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Distinct/group by, order by and filesort
Posted by: Tolan Blundell
Date: February 23, 2010 01:20PM

I've Googled and tested this one until my eyes bleed but I really can't find a solution.

A quick overview. I have an O/RM that uses left outer joins for mapping object inheritance, and can use either joins or subselects for looking up object relationships.

When using joins for object relationships I need to use distinct or order by to limit the results to one row per selected object.

So here's a sample (fetch all articles and subclasses which are published and latest version and attached to 'section' 2):

------------------------------------------------------------
SELECT DISTINCT( article.id ) as TD, article.*, job.*, press_release.*, job.* FROM article 

LEFT OUTER JOIN press_release ON 
    ( article.id=press_release.id 
    AND article.TORM_REVISION_ID=press_release.TORM_REVISION_ID )  

LEFT OUTER JOIN job ON 
    ( article.id=job.id 
    AND article.TORM_REVISION_ID=job.TORM_REVISION_ID ) 

LEFT OUTER JOIN section_to_article ON 
    ( article.id=section_to_article.article_id ) 
    AND (section_to_article.section_id = 2) 

WHERE 
    (article.published = 1) 
    AND ( article.TORM_IS_CURRENT_REVISION = 1 ) 


ORDER BY article.article_date DESC,article.id DESC 

LIMIT 50;

------------------------------------------------------------

Here's the output from mk-visual-explain:

------------------------------------------------------------
Filesort
+- TEMPORARY
   table          temporary(article,press_release,job,section_to_article)
   +- JOIN
      +- Distinct/Not-Exists
      |  +- Bookmark lookup
      |     +- Table
      |     |  table          section_to_article
      |     |  possible_keys  idx_section_to_art_art_id,idx_section_to_art_sec_id
      |     +- Index lookup
      |        key            section_to_article->idx_section_to_art_art_id
      |        possible_keys  idx_section_to_art_art_id,idx_section_to_art_sec_id
      |        key_len        5
      |        ref            my_db.article.id
      |        rows           1
      +- JOIN
         +- Bookmark lookup
         |  +- Table
         |  |  table          job
         |  |  possible_keys  job_composite
         |  +- Index lookup
         |     key            job->job_composite
         |     possible_keys  job_composite
         |     key_len        9
         |     ref            my_db.article.id,my_db.article.TORM_REVISION_ID
         |     rows           1
         +- JOIN
            +- Bookmark lookup
            |  +- Table
            |  |  table          press_release
            |  |  possible_keys  pr_revid,pr_composite
            |  +- Index lookup
            |     key            press_release->pr_composite
            |     possible_keys  pr_revid,pr_composite
            |     key_len        9
            |     ref            my_db.article.id,my_db.article.TORM_REVISION_ID
            |     rows           1
            +- Filter with WHERE
               +- Bookmark lookup
                  +- Table
                  |  table          article
                  |  possible_keys  article_published,art_is_current_rev,art_comp_x3,art_comp_x4
                  +- Index lookup
                     key            article->art_is_current_rev
                     possible_keys  article_published,art_is_current_rev,art_comp_x3,art_comp_x4
                     key_len        2
                     ref            const
                     rows           10257

------------------------------------------------------------

So... I simply can't avoid a filesort with the DISTINCT() present. I've also tried replacing the DISTINCT() with "GROUP BY article.article_date DESC,article.id DESC" (so it matches the order by) but again no dice. I have a huge number of test indexes (obviously this isn't a production box ;):

