Answer
MySQL uses composite indexes left-to-right. If the first column is not used in filtering, the index cannot be used efficiently.
Quick Answer
Composite index (a, b, c): MySQL can use it for queries that filter on (a), (a, b), or (a, b, c) but not on (b) alone or (c) alone (leftmost prefix rule). Put the most selective and equality-matched column first. Range conditions should come last. index on (user_id, created_at) helps: WHERE user_id = 5 ORDER BY created_at - both filter and sort are covered.
S
SugharaIQ Editorial Team
Verified Answer
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Source: SugharaIQ