I have a simple select query:
SELECT *
FROM `inventory`
WHERE account_id = "1"
AND marketplace_id = "2"
AND sku = "3"
AND `date` = "4"
It has a condition in four fields, therefore, the natural index is of these four fields.
I have the following indexes:
and when I explain the query, I get:
Saying it is using the 2nd index from the list, rather than the 3rd or 4th.
If I use USE INDEX(account_id__marketplace_id__sku__date) in my query, it successfully uses it, and filters 100%.
I have tried ANALYZE TABLE and OPTIMIZE TABLE to rebuild the indexes, but the situation stays the same!
From my experience with other tables, if I now drop indexes 2 and 3, and recreate them, such that the largest index is the first in the order, the query will select that index, meaning, it selects the first that matches rather a maximal match.
- MySQL Server version: 5.7.25-google-log
- phpMyAdmin MySQL client version: 5.5.59
Mysql will choose index by the table's data size;
You can try to insert 600000 records into inventory, keep record's value random, and run "Explain"