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Bug#37830 : ORDER BY ASC/DESC - no difference

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Range scan in descending order for c <= <col> <= c type of
ranges was ignoring the DESC flag.
However some engines like InnoDB have the primary key parts 
as a suffix for every secondary key.
When such primary key suffix is used for ordering ignoring 
the DESC is not valid.
But we generally would like to do this because it's faster.
            
Fixed by performing only reverse scan if the primary key is used.
Removed some dead code in the process.
This commit is contained in:
Georgi Kodinov
2008-07-23 14:25:00 +03:00
parent 25dd8b33d7
commit 436f1dc49c
5 changed files with 83 additions and 79 deletions
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15
mysql-test/r/innodb_mysql.result
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@ -1246,4 +1246,19 @@ set global innodb_autoextend_increment=@my_innodb_autoextend_increment;
set @my_innodb_commit_concurrency=@@global.innodb_commit_concurrency;
set global innodb_commit_concurrency=0;
set global innodb_commit_concurrency=@my_innodb_commit_concurrency;
CREATE TABLE t1 (a int, b int, c int, PRIMARY KEY (a), KEY t1_b (b))
ENGINE=InnoDB;
INSERT INTO t1 (a,b,c) VALUES (1,1,1), (2,1,1), (3,1,1), (4,1,1);
INSERT INTO t1 (a,b,c) SELECT a+4,b,c FROM t1;
EXPLAIN SELECT a, b, c FROM t1 WHERE b = 1 ORDER BY a DESC LIMIT 5;
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE t1 range t1_b t1_b 5 NULL 4 Using where
SELECT a, b, c FROM t1 WHERE b = 1 ORDER BY a DESC LIMIT 5;
a b c
8 1 1
7 1 1
6 1 1
5 1 1
4 1 1
DROP TABLE t1;
End of 5.0 tests