Bug#59211: Select Returns Different Value for min(year) Function

get_year_value() contains code to convert 2-digits year to
4-digits.  The fix for Bug#49910 added a check on the size of
the underlying field so that this conversion is not done for
YEAR(4) values. (Since otherwise one would convert invalid
YEAR(4) values to valid ones.)

The existing check does not work when Item_cache is used, since
it is not detected when the cache is based on a Field.  The
reported change in behavior is due to Bug#58030 which added
extra cached items in min/max computations.

The elegant solution would be to implement
Item_cache::real_item() to return the underlying Item.
However, some side effects are observed (change in explain
output) that indicates that such a change is not straight-
forward, and definitely not appropriate for an MRU.

Instead, a Item_cache::field() method has been added in order
to get access to the underlying field.  (This field() method
eliminates the need for Item_cache::eq_def() used in
test_if_ref(), but in order to limit the scope of this fix,
that code has been left as is.)


mysql-test/r/type_year.result:
  Added test case for Bug#59211.
mysql-test/t/type_year.test:
  Added test case for Bug#59211.
sql/item.h:
  Added function Item_cache::field() to get access to the
  underlying Field of a cached field Value.
sql/item_cmpfunc.cc:
  Also check underlying fields of Item_cache, not just Item_Field,
  when checking whether the value is of type YEAR(4) or not.

mysql-test/r/type_year.result

@@ -341,4 +341,18 @@ ta_y	s	tb_y	s
 2001	2001	2001	2001
 DROP TABLE t1;
 #
+# Bug #59211: Select Returns Different Value for min(year) Function
+#
+CREATE TABLE t1(c1 YEAR(4));
+INSERT INTO t1 VALUES (1901),(2155),(0000);
+SELECT * FROM t1;
+c1
+1901
+2155
+0000
+SELECT COUNT(*) AS total_rows, MIN(c1) AS min_value, MAX(c1) FROM t1;
+total_rows	min_value	MAX(c1)
+3	0	2155
+DROP TABLE t1;
+#
 End of 5.1 tests

mysql-test/t/type_year.test

@@ -149,6 +149,16 @@ SELECT ta.y AS ta_y, ta.s, tb.y AS tb_y, tb.s FROM t1 ta, t1 tb HAVING ta_y = tb
 
 DROP TABLE t1;
 
+--echo #
+--echo # Bug #59211: Select Returns Different Value for min(year) Function
+--echo #
+
+CREATE TABLE t1(c1 YEAR(4));
+INSERT INTO t1 VALUES (1901),(2155),(0000);
+SELECT * FROM t1;
+SELECT COUNT(*) AS total_rows, MIN(c1) AS min_value, MAX(c1) FROM t1;
+DROP TABLE t1;
+
 --echo #
 
 --echo End of 5.1 tests

sql/item.h

@@ -2960,11 +2960,10 @@ class Item_cache: public Item_basic_constant
 protected:
   Item *example;
   table_map used_table_map;
-  /*
-    Field that this object will get value from. This is set/used by 
+  /**
+    Field that this object will get value from. This is used by 
     index-based subquery engines to detect and remove the equality injected 
     by IN->EXISTS transformation.
-    For all other uses of Item_cache, cached_field doesn't matter.
   */  
   Field *cached_field;
   enum enum_field_types cached_field_type;
@@ -3021,6 +3020,14 @@ public:
   {
     return this == item;
   }
+
+  /** 
+    If this item caches a field value, return pointer to underlying field.
+
+    @return Pointer to field, or NULL if this is not a cache for a field value.
+  */
+  Field* field() { return cached_field; }
+
   virtual void store(Item *item);
   virtual bool cache_value()= 0;
 };

sql/item_cmpfunc.cc

@@ -1196,9 +1196,12 @@ get_year_value(THD *thd, Item ***item_arg, Item **cache_arg,
          value of 2000.
   */
   Item *real_item= item->real_item();
-  if (!(real_item->type() == Item::FIELD_ITEM &&
-        ((Item_field *)real_item)->field->type() == MYSQL_TYPE_YEAR &&
-        ((Item_field *)real_item)->field->field_length == 4))
+  Field *field= NULL;
+  if (real_item->type() == Item::FIELD_ITEM)
+    field= ((Item_field *)real_item)->field;
+  else if (real_item->type() == Item::CACHE_ITEM)
+    field= ((Item_cache *)real_item)->field();
+  if (!(field && field->type() == MYSQL_TYPE_YEAR && field->field_length == 4))
   {
     if (value < 70)
       value+= 100;