jmserra jmserra - 1 month ago 15
MySQL Question

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!

Answer

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.