Quite common beginners mistake is not to understand how indexing works and so index all columns used in the queries…. separately. So you end up with table which has say 20 indexes but all single column ones. This can be spotted with a glance view. If you have queries with multiple column restrictions in WHERE clause you most likely will need to have multiple column indexes for optimal performance. But wait. Do not go ahead and index all combinations. This would likely be poor choice too
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About Peter Zaitsev
Peter managed the High Performance Group within MySQL until 2006, when he founded Percona. Peter has a Master's Degree in Computer Science and is an expert in database kernels, computer hardware, and application scaling.