Bug #18539: uncompress(d) is null: impossible?

  The UNCOMPRESS() function was not marked as maybe_null, even though
  it returns NULL on invalid data. This confused the optimizer.

mysql-test/r/func_compress.result

@@ -79,3 +79,31 @@ uncompress(a)	uncompressed_length(a)
 NULL	NULL
 a	1
 drop table t1;
+create table t1 (a varchar(32) not null);
+insert into t1 values ('foo');
+explain select * from t1 where uncompress(a) is null;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	
+Warnings:
+Error	1256	Uncompressed data size too large; the maximum size is 104857600 (probably, length of uncompressed data was corrupted)
+select * from t1 where uncompress(a) is null;
+a
+foo
+Warnings:
+Error	1256	Uncompressed data size too large; the maximum size is 104857600 (probably, length of uncompressed data was corrupted)
+explain select *, uncompress(a) from t1;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	system	NULL	NULL	NULL	NULL	1	
+select *, uncompress(a) from t1;
+a	uncompress(a)
+foo	NULL
+Warnings:
+Error	1256	Uncompressed data size too large; the maximum size is 104857600 (probably, length of uncompressed data was corrupted)
+select *, uncompress(a), uncompress(a) is null from t1;
+a	uncompress(a)	uncompress(a) is null
+foo	NULL	1
+Warnings:
+Error	1256	Uncompressed data size too large; the maximum size is 104857600 (probably, length of uncompressed data was corrupted)
+Error	1256	Uncompressed data size too large; the maximum size is 104857600 (probably, length of uncompressed data was corrupted)
+drop table t1;
+End of 5.0 tests

mysql-test/t/func_compress.test

@@ -57,3 +57,17 @@ select uncompress(a), uncompressed_length(a) from t1;
 drop table t1;
 
 # End of 4.1 tests
+
+#
+# Bug #18539: uncompress(d) is null: impossible?
+#
+create table t1 (a varchar(32) not null);
+insert into t1 values ('foo');
+explain select * from t1 where uncompress(a) is null;
+select * from t1 where uncompress(a) is null;
+explain select *, uncompress(a) from t1;
+select *, uncompress(a) from t1;
+select *, uncompress(a), uncompress(a) is null from t1;
+drop table t1;
+
+--echo End of 5.0 tests

sql/item_strfunc.h

@@ -784,7 +784,7 @@ class Item_func_uncompress: public Item_str_func
   String buffer;
 public:
   Item_func_uncompress(Item *a): Item_str_func(a){}
-  void fix_length_and_dec(){max_length= MAX_BLOB_WIDTH;}
+  void fix_length_and_dec(){ maybe_null= 1; max_length= MAX_BLOB_WIDTH; }
   const char *func_name() const{return "uncompress";}
   String *val_str(String *) ZLIB_DEPENDED_FUNCTION
 };