+ * Although session variables are not transactional, we don't + * want (and we cannot) to run cleaning immediately (when we + * got sinval message). The value of session variables can + * be still used or the operation that emits cleaning can be + * reverted. Unfortunatelly, this check can be done only in + * when transaction is committed (the check against system + * catalog requires transaction state).
This was the original idea, but since there's now locking to make all DDL safe, the metadata should be considered fully transactional and no session should still be able to use a concurrently dropped variable. Also, the invalidation messages are not sent until the transaction is committed. So is that approach still needed (at least for things outside ON COMMIT DROP / ON TRANSACTION END RESET
I think this is still necessary. The lock protects the variable against drop from the second session, but not for reverted deletion from the current session.
This implementation is due Tomas's request for
CREATE VARIABLE xx AS int;
LET xx = 100;
BEGIN;
DROP VARIABLE xx;
ROLLBACK;
SELECT xx; --> 100
and the variable still holds the last value before DROP
Personally, this is a corner case (for me, and I think so for users it is not too interesting, and important), and this behavior is not necessary - originally I implemented just the RESET variable in this case. On the other hand, this is a nice feature, and there is an analogy with TRUNCATE behavior.
More, I promised, as a second step, implementation of optional transactional behavior of session variables. And related code is necessary for it. So I prefer to use related code without change.
