Query:
SELECT *, history_count as `count`
FROM pdf_history
WHERE 1 AND history_date>=1426180929 AND history_count!=0
EXPLAIN
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE pdf_history ALL history_date,history_count NULL NULL NULL 697 Using where
One of the reason optimizer choose not to use the index is when the filter doesn't reduce the search space.
for example if
history_date>=1426180929
or
history_count!=0
already bring all the records then using the index doesnt really help.
My suggestion do this both querys and check the ANALYZE
to see how many of the 600 records are match that filter
SELECT count(*) FROM pdf_history WHERE history_date>=1426180929;
SELECT count(*) FROM pdf_history WHERE history_count!=0;
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