OR

In general OR kills optimization.

WHERE a = 12 OR b = 78

cannot use INDEX(a,b), and may or may not use INDEX(a), INDEX(b) via “index merge”. Index merge is better than nothing, but only barely.

WHERE x = 3 OR x = 5

is turned into

WHERE x IN (3, 5)

which may use an index with x in it.

Found a mistake? Have a question or improvement idea? Let me know.

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