Subject READ ONLY READ COMMITTED and consistency
Author Michael Ludwig
In a transaction configured with READ ONLY READ COMMITTED, is it
possible to see just part of a commit?

Imagine a rather long-running SELECT and an intervening UPDATE changing
two rows which will both be part of our SELECT's result set:

SELECT a FROM MyTable WHERE <going to take a while>;

While the SELECT is fetching data, in another transaction:

UPDATE MyTable SET a = :x WHERE <lightning-fast index lookup>;
-- affecting two rows in MyTable

My SELECT is still running and fetching data at this point.

Is it possible for my RO/RC transaction to see one of the two rows
affected by the intervening UPDATE in its state before the intervening
COMMIT, and the other in its state after that COMMIT?

This would not, if I understand correctly, constitute a DIRTY READ
(which Firebird does not allow), as both versions are committed, just
not by the same transaction.

It might nevertheless be undesirable.
Michael Ludwig