Best SQL indexes for join table

Question

With performance improvements in mind, I was wondering if and which indexes are helpful on a join table (specifically used in a Rails 3 has_and_belongs_to_many context).

Answer 1

Depends on how you are going to query the data.

Assuming you want to search for all of these...

• WHERE bar_id = ?
• WHERE foo_id = ?
• WHERE bar_id = ? AND foo_id = ?

...then you should probably go with an index on {bar_id, foo_id} and an index on {foo_id}.

While you could also create a third index on {bar_id}, the price of maintaining additional index would probably outweigh the benefit of better clustering in the smaller index.

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Also, how do you plan to cover your queries with indexes? Some of the alternatives, such as...

• {foo_id, bar_id} and {bar_id}
• {foo_id, bar_id} and {bar_id, foo_id}

...might cover certain kinds of queries better.

Covering is a balancing act - sometimes adding a field to an index just for covering purposes is justified, sometimes it's not. You won't know until you measure on realistic amounts of data.

(Disclaimer: I'm not familiar with Ruby. This answer is purely from the database perspective.)