BUG#19055: There may exist a SEL_TREE objects with type==KEY and keys[i]==NULL for any i.

(for example, such objects can be returned from get_mm_parts() for "string_field = int_val).
Make find_used_partitions_imerge() to handle such SEL_TREE objects.

mysql-test/r/partition_pruning.result

@@ -670,3 +670,11 @@ select * from t1 where a like 'n%';
 a
 na
 drop table t1;
+create table t1 (s1 varchar(15)) partition by key (s1);
+select * from t1 where s1 = 0 or s1 is null;
+s1
+insert into t1 values ('aa'),('bb'),('0');
+explain partitions select * from t1 where s1 = 0 or s1 is null;
+id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	p0	ALL	NULL	NULL	NULL	NULL	3	Using where
+drop table t1;

mysql-test/t/partition_pruning.test

@@ -556,7 +556,7 @@ drop table t1;
 # being fixed.
 
 #
-#BUG 17946 Like searches fail with partitioning
+# BUG#17946 Like searches fail with partitioning
 #
 create table t1 (a char(32) primary key)
 partition by key()
@@ -566,3 +566,11 @@ select * from t1;
 select * from t1 where a like 'n%';
 drop table t1;
 
+
+# BUG#19055 Crashes for varchar_col=NUMBER or varchar_col IS NULL
+create table t1 (s1 varchar(15)) partition by key (s1);
+select * from t1 where s1 = 0 or s1 is null;
+insert into t1 values ('aa'),('bb'),('0');
+explain partitions select * from t1 where s1 = 0 or s1 is null;
+drop table t1;
+

sql/opt_range.cc

@@ -374,6 +374,12 @@ public:
     keys_map.clear_all();
     bzero((char*) keys,sizeof(keys));
   }
+  /*
+    Note: there may exist SEL_TREE objects with sel_tree->type=KEY and
+    keys[i]=0 for all i. (SergeyP: it is not clear whether there is any
+    merit in range analyzer functions (e.g. get_mm_parts) returning a
+    pointer to such SEL_TREE instead of NULL)
+  */
   SEL_ARG *keys[MAX_KEY];
   key_map keys_map;        /* bitmask of non-NULL elements in keys */
 
@@ -2580,7 +2586,8 @@ int find_used_partitions_imerge(PART_PRUNE_PARAM *ppar, SEL_IMERGE *imerge)
     ppar->cur_part_fields= 0;
     ppar->cur_subpart_fields= 0;
     init_all_partitions_iterator(ppar->part_info, &ppar->part_iter);
-    if (-1 == (res |= find_used_partitions(ppar, (*ptree)->keys[0])))
+    SEL_ARG *key_tree= (*ptree)->keys[0];
+    if (!key_tree || (-1 == (res |= find_used_partitions(ppar, key_tree))))
       return -1;
   }
   return res;
@@ -5104,7 +5111,7 @@ get_mm_parts(RANGE_OPT_PARAM *param, COND *cond_func, Field *field,
       tree->keys_map.set_bit(key_part->key);
     }
   }
-
+  
   DBUG_RETURN(tree);
 }