(From #1875 (comment):)

All this seems to be in line with the general policy that stuff in EF.Functions corresponds directly to SQL functions (as much as possible) [...]

@roji Is there an existing/recommended pattern for adding convenience methods that the database does not implement (but we translate into working SQL anyway) and that complement methods in EF.Functions?

An example would be from the OP:

Add the following convenience translations:

• DateDiffMillisecond
• DateDiffTick
• DateDiffNanosecond

Hmm, I'm not sure we have such convenience methods at the moment - I'd have to look.

OTOH I'd introduce these on EF.Functions alongside methods that translate directly to database functions - I'm not sure the user should care whether something translates directly to a SQL function or to a more elaborate construct.

BTW note that an alternative to providing an EF.Functions convenience thing, is to pattern-match a more complex construct in the query tree, i.e. where we avoid introducing a special function. That may be preferable - as long as it makes sense and allows expressing the same thing without a special thing...