However when the 'sex' column is involved:
mysql> select count(*) from sex where id>459000 and id <=460000 and sex = 'M'; +----------+ | count(*) | +----------+ | 504 | +----------+ 1 row in set (5.09 sec)
Any way to make this faster ?
Well, MySql can only use 1 index per table to optimize a query.
It's apparently using the index on ID, so it then needs to examine all records in the right ID range to see if they meet the sex='M' condition.
You could build an index on both fields as one index, and MySql should be able to use it to resolve both parts of the query.
create index id_and_sex_index on sex (id,sex);
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