Comments on: Covering index and prefix indexes

By: L.A

L.A — Tue, 19 Apr 2011 16:42:52 +0000

I have a few questions about your last query:

First, in your explain output, you have

key: i

but “extra” doesn’t say “using index”. If index is not used, shouldn’t you be getting key: null?
Or does it mean that index is being used, just not the optimal one?

Second, how would you change your indexing so that your last query would work optimally?

Thanks.

By: The MySQL Query Performance Improvement Process | Pablowe

The MySQL Query Performance Improvement Process | Pablowe — Sun, 03 Aug 2008 22:28:28 +0000

[...] can bog down the network if the unneeded columns are large, and eliminate the possibility of using covering indexes that could eliminate the need to read from disk when retrieving the result set. Additionally, a [...]

By: peter

peter — Fri, 24 Nov 2006 08:42:19 +0000

Oh yes. Sorry that is what I meant. Fixed now

By: Maarten

Maarten — Fri, 24 Nov 2006 08:15:59 +0000

The query of you’re last example reeds EXPLAIN SELECT j FROM t WHERE i=5 AND k LIKE “a%”, but don’t you mean EXPLAIN SELECT k FROM t WHERE i=5 AND j LIKE “a%” ?