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Fine-Tuning MySQL Full-Text Search With InnoDB

If you are using FULLTEXT indexes in MySQL and plan to switch from MyISAM to InnoDB then you should review the reference manual section on Fine-Tuning MySQL Full-Text Search to see what configuration changes may be required. As I mentioned in yesterday’s post when comparing query results on my database with FULLTEXT indexes in MyISAM versus InnoDB I got different results. Specifically, the InnoDB tables were returning fewer results for certain queries with short FULLTEXT search terms. Here’s an example of a query that returned fewer results on InnoDB:

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select id
from flite.ad_index
where match(name,description,keywords) against('+v1*' IN BOOLEAN MODE);

The issue was that all of the fine tuning I had done before was limited to MyISAM, so it didn’t affect InnoDB. In the past I configured MySQL FULLTEXT search to index words as short as 1 character (the default is 3), and to index common words (not to use any stopword list). These are the relevant variables I set in in my.cnf:

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ft_min_word_len = 1
ft_stopword_file = ''

InnoDB has its own variables to control stopwords and minimum word length, so I needed to set these variables when I changed the tables from MyISAM to InnoDB:

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innodb_ft_min_token_size = 1
innodb_ft_enable_stopword = OFF

Since those variables are not dynamic, I had to restart MySQL for them to take effect. Furthermore, I needed to rebuild the FULLTEXT indexes on the relevant tables. This is how the manual instructs you to rebuld the indexes:

To rebuild the FULLTEXT indexes for an InnoDB table, use ALTER TABLE with the DROP INDEX and ADD INDEX options to drop and re-create each index.

Rather than drop and recreate the indexes, I just used ALTER TABLE ... FORCE to rebuild the table (and indexes), like this:

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alter table flite.ad_index force;

After making those changes I re-ran pt-upgrade, and now I am getting the same set of rows back from MyISAM and InnoDB. The order of the rows is slightly different in some cases, but as I mentioned yesterday that is expected behavior.

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