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Asked: 2026-06-09T00:32:24+00:00

So whenever I EXPLAIN my query, I often get instances in which it states

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So whenever I EXPLAIN my query, I often get instances in which it states certain fields as possible keys but then the key table will be null…

Why would MySQL do this and decide to use no keys when in fact it can use a possible key?

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  1. Editorial Team

    It can’t always use a key, for instance:

    SELECT * FROM A, B where A.id = B.id;

    Although id is a possible key for both lookups, it can’t be used for both. One of the tables must do a full scan.

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