MDEV-3928: Assertion `example' failed in Item_cache::is_expensive_processor with a 2-level IN subquery

Analysis:
The following call stack shows that it is possible to set Item_cache::value_cached, and the relevant value
without setting Item_cache::example.

#0 Item_cache_temporal::store_packed at item.cc:8395
#1 get_datetime_value at item_cmpfunc.cc:915
#2 resolve_const_item at item.cc:7987
#3 propagate_cond_constants at sql_select.cc:12264
#4 propagate_cond_constants at sql_select.cc:12227
#5 optimize_cond at sql_select.cc:13026
#6 JOIN::optimize at sql_select.cc:1016
#7 st_select_lex::optimize_unflattened_subqueries at sql_lex.cc:3161
#8 JOIN::optimize_unflattened_subqueries at opt_subselect.cc:4880
#9 JOIN::optimize at sql_select.cc:1554

The fix is to set Item_cache_temporal::example even when the value is
set directly by Item_cache_temporal::store_packed. This makes the
Item_cache_temporal object consistent.

mysql-test/r/subselect4.result

@@ -2279,5 +2279,31 @@ MAX(a)	bb
 NULL	NULL
 drop table t1, t2;
 set optimizer_switch=@subselect4_tmp;
+#
+# MDEV-3928  Assertion `example' failed in Item_cache::is_expensive_processor with a 2-level IN subquery 
+#
+CREATE TABLE t1 (a1 INT, b1 TIME) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (4,'21:22:34'),(6,'10:50:38');
+CREATE TABLE t2 (a2 INT, b2 TIME) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (8, '06:17:39');
+CREATE TABLE t3 (a3 INT, b3 TIME) ENGINE=MyISAM;
+INSERT INTO t3 VALUES (1,'00:00:01'),(7,'00:00:02');
+EXPLAIN
+SELECT * FROM t1 WHERE a1 IN ( 
+SELECT a2 FROM t2 WHERE a2 IN ( 
+SELECT a3 FROM t3 WHERE b2 = b1 AND b2 <= b1 ORDER BY b3 
+)
+);
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	system	NULL	NULL	NULL	NULL	1	
+1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	2	Using where
+3	DEPENDENT SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	2	Using where
+SELECT * FROM t1 WHERE a1 IN ( 
+SELECT a2 FROM t2 WHERE a2 IN ( 
+SELECT a3 FROM t3 WHERE b2 = b1 AND b2 <= b1 ORDER BY b3 
+)
+);
+a1	b1
+drop table t1, t2, t3;
 SET optimizer_switch= @@global.optimizer_switch;
 set @@tmp_table_size= @@global.tmp_table_size;

mysql-test/t/subselect4.test

@@ -1858,5 +1858,33 @@ drop table t1, t2;
 
 set optimizer_switch=@subselect4_tmp;
 
+--echo #
+--echo # MDEV-3928  Assertion `example' failed in Item_cache::is_expensive_processor with a 2-level IN subquery 
+--echo #
+
+CREATE TABLE t1 (a1 INT, b1 TIME) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (4,'21:22:34'),(6,'10:50:38');
+
+CREATE TABLE t2 (a2 INT, b2 TIME) ENGINE=MyISAM;
+INSERT INTO t2 VALUES (8, '06:17:39');
+
+CREATE TABLE t3 (a3 INT, b3 TIME) ENGINE=MyISAM;
+INSERT INTO t3 VALUES (1,'00:00:01'),(7,'00:00:02');
+
+EXPLAIN
+SELECT * FROM t1 WHERE a1 IN ( 
+  SELECT a2 FROM t2 WHERE a2 IN ( 
+    SELECT a3 FROM t3 WHERE b2 = b1 AND b2 <= b1 ORDER BY b3 
+  )
+);
+
+SELECT * FROM t1 WHERE a1 IN ( 
+  SELECT a2 FROM t2 WHERE a2 IN ( 
+    SELECT a3 FROM t3 WHERE b2 = b1 AND b2 <= b1 ORDER BY b3 
+  )
+);
+
+drop table t1, t2, t3;
+
 SET optimizer_switch= @@global.optimizer_switch;
 set @@tmp_table_size= @@global.tmp_table_size;

sql/item.cc

@@ -8388,9 +8388,10 @@ int Item_cache_temporal::save_in_field(Field *field, bool no_conversions)
 }
 
 
-void Item_cache_temporal::store_packed(longlong val_arg)
+void Item_cache_temporal::store_packed(longlong val_arg, Item *example)
 {
   /* An explicit values is given, save it. */
+  store(example);
   value_cached= true;
   value= val_arg;
   null_value= false;

sql/item.h

@@ -3799,7 +3799,7 @@ public:
   bool cache_value();
   bool get_date(MYSQL_TIME *ltime, uint fuzzydate);
   int save_in_field(Field *field, bool no_conversions);
-  void store_packed(longlong val_arg);
+  void store_packed(longlong val_arg, Item *example);
   /*
     Having a clone_item method tells optimizer that this object
     is a constant and need not be optimized further.
@@ -3808,7 +3808,7 @@ public:
   Item *clone_item()
   {
     Item_cache_temporal *item= new Item_cache_temporal(cached_field_type);
-    item->store_packed(value);
+    item->store_packed(value, example);
     return item;
   }
 };

sql/item_cmpfunc.cc

@@ -912,7 +912,7 @@ get_datetime_value(THD *thd, Item ***item_arg, Item **cache_arg,
     if (save_arena)
       thd->set_query_arena(save_arena);
 
-    cache->store_packed(value);
+    cache->store_packed(value, item);
     *cache_arg= cache;
     *item_arg= cache_arg;
   }