How many database indexes is too many?

Question

I\'m working on a project with a rather large Oracle database (although my question applies equally well to other databases). We have a web interface which allows users to

Answer 1

In addition to the points everyone else has raised, the Cost Based Optimizer incurs a cost when creating a plan for an SQL statement if there are more indexes because there are more combinations for it to consider. You can reduce this by correctly using bind variables so that SQL statements stay in the SQL cache. Oracle can then do a soft parse and re-use the plan it found last time.

As always, nothing is simple. If there are skewed columns and histograms involved then this can be a bad idea.

In our web applications we tend to limit the combinations of searches that we allow. Otherwise you would have to test literally every combination for performance to ensure you did not have a lurking problem that someone will find one day. We have also implemented resource limits to stop this causing issues elsewhere in the application should something go wrong.