When should database indexes be rebuilt?

Question

I was reading about refactoring a large slow SQL Query over here, and the current highest response is from Mitch Wheat, who wants to make sure the query uses indexes for the

Answer 1

There is no need to 'rebuild' an index. They are always kept up-to-date. Maybe he was referring to rebuilding the table. Depending on your usage patterns and schema, you can get fragmented pages in InnoDB, and I think in MyISAM also. Rebuilding the table can improve performance by getting rid of fragmentation of your data on disk. I don't use MyISAM tables regularly, but I believe it is recommended to run 'OPTIMIZE TABLE' with certain usage patterns. See the MySQL docs on OPTIMIZE TABLE for some good info on both MyISAM and InnoDB.

I'm not as familiar with MyISAM intricacies, but with InnoDB it is true that statistics can get out of date. The database keeps estimated statistics on how your data is distributed for a given index, and it's possible for those to get out-of-date, but MySQL/InnoDB has some built in functionality to try to keep statistics current. You usually don't have to worry about it.

So if you are using InnoDB, the answer is no, you typically don't need to actively do anything to keep your indexes performing well. I'm not as sure with MyISAM, I think it's more common to need to optimize those tables regularly.