[hobs]

Generally any equality is fine, but you'd usually do something like on mycustomfunction(a) = mycustomfuntion(b)

And if you do this, it will be incredibly slow because you cant use any indexes in this case.

[mhotchen]

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...

[hobs]

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.

[TeMPOraL]

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

[sk1pper]

You can create indexes on expressions, like mycustomfunction(a), in Postgres at least.