Fixed bug #18206.

The bug report revealed two problems related to min/max optimization:
1. If the length of a constant key used in a SARGable condition for
for the MIN/MAX fields is greater than the length of the field an 
unwanted warning on key truncation is issued;
2. If MIN/MAX optimization is applied to a partial index, like INDEX(b(4))
than can lead to returning a wrong result set.

mysql-test/r/func_group.result

@@ -916,3 +916,27 @@ select count(*), min(7), max(7) from t2m, t1i;
 count(*)	min(7)	max(7)
 0	NULL	NULL
 drop table t1m, t1i, t2m, t2i;
+CREATE TABLE t1 (id int PRIMARY KEY, b char(3), INDEX(b));
+INSERT INTO t1 VALUES (1,'xx'), (2,'aa');
+SELECT * FROM t1;
+id	b
+1	xx
+2	aa
+SELECT MAX(b) FROM t1 WHERE b < 'ppppp';
+MAX(b)
+aa
+SHOW WARNINGS;
+Level	Code	Message
+SELECT MAX(b) FROM t1 WHERE b < 'pp';
+MAX(b)
+aa
+DROP TABLE t1;
+CREATE TABLE t1 (id int PRIMARY KEY, b char(16), INDEX(b(4)));
+INSERT INTO t1 VALUES (1, 'xxxxbbbb'), (2, 'xxxxaaaa');
+SELECT MAX(b) FROM t1;
+MAX(b)
+xxxxbbbb
+EXPLAIN SELECT MAX(b) FROM t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	2	
+DROP TABLE t1;

mysql-test/t/func_group.test

@@ -598,4 +598,23 @@ select count(*), min(7), max(7) from t2m, t1i;
 
 drop table t1m, t1i, t2m, t2i;
 
+#
+# Bug #18206: min/max optimization cannot be applied to partial index
+#
+
+CREATE TABLE t1 (id int PRIMARY KEY, b char(3), INDEX(b));
+INSERT INTO t1 VALUES (1,'xx'), (2,'aa');
+SELECT * FROM t1;
+
+SELECT MAX(b) FROM t1 WHERE b < 'ppppp';
+SHOW WARNINGS;
+SELECT MAX(b) FROM t1 WHERE b < 'pp';
+DROP TABLE t1;
+
+CREATE TABLE t1 (id int PRIMARY KEY, b char(16), INDEX(b(4)));
+INSERT INTO t1 VALUES (1, 'xxxxbbbb'), (2, 'xxxxaaaa');
+SELECT MAX(b) FROM t1;
+EXPLAIN SELECT MAX(b) FROM t1;
+DROP TABLE t1;
+
 # End of 4.1 tests

sql/opt_sum.cc

@@ -543,6 +543,10 @@ static bool matching_cond(bool max_fl, TABLE_REF *ref, KEY *keyinfo,
       break;                        // Found a part od the key for the field
   }
 
+#if 0
+  if (part->length != (((Item_field*) args[0])->field)->field_length)
+    return 0;
+#endif
   bool is_field_part= part == field_part;
   if (!(is_field_part || eq_type))
     return 0;
@@ -582,7 +586,8 @@ static bool matching_cond(bool max_fl, TABLE_REF *ref, KEY *keyinfo,
     }
     else
     {
-      store_val_in_field(part->field, args[between && max_fl ? 2 : 1]);
+      store_val_in_field(part->field, args[between && max_fl ? 2 : 1],
+                         CHECK_FIELD_IGNORE);
       if (part->null_bit) 
         *key_ptr++= (byte) test(part->field->is_null());
       part->field->get_key_image((char*) key_ptr, part->length,
@@ -638,6 +643,8 @@ static bool matching_cond(bool max_fl, TABLE_REF *ref, KEY *keyinfo,
         field BETWEEN const1 AND const2
      3. all references to the columns from the same table as column field
         occur only in conjucts mentioned above.
+     4. each of k first components the index is not partial, i.e. is not
+        defined on a fixed length proper prefix of the field.
 
      If such an index exists the function through the ref parameter
      returns the key value to find max/min for the field using the index,
@@ -647,8 +654,8 @@ static bool matching_cond(bool max_fl, TABLE_REF *ref, KEY *keyinfo,
       of the whole search key)
 
   NOTE
-   This function may set table->key_read to 1, which must be reset after
-   index is used! (This can only happen when function returns 1)
+    This function may set table->key_read to 1, which must be reset after
+    index is used! (This can only happen when function returns 1)
 
   RETURN
     0   Index can not be used to optimize MIN(field)/MAX(field)
@@ -682,6 +689,10 @@ static bool find_key_for_maxmin(bool max_fl, TABLE_REF *ref,
       if (!(table->file->index_flags(idx, jdx, 0) & HA_READ_ORDER))
         return 0;
 
+        /* Check whether the index component is partial */
+      if (part->length < table->field[part->fieldnr-1]->pack_length())
+        break;
+
       if (field->eq(part->field))
       {
         ref->key= idx;

sql/sql_select.cc

@@ -3434,7 +3434,7 @@ get_store_key(THD *thd, KEYUSE *keyuse, table_map used_tables,
 */
 
 bool
-store_val_in_field(Field *field,Item *item)
+store_val_in_field(Field *field, Item *item, enum_check_fields check_flag)
 {
   bool error;
   THD *thd=current_thd;
@@ -3445,7 +3445,7 @@ store_val_in_field(Field *field,Item *item)
     with select_insert, which make count_cuted_fields= 1
    */
   enum_check_fields old_count_cuted_fields= thd->count_cuted_fields;
-  thd->count_cuted_fields= CHECK_FIELD_WARN;
+  thd->count_cuted_fields= check_flag;
   error= item->save_in_field(field, 1);
   thd->count_cuted_fields= old_count_cuted_fields;
   return error || cuted_fields != thd->cuted_fields;
@@ -7097,7 +7097,7 @@ static bool test_if_ref(Item_field *left_item,Item *right_item)
 	    field->real_type() != FIELD_TYPE_VAR_STRING &&
 	    (field->type() != FIELD_TYPE_FLOAT || field->decimals() == 0))
 	{
-	  return !store_val_in_field(field,right_item);
+	  return !store_val_in_field(field, right_item, CHECK_FIELD_WARN);
 	}
       }
     }

sql/sql_select.h

@@ -335,7 +335,7 @@ extern const char *join_type_str[];
 void TEST_join(JOIN *join);
 
 /* Extern functions in sql_select.cc */
-bool store_val_in_field(Field *field,Item *val);
+bool store_val_in_field(Field *field, Item *val, enum_check_fields check_flag);
 TABLE *create_tmp_table(THD *thd,TMP_TABLE_PARAM *param,List<Item> &fields,
 			ORDER *group, bool distinct, bool save_sum_fields,
 			ulong select_options, ha_rows rows_limit,