Re: Sort Performance
Without the ORDER BY,
the full table is scanned is "sequential" manner.
With the ORDER BY,
the full index is scanned in a sorted order, and the selected rows are retrieved selectively one by one "randomly" based on the sorted order. No actual sorting is required.
This index `security` on a 255 varchar can be as large as 130 MB in size with 500,000 records, based on your table structure, this is close to one third of your total table size.
Anyway, I still do not have an answer for the great difference in the time taken. Maybe it is time to check your server buffer size, maybe "innodb_buffer_pool_size", not sure if this is the correct one or not, need to check.
You should do an EXPLAIN on the IGNORE INDEX as well, because sometimes MySQL do not follow that instruction strictly. Maybe the index is still used.
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