
+explain select distinct a from t1 where a = 2e0;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	Using where; Using temporary
 drop table t1;
 create table t1 (f1 varchar(40));
 insert into t1 values ('2010-10-10 00:00:00.0001'),('2010-10-10 00:00:00.0002'),('2010-10-10 00:00:00.0003');

mysql-test/t/distinct.test

@@ -622,6 +622,25 @@ create table t1 (a varchar(100));
 insert t1 values ('2010-10-10'), ('20101010');
 select * from t1 where a = DATE('2010-10-10');
 select distinct a from t1 where a = DATE('2010-10-10');
+explain select distinct a from t1 where a = DATE('2010-10-10');
+drop table t1;
+#
+# test_if_equality_guarantees_uniqueness() and different type combinations
+#
+--echo # date = string
+create table t1 (a date);
+insert t1 values ('2010-10-10'), ('20101010');
+explain select distinct a from t1 where a = '2010-10-10';
+drop table t1;
+--echo # double = string
+create table t1 (a double);
+insert t1 values (2), (2);
+explain select distinct a from t1 where a = '2';
+--echo # double = int
+explain select distinct a from t1 where a = 2;
+--echo # string = double
+alter table t1 modify a varchar(100);
+explain select distinct a from t1 where a = 2e0;
 drop table t1;
 
 #

sql/sql_select.cc

@@ -12171,15 +12171,15 @@ remove_eq_conds(THD *thd, COND *cond, Item::cond_result *cond_value)
   Checks if an equality predicate can be used to take away 
   DISTINCT/GROUP BY because it is known to be true for exactly one 
   distinct value (e.g. <expr> == <const>).
-  Arguments must be of the same type because e.g. 
-  <string_field> = <int_const> may match more than 1 distinct value from 
-  the column.
-  Additionally, strings must have the same collation.
-  Or the *field* must be a datetime - if the constant is a datetime
-  and a field is not - this is not enough, consider:
-    create table t1 (a varchar(100));
-    insert t1 values ('2010-01-02'), ('2010-1-2'), ('20100102');
-    select distinct t1 from t1 where a=date('2010-01-02');
+  Arguments must be compared in the native type of the left argument
+  and (for strings) in the native collation of the left argument.
+  Otherwise, for example,
+  <string_field> = <int_const> may match more than 1 distinct value or
+  the <string_field>.
+
+  @note We don't need to aggregate l and r collations here, because r -
+  the constant item - has already been converted to a proper collation
+  for comparison. We only need to compare this collation with field's collation.
 
   @retval true    can be used
   @retval false   cannot be used
@@ -12188,13 +12188,9 @@ static bool
 test_if_equality_guarantees_uniqueness(Item *l, Item *r)
 {
   return r->const_item() &&
-    /* the field is a date (the const will be converted to a date) */
-     (l->cmp_type() == TIME_RESULT ||
-      /* or arguments are of the same result type */
-      (r->result_type() == l->result_type() &&
-       /* and must have the same collation if compared as strings */
-       (l->result_type() != STRING_RESULT ||
-        l->collation.collation == r->collation.collation)));
+    item_cmp_type(l->cmp_type(), r->cmp_type()) == l->cmp_type() &&
+    (l->cmp_type() != STRING_RESULT ||
+     l->collation.collation == r->collation.collation);
 }
 
 /**