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Good news! Many database systems support indexes on functions, including on user-defined functions. The only one I've used is Postgres[0] but apparently MySQL added support recently as well[1]

[0] https://www.postgresql.org/docs/current/indexes-expressional...

[1] https://dev.mysql.com/doc/refman/8.0/en/create-index.html#cr...

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That's cool, but then I'd assume you have to make an index for each function. In general if a person is asking basic questions about sargability, I will steer them away from functions on a predicate (it's usually my last choice.)

Often you can just think a bit harder about your set based logic and make an index that's reusable by other queries.

In other SQL's you can also make a materialized view, which may be able to persist some of these things, its just like keeping n+1 copies of your data to satisfy a different set of relations/projections.

SQLite has it too (caveats, like always, apply): https://www.sqlite.org/expridx.html.

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