Issue #25: Make ORDER BY DESC faster

Added support for reverse-ordered Column Families. If a Column Family
name starts with "rev:", then the data in it is stored in the reverse
order.

mysql-test/r/rocksdb_cf_reverse.result

@@ -0,0 +1,120 @@
+create table t0 (a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+create table t1 (
+pk int primary key,
+a int not null,
+b int not null,
+key(a) comment 'rev:foo',
+key(b) comment 'bar'
+) engine=rocksdb;
+insert into t1 select a,a,a  from t0;
+insert into t1 select a+10,a+10,a+10  from t0;
+# Primary key is not in a reverse-ordered CF, so full table scan
+#   returns rows in ascending order:
+select * from t1;
+pk	a	b
+0	0	0
+1	1	1
+2	2	2
+3	3	3
+4	4	4
+5	5	5
+6	6	6
+7	7	7
+8	8	8
+9	9	9
+10	10	10
+11	11	11
+12	12	12
+13	13	13
+14	14	14
+15	15	15
+16	16	16
+17	17	17
+18	18	18
+19	19	19
+explain 
+select a from t1 order by a limit 5;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	a	4	NULL	5	Using index
+select a from t1 order by a limit 5;
+a
+0
+1
+2
+3
+4
+explain 
+select b from t1 order by b limit 5;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	b	4	NULL	5	Using index
+select a from t1 order by a limit 5;
+a
+0
+1
+2
+3
+4
+explain 
+select a from t1 order by a desc limit 5;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	a	4	NULL	5	Using index
+select a from t1 order by a desc limit 5;
+a
+19
+18
+17
+16
+15
+explain 
+select b from t1 order by b desc limit 5;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	b	4	NULL	5	Using index
+select b from t1 order by b desc limit 5;
+b
+19
+18
+17
+16
+15
+drop table t1;
+#
+# Try a primary key in a reverse-ordered CF.
+#
+create table t2 (
+pk int,
+a int not null,
+primary key(pk) comment 'rev:cf1'
+) engine=rocksdb;
+insert into t2 select a,a from t0;
+# Primary key is in a reverse-ordered CF, so full table scan
+#   returns rows in descending order:
+select * from t2;
+pk	a
+9	9
+8	8
+7	7
+6	6
+5	5
+4	4
+3	3
+2	2
+1	1
+0	0
+set autocommit=0;
+begin;
+delete from t2 where a=3 or a=7;
+select * from t2;
+pk	a
+9	9
+8	8
+6	6
+5	5
+4	4
+2	2
+1	1
+0	0
+rollback;
+set autocommit=1;
+drop table t2;
+drop table t0;

mysql-test/t/rocksdb_cf_reverse.test

@@ -0,0 +1,65 @@
+#
+# RocksDB-SE tests for reverse-ordered Column Families
+#
+
+create table t0 (a int);
+insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
+
+create table t1 (
+  pk int primary key,
+  a int not null,
+  b int not null,
+  key(a) comment 'rev:foo',
+  key(b) comment 'bar'
+) engine=rocksdb;
+
+insert into t1 select a,a,a  from t0;
+insert into t1 select a+10,a+10,a+10  from t0;
+
+--echo # Primary key is not in a reverse-ordered CF, so full table scan
+--echo #   returns rows in ascending order:
+select * from t1;
+
+explain 
+select a from t1 order by a limit 5;
+select a from t1 order by a limit 5;
+
+explain 
+select b from t1 order by b limit 5;
+select a from t1 order by a limit 5;
+
+explain 
+select a from t1 order by a desc limit 5;
+select a from t1 order by a desc limit 5;
+
+explain 
+select b from t1 order by b desc limit 5;
+select b from t1 order by b desc limit 5;
+
+drop table t1;
+
+--echo #
+--echo # Try a primary key in a reverse-ordered CF.
+--echo #
+
+create table t2 (
+  pk int,
+  a int not null,
+  primary key(pk) comment 'rev:cf1'
+) engine=rocksdb;
+
+insert into t2 select a,a from t0;
+--echo # Primary key is in a reverse-ordered CF, so full table scan
+--echo #   returns rows in descending order:
+select * from t2;
+
+set autocommit=0;
+begin;
+delete from t2 where a=3 or a=7;
+select * from t2;
+rollback;
+set autocommit=1;
+
+drop table t2;
+drop table t0;
+