MySql - slow sending data phase

Question

One of my queries on MySQL 5.0.45 is running slow in \"sending data\" phase. The query is a simple select, returns about 300 integer ID fields as result set.

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Answer 1

I had two index (date_index and id) ,

i had WHERE date_index>NOW() - INTERVAL 24 HOURS and an ORDER BY id in query, MySql preferred id as index and it didn't use date_index that caused long query time for big tables.

i found it in a legacy system after 5 years that tables was grown.