Article:		Emulating Analytic (AKA Ranking) Functions with MySQL
Subject:		MySQL version
Date:		2007-04-02 00:24:19
From:		sfaroult
Response to: Very good article
If you want all the gory details the tests were performed with MySQL 5.0.27-standard and Oracle 10.2.0.1.0. Both were running on a double Xeon 2.4 GHz test machine under what initially was a Mandriva 2007.0 that is in a perpetual state of flux and that Oracle anyway believes to be a redhat-4. Sample data available on request :-).

Showing messages 1 through 3 of 3.

• MySQL version
  2007-04-04 20:10:40  jaypipes [View]
  
  
  Hi there! Great article, Stephane! Reminds me of the examples I put into Chapter 8 of "Pro MySQL"...
  
  Couple things I wanted to note, however.
  
  First, the number one reason you see such a performance difference with adding an index *isn't* actually the (correct) point you make about the subquery being fired once for each subset row. The reason is because you are using the InnoDB storage engine, and InnoDB has a known issue with doing a SELECT COUNT(*) when there is no WHERE condition on an indexed field. InnoDB is forced to do a full table scan, as you point out, but for a different reason, each time a SELECT COUNT(*) is used in this manner. See http://dev.mysql.com/doc/refman/5.0/en/innodb-restrictions.html for more info on that.
  
  Second, is that you can get *much* better performance from your MySQL SELECTs by avoiding correlated subqueries and rewriting as a derived table... It's a bug in the MySQL optimizer unfortunately. Even better performance would be from creating a temporary table in place of the derived table and having an appropriate index on the temporary table. Unfortunately, a derived table is created internally with no indexes.
  
  Here's your correlated subquery rewritten as a derived table. Let me know if you get much better performance from it, though I think the biggest performance benefit comes from adding an index because of the InnoDB limitation mentioned above:
  
  original SQL:
  
  select *
  from (select a.DEPTNO, a.EMPNO, a.LASTNAME, a.FIRSTNAME, a.SAL,
  (select 1 + count(*)
  from EMPLOYEES b
  where b.DEPTNO = a.DEPTNO
  and b.SAL > a.SAL) RANK
  from EMPLOYEES as a) as x
  where x.RANK <= 5
  order by x.DEPTNO, x.RANK;
  
  Rewritten SQL:
  select a.DEPTNO, a.EMPNO, a.LASTNAME, a.FIRSTNAME, a.SAL, x.rank as RANK
  from EMPLOYEES as a
  inner join (
  # Get the ranks of each employee within the dept
  select e1.EMPNO, count(DISTINCT e2.SAL) as rank
  from EMPLOYEES e1
  inner join EMPLOYEES e2
  on e1.DEPTNO = e2.DEPTNO
  and e1.SAL <= e2.SAL
  group by e1.EMPNO
  ) as x
  on a.EMPNO = x.EMPNO
  order by a.DEPTNO, RANK;
  
  Do the comparison after adding that index, of course. :)
  
  Cheers,
  
  Jay Pipes
  jay at mysql dot com

• MySQL version
  2007-04-04 19:51:13  jaypipes [View]
  
  
  Hi there! Great article, Stephane! Reminds me of the examples I put into Chapter 8 of "Pro MySQL"...
  
  Couple things I wanted to note, however.
  
  First, the number one reason you see such a performance difference with adding an index *isn't* actually the (correct) point you make about the subquery being fired once for each subset row. The reason is because you are using the InnoDB storage engine, and InnoDB has a known issue with doing a SELECT COUNT(*) when there is no WHERE condition on an indexed field. InnoDB is forced to do a full table scan, as you point out, but for a different reason, each time a SELECT COUNT(*) is used in this manner. See http://dev.mysql.com/doc/refman/5.0/en/innodb-restrictions.html for more info on that.
  
  Second, is that you can get *much* better performance from your MySQL SELECTs by avoiding correlated subqueries and rewriting as a derived table... It's a bug in the MySQL optimizer unfortunately. Even better performance would be from creating a temporary table in place of the derived table and having an appropriate index on the temporary table. Unfortunately, a derived table is created internally with no indexes.
  
  Here's your correlated subquery rewritten as a derived table. Let me know if you get much better performance from it, though I think the biggest performance benefit comes from adding an index because of the InnoDB limitation mentioned above:
  
  original SQL:
  
  select *
  from (select a.DEPTNO, a.EMPNO, a.LASTNAME, a.FIRSTNAME, a.SAL,
  (select 1 + count(*)
  from EMPLOYEES b
  where b.DEPTNO = a.DEPTNO
  and b.SAL > a.SAL) RANK
  from EMPLOYEES as a) as x
  where x.RANK <= 5
  order by x.DEPTNO, x.RANK;
  
  Rewritten SQL:
  
  select a.DEPTNO, a.EMPNO, a.LASTNAME, a.FIRSTNAME, a.SAL, x.emp_count + 1 as RANK
  from EMPLOYEES as a
  inner join (
  select count(*) as emp_count
  from EMPLOYEES
  ) as x
  on x.DEPTNO = a.DEPTNO
  and x.SAL > a.SAL
  where RANK <= 5
  order by a.DEPTNO, RANK;
  
  Do the comparison after adding that index, of course. :)
  
  Cheers,
  
  Jay Pipes
  jay at mysql dot com
• MySQL version
  2007-04-06 10:16:23  Stephane Faroult | [View]
  
  
  Jay,
  
  Where did I say I was using InnoDB? I didn't want to compare Oracle to Oracle ;-). Actually, as my point was a purely SQL point, I haven't specified any storage engine - which means that my table is of the default MyISAM type. That said, the rewriting you suggest is indeed interesting. I have used the code of your second rewriting, added the missing condition on RANK and ran it twice, one without the index, and one with the index. There is a noted improvement WITHOUT the index on the query with the subquery in the SELECT list, as could indeed be expected:
  30 rows in set (1 min 48.16 sec)
  
  With the index, though, it's quite comparable:
  30 rows in set (33.57 sec)
  
  The problem with the index is the way I have generated my data ( ... randomly). As a result, employees from a same department are spread all over the table. We might expect greater efficiency if employees were physically clustered by DEPTNO (by "clustered" I mean of course no more than "if the rows were close from each other").