Fixed bug mdev-11608.

The fix for bug mdev-11488 introduced the virtual method
convert_to_basic_const_item for the class Item_cache.
The implementation of this method for the class Item_cache_str
was not quite correct: the server could crash if the cached item
was null.
A similar problem could appear for the implementation of
this method for the class Item_cache_decimal. Although I could not
reproduce the problem I decided to change the code appropriately.

mysql-test/r/derived_cond_pushdown.result

@@ -8178,3 +8178,66 @@ EXPLAIN
 }
 DROP VIEW v1;
 DROP TABLE t1;
+#
+# MDEV-11608: pushdown of the predicate with cached null value
+#
+CREATE TABLE t1 (c VARCHAR(3));
+CREATE ALGORITHM=TEMPTABLE VIEW v1 AS SELECT * FROM t1;
+INSERT INTO t1 VALUES ('foo'),('bar');
+CREATE TABLE t2 (c VARCHAR(3));
+INSERT INTO t2 VALUES ('foo'),('xyz');
+SELECT * FROM v1 WHERE v1.c IN ( SELECT MIN(c) FROM t2 WHERE 0 );
+c
+EXPLAIN FORMAT=JSON
+SELECT * FROM v1 WHERE v1.c IN ( SELECT MIN(c) FROM t2 WHERE 0 );
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "<subquery2>",
+      "access_type": "system",
+      "rows": 1,
+      "filtered": 100,
+      "materialized": {
+        "unique": 1,
+        "query_block": {
+          "select_id": 2,
+          "table": {
+            "message": "Impossible WHERE"
+          }
+        }
+      }
+    },
+    "table": {
+      "table_name": "<derived3>",
+      "access_type": "ALL",
+      "rows": 2,
+      "filtered": 100,
+      "attached_condition": "v1.c = NULL",
+      "materialized": {
+        "query_block": {
+          "select_id": 3,
+          "table": {
+            "table_name": "t1",
+            "access_type": "ALL",
+            "rows": 2,
+            "filtered": 100,
+            "attached_condition": "t1.c = NULL"
+          }
+        }
+      }
+    }
+  }
+}
+DROP VIEW v1;
+DROP TABLE t1,t2;
+CREATE TABLE t1 (d DECIMAL(10,2));
+CREATE ALGORITHM=TEMPTABLE VIEW v1 AS SELECT * FROM t1;
+INSERT INTO t1 VALUES (5.37),(1.1);
+CREATE TABLE t2 (d DECIMAL(10,2));
+INSERT INTO t2 VALUES ('1.1'),('2.23');
+SELECT * FROM v1 WHERE v1.d IN ( SELECT MIN(d) FROM t2 WHERE 0 );
+d
+DROP VIEW v1;
+DROP TABLE t1,t2;

mysql-test/t/derived_cond_pushdown.test

@@ -1289,3 +1289,33 @@ SELECT * FROM v1 WHERE NULLIF(1, i);
 
 DROP VIEW v1;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-11608: pushdown of the predicate with cached null value
+--echo #
+
+CREATE TABLE t1 (c VARCHAR(3));
+CREATE ALGORITHM=TEMPTABLE VIEW v1 AS SELECT * FROM t1;
+INSERT INTO t1 VALUES ('foo'),('bar');
+ 
+CREATE TABLE t2 (c VARCHAR(3));
+INSERT INTO t2 VALUES ('foo'),('xyz');
+ 
+SELECT * FROM v1 WHERE v1.c IN ( SELECT MIN(c) FROM t2 WHERE 0 );
+EXPLAIN FORMAT=JSON
+SELECT * FROM v1 WHERE v1.c IN ( SELECT MIN(c) FROM t2 WHERE 0 );
+
+DROP VIEW v1;
+DROP TABLE t1,t2;
+
+CREATE TABLE t1 (d DECIMAL(10,2));
+CREATE ALGORITHM=TEMPTABLE VIEW v1 AS SELECT * FROM t1;
+INSERT INTO t1 VALUES (5.37),(1.1);
+ 
+CREATE TABLE t2 (d DECIMAL(10,2));
+INSERT INTO t2 VALUES ('1.1'),('2.23');
+ 
+SELECT * FROM v1 WHERE v1.d IN ( SELECT MIN(d) FROM t2 WHERE 0 );
+
+DROP VIEW v1;
+DROP TABLE t1,t2;

sql/item.cc

@@ -9703,12 +9703,15 @@ my_decimal *Item_cache_decimal::val_decimal(my_decimal *val)
 Item *Item_cache_decimal::convert_to_basic_const_item(THD *thd)
 {
   Item *new_item;
-  my_decimal decimal_value;
-  my_decimal *result= val_decimal(&decimal_value);
   DBUG_ASSERT(value_cached || example != 0);
-  new_item= null_value ?
-            (Item*) new (thd->mem_root) Item_null(thd) :
-	    (Item*) new (thd->mem_root) Item_decimal(thd, result);
+  if (null_value)
+    new_item= (Item*) new (thd->mem_root) Item_null(thd);
+  else
+  {
+     my_decimal decimal_value;
+     my_decimal *result= val_decimal(&decimal_value);
+     new_item= (Item*) new (thd->mem_root) Item_decimal(thd, result);
+  }
   return new_item;
 } 
 
@@ -9795,14 +9798,14 @@ bool Item_cache_row::allocate(THD *thd, uint num)
 Item *Item_cache_str::convert_to_basic_const_item(THD *thd)
 {
   Item *new_item;
-  char buff[MAX_FIELD_WIDTH];
-  String tmp(buff, sizeof(buff), value->charset());
-  String *result= val_str(&tmp);
   DBUG_ASSERT(value_cached || example != 0);
   if (null_value)
     new_item= (Item*) new (thd->mem_root) Item_null(thd);
   else
   {
+    char buff[MAX_FIELD_WIDTH];
+    String tmp(buff, sizeof(buff), value->charset());
+    String *result= val_str(&tmp);
     uint length= result->length();
     char *tmp_str= thd->strmake(result->ptr(), length);
     new_item= new (thd->mem_root) Item_string(thd, tmp_str, length,