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mysqlsqlindexingquery-optimization

Optimizing MySQL LIKE '%string%' queries in innoDB

Having this table:

CREATE TABLE `example` (
`id` int(11) unsigned NOT NULL auto_increment,
`keywords` varchar(200) NOT NULL,
PRIMARY KEY  (`id`)
) ENGINE=InnoDB;

We would like to optimize the following query:

SELECT id FROM example WHERE keywords LIKE '%whatever%'

The table is InnoDB, (so no FULLTEXT for now) which would be the best index to use in order to optimize such query?

We've tried a simple : 

ALTER TABLE `example` ADD INDEX `idxSearch` (`keywords`);

But an explain query shows that need to scan the whole table if our queries where LIKE 'whatever%' instead, this index performs well, but otherwise has no value.

Is there anyway to optimize this for innoDB ?

Thanks!

Solution

  • Indexes are built from the start of the string towards the end. When you use LIKE 'whatever%' type clause, MySQL can use those start-based indexes to look for whatever very quickly.

    But switching to LIKE '%whatever%' removes that anchor at the start of the string. Now the start-based indexes can't be used, because your search term is no longer anchored at the start of the string - it's "floating" somewhere in the middle and the entire field has to be search. Any LIKE '%... query can never use indexes.

    That's why you use fulltext indexes if all you're doing are 'floating' searches, because they're designed for that type of usage.

    Of major note: InnoDB now supports fulltext indexes as of version 5.6.4. So unless you can't upgrade to at least 5.6.4, there's nothing holding you back from using InnoDB *AND fulltext searches.