Two duplicate indexes with the same columns

Question

I am looking at a table in our database (I didn\'t make the table), and I see that there are 2 indexes that are exactly the same (I don\'t know why this was done) just named

Answer 1

Yes, it can have an effect.

Of course the two indexes take extra space on disk and also in memory if they are used.

But they also cause the query optimizer to do more work to calculate the benefit of each index during every SELECT. The more indexes you have, the more cases it has to compare. So it pays off to eliminate truly redundant indexes.

As others have also noted, indexes are updated during INSERT/UPDATE/DELETE operations, so the more indexes you have, the more overhead this represents. Indexes that get a lot of use justify their own overhead, but duplicate indexes take more overhead with no additional benefit to match.

If you're interested, Percona Toolkit has a tool pt-duplicate-key-checker that searches all your tables for cases like this.