Question

Does anyone know how function indexes work in databases?

Answer 1

If you are talking about taking the result of a function when creating the index, this feature is used from SQL like this:

CREATE INDEX index_name ON table_name (function_name(column_name)); 

This can be used by the planner for queries like:

SELECT foo FROM table_name WHERE bar = function_name(column_name); 

For exact details see the given RDBMS’s documentation (for example in PostgreSQL it is called indexes on expressions).