Fix bug lp:1008487

Analysis:
The crash is a result of Item_cache_temporal::example not being set
(it is NULL). It turns out that the value of Item_cache_temporal
may be set directly by calling Item_cache_temporal::store_packed
without ever setting the "example" of this Item_cache. Therefore
the failing assertion is too narrow.

Solution:
Remove the assert.
In principle we could overwrite this method for Item_cache_temporal,
but it doesn't make sense just for this assert.

mysql-test/r/type_date.result

@@ -339,3 +339,17 @@ select @a;
 @a
 0111-01-01
 #
+#
+# BUG LP:1008487 virtual bool Item_cache::is_expensive(): Assertion `example' failed
+#
+create table t1(a date,key(a));
+insert into t1 values ('2012-01-01'),('2012-02-02');
+explain
+select 1 from t1 as t1_0 inner join t1 as t2 on (t1_0.a <=> now()) join t1 on 1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1_0	ref	a	a	4	const	0	Using where; Using index
+1	SIMPLE	t2	index	NULL	a	4	NULL	2	Using index; Using join buffer (flat, BNL join)
+1	SIMPLE	t1	index	NULL	a	4	NULL	2	Using index; Using join buffer (incremental, BNL join)
+select 1 from t1 as t1_0 inner join t1 as t2 on (t1_0.a <=> now()) join t1 on 1;
+1
+drop table t1;

mysql-test/t/type_date.test

@@ -317,3 +317,13 @@ set @a=(select min(makedate('111','1'))) ;
 select @a;
 --echo #
 
+--echo #
+--echo # BUG LP:1008487 virtual bool Item_cache::is_expensive(): Assertion `example' failed
+--echo #
+create table t1(a date,key(a));
+insert into t1 values ('2012-01-01'),('2012-02-02');
+explain
+select 1 from t1 as t1_0 inner join t1 as t2 on (t1_0.a <=> now()) join t1 on 1;
+select 1 from t1 as t1_0 inner join t1 as t2 on (t1_0.a <=> now()) join t1 on 1;
+drop table t1;
+

sql/item.h

@@ -3889,7 +3889,6 @@ class Item_cache: public Item_basic_constant
   bool is_null() { return null_value; }
   virtual bool is_expensive()
   {
-    DBUG_ASSERT(example);
     if (value_cached)
       return false;
     return example->is_expensive();