Different encoding behavior for PersistDbSpecific with persistent-postgresql and persistent-mysql

What is the discrepancy?

persistent-mysql and persistent-postgresql encode PersistDbSpecific differently.

persistent-mysql uses Plain:

https://github.com/yesodweb/persistent/blob/31e25c41c648fbbf5cca7cb79665482d008931bf/persistent-mysql/Database/Persist/MySQL.hs#L239

However, persistent-postgresql uses Escape (effectively, treating it as a string-like value):

https://github.com/yesodweb/persistent/blob/31e25c41c648fbbf5cca7cb79665482d008931bf/persistent-postgresql/Database/Persist/Postgresql.hs#L554

When was this introduced?

The behavior for PostgreSQL changed with 9287943c1dd4ca0c9822a715c057f4fb2ab6888e.

Why is this a problem?

With the current behavior it is not possible to encode something like point '(2.0,0)' for PostgreSQL.

(I would also be very surprised if the example given in the docs for PersistDbSpecific is still anywhere close working; but I haven't tried that specifically.)

How to address this?

I'm not sure how to best address this. Altering how PersistDbSpecific is encoded would be a breaking change that isn't detected by the type checker. One solution might be to remove PersistDbSpecific and add two new constructors:

• PersistDbSpecificEscape, which uses Escape (current behavior of persistent-postgresql)
• PersistDbSpecificPlain, which uses Plain (current behavior of persistent-mysql)

Compare the PersistDbSpecific handling in persistent-mysql/Database/Persist/MySQL.hs and persistent-postgresql/Database/Persist/Postgresql.hs, including the referenced PostgreSQL change 9287943c1dd4ca0c9822a715c057f4fb2ab6888e. First establish the intended distinction between Plain and Escape, then determine an agreed backward-compatible API and verify that the documented PostgreSQL example works with the resulting behavior.