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I don't know mysql/mariaDB that well, but I know in ms sql server that doing a DISTINCT on a Function([Column]) would not use an index on [Column].

In this case it would be much faster to have an intermediate table or column with that statistic that could then be properly indexed.

It even has a name - SARGable - able to use an index on the [S]earch [ARG]uments but still easy to get tripped up.

Depending on the function. The query optimizer is smart enough to recognize many starts-with scenarios, as one example.