Featured in Perl Weekly

Featured in Perl Weekly

Friday, October 14, 2011

MySQL `FORCE INDEX` best practices?

Hi,

Almost everywhere I read the use of FORCE INDEX is highly discouraged and I perfectly understand and know why - there are immensely huge chances that MySQL knows better what indexes to choose than the (average) developer.

However, recently I have found a case where FORCE INDEX improved my execution times in the range of hundred of times:


  •  JOIN on 4 tables
  •  first table has about 500 000 records
  •  2 of the INNER JOINed table have more than 1 milion records
  •  first table has a field called published_date, stored as varchar in YMD format (could not changed to datetime)
  •  needed a range on published_date of at most 5 000 records
  •  some other GROUP BY and ORDER BY clauses on the first table on different fields than published_date were needed for this query


Although I've rewritten the query in many ways, I was not able to get execution times smaller than 130 seconds (with highest being over 700 ). After using FORCE INDEX with published_date, the execution time dropped below 5 seconds.

It took me a few days to remember about the infamous FORCE INDEX option.

Based on suggestions I received on StackOverflow, I have issued ANALIZE TABLE and OPTIMIZE TABLE on the annoing table that does not want to use the index - unfortunately, it stiil doesn't use that index, so for now FORCE INDEX remains the only viable option. Below is the output for EXPLAIN after each step I took in my tryings to optimize the table:

 without FORCE INDEX on table with alias a  
 id     select_type     table     type     possible_keys     key     key_len     ref     rows     Extra  
 1     SIMPLE     am2     range     PRIMARY,idx_meta_article     idx_meta_article     4     NULL     275228     Using where; Using index; Using temporary; Using f...  
 1     SIMPLE     a     eq_ref     PRIMARY,serial_issue_date_productid,pub_date,idx_d...     PRIMARY     4     mydb_toto.am2.ArticleID     1     Using where  
 1     SIMPLE     ai     ref     PRIMARY,idx_iso_article     PRIMARY     4     mydb_toto.a.serial     11523     Using where; Using index  
 1     SIMPLE     m     range     PRIMARY,meta_articles_type     meta_articles_type     4     NULL     96     Using where  
 1     SIMPLE     am     eq_ref     PRIMARY,idx_meta_article     PRIMARY     8     mydb_toto.a.serial,mydb_toto.m.meta_id     1     Using where; Using index  
 with FORCE INDEX on table with alias a  
 id     select_type     table     type     possible_keys     key     key_len     ref     rows     Extra  
 1     SIMPLE     a     range     pub_date     pub_date     11     NULL     17679     Using where; Using temporary; Using filesort  
 1     SIMPLE     am2     ref     PRIMARY,idx_meta_article     PRIMARY     4     mydb_toto.a.serial     21930     Using where; Using index  
 1     SIMPLE     ai     ref     PRIMARY,idx_iso_article     PRIMARY     4     mydb_toto.a.serial     11523     Using where; Using index  
 1     SIMPLE     m     range     PRIMARY,meta_articles_type     meta_articles_type     4     NULL     96     Using where  
 1     SIMPLE     am     eq_ref     PRIMARY,idx_meta_article     PRIMARY     8     mydb_toto.am2.ArticleID,mydb_toto.m.meta_id     1     Using where; Using index  
 after ANALYZE TABLE, without FORCE INDEX:  
 id     select_type     table     type     possible_keys     key     key_len     ref     rows     Extra  
 1     SIMPLE     am2     range     PRIMARY,idx_meta_article     idx_meta_article     4     NULL     275228     Using where; Using index; Using temporary; Using f...  
 1     SIMPLE     a     eq_ref     PRIMARY,serial_issue_date_productid,pub_date,idx_d...     PRIMARY     4     mydb_toto.am2.ArticleID     1     Using where  
 1     SIMPLE     ai     ref     PRIMARY,idx_iso_article     PRIMARY     4     mydb_toto.a.serial     11523     Using where; Using index  
 1     SIMPLE     m     range     PRIMARY,meta_articles_type     meta_articles_type     4     NULL     96     Using where  
 1     SIMPLE     am     eq_ref     PRIMARY,idx_meta_article     PRIMARY     8     mydb_toto.a.serial,mydb_toto.m.meta_id     1     Using where; Using index  
 after OPTIMIZE TABLE, without FORCE INDEX:  
 id     select_type     table     type     possible_keys     key     key_len     ref     rows     Extra  
 1     SIMPLE     am2     range     PRIMARY,idx_meta_article     idx_meta_article     4     NULL     275228     Using where; Using index; Using temporary; Using f...  
 1     SIMPLE     a     eq_ref     PRIMARY,serial_issue_date_productid,pub_date,idx_d...     PRIMARY     4     mydb_toto.am2.ArticleID     1     Using where  
 1     SIMPLE     ai     ref     PRIMARY,idx_iso_article     PRIMARY     4     mydb_toto.a.serial     11523     Using where; Using index  
 1     SIMPLE     m     range     PRIMARY,meta_articles_type     meta_articles_type     4     NULL     96     Using where  
 1     SIMPLE     am     eq_ref     PRIMARY,idx_meta_article     PRIMARY     8     mydb_toto.a.serial,mydb_toto.m.meta_id     1     Using where; Using index  
 after OPTIMIZE TABLE and ANALYZE TABLE, with FORCE INDEX  
 id     select_type     table     type     possible_keys     key     key_len     ref     rows     Extra  
 1     SIMPLE     a     range     pub_date     pub_date     11     NULL     17679     Using where; Using temporary; Using filesort  
 1     SIMPLE     am2     ref     PRIMARY,idx_meta_article     PRIMARY     4     mydb_toto.a.serial     21930     Using where; Using index  
 1     SIMPLE     ai     ref     PRIMARY,idx_iso_article     PRIMARY     4     mydb_toto.a.serial     11523     Using where; Using index  
 1     SIMPLE     m     range     PRIMARY,meta_articles_type     meta_articles_type     4     NULL     96     Using where  
 1     SIMPLE     am     eq_ref     PRIMARY,idx_meta_article     PRIMARY     8     mydb_toto.am2.ArticleID,mydb_toto.m.meta_id     1     Using where; Using index  

Questions:


  •  What are other use cases you found where FORCE INDEX saved you?
  • Do you have some best practices when you consider using FORCE INDEX?