Increase limit of partial key length in MEMORY storage engine

to the same as a full key. (Bug #10566)

mysql-test/r/heap.result

@@ -291,3 +291,8 @@ a	b
 1	7
 1	8
 drop table t1;
+create table t1 (c char(255), primary key(c(90)));
+insert into t1 values ("abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz");
+insert into t1 values ("abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz");
+ERROR 23000: Duplicate entry 'abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijkl' for key 1
+drop table t1;

mysql-test/t/heap.test

@@ -224,3 +224,12 @@ insert t1 (a) values (1);
 insert t1 (a) values (1);
 select * from t1;                                                               
 drop table t1;
+
+#
+# Bug #10566: Verify that we can create a prefixed key with length > 255
+#
+create table t1 (c char(255), primary key(c(90)));
+insert into t1 values ("abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz");
+--error 1062
+insert into t1 values ("abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz");
+drop table t1;

sql/ha_heap.h

@@ -53,6 +53,7 @@ public:
   }
   const key_map *keys_to_use_for_scanning() { return &btree_keys; }
   uint max_supported_keys()          const { return MAX_KEY; }
+  uint max_supported_key_part_length() const { return MAX_KEY_LENGTH; }
   double scan_time() { return (double) (records+deleted) / 20.0+10; }
   double read_time(uint index, uint ranges, ha_rows rows)
   { return (double) rows /  20.0+1; }