Re: Index causing much slower query

[Scott Noyes]

> Perhaps the index with (intcol, datetimecol) is too big to fit in memory while the other index (intcol) fits in memory?

That's a likely cause; partitions seem to be a good fit for data warehouses where keys would be too large to fit in memory, meaning lots of disk access.

Some benchmarks on large data sets without primary keys are discussed in Giuseppe's article here:

http://dev.mysql.com/tech-resources/articles/testing-partitions-large-db.html