Steps to reproduce the issue

Apply PR #13291 which fixes another search related issue.
Add a Search module (not Smart Search) and use it to search for any word.

Expected result

No error.

Actual result

Additional comments

Examining the debug information we see that the error is caused by comparing two column of different types, #__fields_values.item_id (varchar) and #__content.id (integer), which can't be done on PostgreSQL without typecasting.

@laoneo I presume there was a reason for making #__fields_values.item_id column a varchar, like enabling support for third party extensions with non-numerical ids?