[Sirupsen]

I'm not completely sure I understand your comment, so pardon me if I misunderstand. I don't think a foreign data wrapper would fundamentally to be more efficient with whatever table is ~foreign~, especially for an OUTER JOIN? Unless you're basically implementing something similar to data-diff with an OUTER JOIN with FDW, which seems possible

If you're doing in-database diffs, however, a join-based approach will likely outperform data-diff though.

Ideally databases would have support a standard MERKLE TREE INDEX so we could get extremely fast comparisons.

[cryptonector]

A naive FULL OUTER JOIN is O(N), which is not efficient, indeed.

An RDBMS could implement something like the rsync algorithm, or history tables, etc., to speed up a FULL OUTER JOIN.

The point is that FULL OUTER JOIN is the SQL table source "diff" primitive. Thus it seems natural to use that and let the RDBMS optimize it.