------------------------------------------------------------
+---------+------------+--------------------+--------------+--------------------------+-----------+-------------+----------+--------+------+------------+---------+
| Table   | Non_unique | Key_name           | Seq_in_index | Column_name              | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------+------------+--------------------+--------------+--------------------------+-----------+-------------+----------+--------+------+------------+---------+
| article |          1 | torm_class_article |            1 | TORM_CLASS               | A         |           4 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | article_published  |            1 | published                | A         |           2 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | article_date       |            1 | article_date             | A         |        3726 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | idx_article_dirty  |            1 | dirty                    | A         |           3 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | article_composite  |            1 | id                       | A         |        NULL |     NULL | NULL   |      | BTREE      |         | 
| article |          1 | article_composite  |            2 | TORM_REVISION_ID         | A         |        NULL |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | article_composite2 |            1 | id                       | A         |        7452 |     NULL | NULL   |      | BTREE      |         | 
| article |          1 | article_composite2 |            2 | TORM_REVISION_ID         | A         |       22356 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | article_composite2 |            3 | TORM_CLASS               | A         |       22356 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | article_composite2 |            4 | TORM_IS_CURRENT_REVISION | A         |       22356 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | article_composite2 |            5 | published                | A         |       22356 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_is_current_rev |            1 | TORM_IS_CURRENT_REVISION | A         |           6 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x2        |            1 | TORM_CLASS               | A         |           4 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x2        |            2 | TORM_IS_CURRENT_REVISION | A         |          12 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x2        |            3 | article_date             | A         |        4471 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x2        |            4 | id                       | A         |       11178 |     NULL | NULL   |      | BTREE      |         | 
| article |          1 | art_comp_x3        |            1 | published                | A         |           2 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x3        |            2 | TORM_CLASS               | A         |           6 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x3        |            3 | TORM_IS_CURRENT_REVISION | A         |          18 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x3        |            4 | article_date             | A         |        5589 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x3        |            5 | id                       | A         |       11178 |     NULL | NULL   |      | BTREE      |         | 
| article |          1 | art_comp_x4        |            1 | published                | A         |           2 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x4        |            2 | TORM_CLASS               | A         |           6 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x4        |            3 | TORM_IS_CURRENT_REVISION | A         |          18 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x4        |            4 | article_date             | A         |        5589 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x4        |            5 | id                       | A         |       11178 |     NULL | NULL   |      | BTREE      |         | 
| article |          1 | art_comp_x5        |            1 | TORM_CLASS               | A         |           4 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x5        |            2 | TORM_IS_CURRENT_REVISION | A         |          12 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x5        |            3 | article_date             | A         |        4471 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x5        |            4 | id                       | A         |       11178 |     NULL | NULL   |      | BTREE      |         | 
| article |          1 | art_id             |            1 | id                       | A         |        7452 |     NULL | NULL   |      | BTREE      |         | 
| article |          1 | art_comp_x6        |            1 | id                       | A         |        7452 |     NULL | NULL   |      | BTREE      |         | 
| article |          1 | art_comp_x6        |            2 | published                | A         |       11178 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x6        |            3 | TORM_CLASS               | A         |       11178 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x6        |            4 | TORM_IS_CURRENT_REVISION | A         |       11178 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x7        |            1 | TORM_CLASS               | A         |           4 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x7        |            2 | TORM_IS_CURRENT_REVISION | A         |          12 |     NULL | NULL   | YES  | BTREE      |         | 
| article |          1 | art_comp_x7        |            3 | id                       | A         |       11178 |     NULL | NULL   |      | BTREE      |         | 
| article |          1 | art_search_head    |            1 | headline                 | NULL      |       11178 |     NULL | NULL   | YES  | FULLTEXT   |         | 
| article |          1 | art_search_summ    |            1 | summary                  | NULL      |       11178 |     NULL | NULL   | YES  | FULLTEXT   |         | 
| article |          1 | art_search_body    |            1 | body                     | NULL      |       22356 |     NULL | NULL   | YES  | FULLTEXT   |         | 
+---------+------------+--------------------+--------------+--------------------------+-----------+-------------+----------+--------+------+------------+---------+


Any thoughts? I'm about at my wits end and ready to go back to subselects. It's a shame because generally the system performs better with the joins but there are a lot of queries of this class doing big filesorts and wiping out any performance gains elsewhere...

Thanks!



Edited 2 time(s). Last edit at 02/23/2010 02:58PM by Tolan Blundell.

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Distinct/group by, order by and filesort
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