Allow keys with 0 segements in MyISAM

This fixed a bug in SELECT DISTINCT when all selected parts where constants. (Bug #3203)

myisam/mi_create.c

@@ -366,7 +366,13 @@ int mi_create(const char *name,uint keys,MI_KEYDEF *keydefs,
       my_errno=HA_WRONG_CREATE_OPTION;
       goto err;
     }
-    if ((keydef->flag & (HA_NOSAME | HA_NULL_PART_KEY)) == HA_NOSAME)
+    /*
+      key_segs may be 0 in the case when we only want to be able to
+      add on row into the table. This can happen with some DISTINCT queries
+      in MySQL
+    */
+    if ((keydef->flag & (HA_NOSAME | HA_NULL_PART_KEY)) == HA_NOSAME &&
+	key_segs)
       share.state.rec_per_key_part[key_segs-1]=1L;
     length+=key_length;
     keydef->block_length= MI_BLOCK_SIZE(length,pointer,MI_MAX_KEYPTR_SIZE);

mysql-test/r/myisam.result

@@ -431,3 +431,25 @@ a	concat(b,'.')
 1	a.
 3	a  .
 drop table t1;
+create table t1 (a int not null);
+create table t2 (a int not null, primary key (a));
+insert into t1 values (1);
+insert into t2 values (1),(2);
+select sql_big_result distinct t1.a from t1,t2 order by t2.a;
+a
+1
+select distinct t1.a from t1,t2 order by t2.a;
+a
+1
+select sql_big_result distinct t1.a from t1,t2;
+a
+1
+explain select sql_big_result distinct t1.a from t1,t2 order by t2.a;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	system	NULL	NULL	NULL	NULL	1	Using temporary
+t2	index	NULL	PRIMARY	4	NULL	2	Using index; Distinct
+explain select distinct t1.a from t1,t2 order by t2.a;
+table	type	possible_keys	key	key_len	ref	rows	Extra
+t1	system	NULL	NULL	NULL	NULL	1	Using temporary
+t2	index	NULL	PRIMARY	4	NULL	2	Using index; Distinct
+drop table t1,t2;

mysql-test/t/myisam.test

@@ -422,3 +422,17 @@ update t1 set b='b  ' where a > 1;
 delete from t1 where b='b';
 select a,concat(b,'.') from t1;
 drop table t1;
+
+#
+# Test keys with 0 segments. (Bug #3203)
+#
+create table t1 (a int not null);
+create table t2 (a int not null, primary key (a));
+insert into t1 values (1);
+insert into t2 values (1),(2);
+select sql_big_result distinct t1.a from t1,t2 order by t2.a;
+select distinct t1.a from t1,t2 order by t2.a;
+select sql_big_result distinct t1.a from t1,t2;
+explain select sql_big_result distinct t1.a from t1,t2 order by t2.a;
+explain select distinct t1.a from t1,t2 order by t2.a;
+drop table t1,t2;