Question

I’m creating a MySQL table in Rails which has about 170 boolean fields, each of which needs to be searchable and hence indexed, but when I create the indexes I get the error message:

To many keys specified; max 64 keys allowed

Is this limit hard coded or is there a config switch I can flip which wold get around it?

Or do I need to refactor the table? The obvious way to me looks to be to join multiple tables together and run a query such as

Table.find.all(:conditions => "join1.fieldx = true and join2.fieldy = true")

Are there any performance or other gotchas with such a strategy?

Answer 1

Classify the boolean fields into groups and maintain separate tables for those groups. Make joins in your query to retrieve results. Use the EXPLAIN EXTENDED SELECT to optimize the indexes for your queries.

Also, try to maintain covering indexes because MySQL uses only one index per table.

http://www.mysqlperformanceblog.com/2009/06/05/a-rule-of-thumb-for-choosing-column-order-in-indexes

EDIT 01:

As Daniel mentioned in his answer, the index cardianality for Boolean values is not going to help you any way. Sometime things get worse when you use index for such columns.

Instead of using 170 boolean columns, you can use 170 tables with reference to the primary key of the parent data.

Let’s say that your parent table is students and the primary key is student_id.

Have separate tables for 170 subjects they undertake in their schooling age.

If a student successfully passes the English subject in his Form 1, insert the appropriate student_id in the form_1_english table. This way, you have only unique values in that column and an index on that column will be much more efficient.