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Say I have a simple query like this:
SELECT * FROM topics ORDER BY last_post_id DESC In accordance to MySQL’s Order-By Optimization Docs, an index on last_post_id should be utilized. But EXPLAINing the query tells the contrary:
id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE topic_info ALL NULL NULL NULL NULL 13 Using filesort Why is my index not being utilized?
Do you really only have 13 rows? The database may be deciding that simply sorting them is quicker than going via the index.