At Sun, 17 Jan 2021 23:02:18 -0800, Noah Misch <n...@leadboat.com> wrote in > On Sun, Jan 17, 2021 at 10:36:31PM -0800, Noah Misch wrote: > > I wrote the above based on the "PageGetLSN(page) > (snapshot)->lsn" check in > > TestForOldSnapshot(). If the LSN isn't important, what else explains > > RelationAllowsEarlyPruning() checking RelationNeedsWAL()? > > Thinking about it more, some RelationAllowsEarlyPruning() callers need > different treatment. Above, I was writing about the case of deciding whether > to do early pruning. The other RelationAllowsEarlyPruning() call sites are > deciding whether the relation might be lacking old data. For that case, we > should check RELPERSISTENCE_PERMANENT, not RelationNeedsWAL(). Otherwise, we > could fail to report an old-snapshot error in a case like this: > > setup: CREATE TABLE t AS SELECT ...; > xact1: BEGIN ISOLATION LEVEL REPEATABLE READ; SELECT 1; -- take snapshot > xact2: DELETE FROM t; > (plenty of time passes) > xact3: SELECT count(*) FROM t; -- early pruning > xact1: SAVEPOINT q; SELECT count(*) FROM t; ROLLBACK TO q; -- "snapshot too > old" > xact1: ALTER TABLE t SET TABLESPACE something; -- start skipping WAL > xact1: SELECT count(*) FROM t; -- no error, wanted "snapshot too old" > > Is that plausible?
Thank you for the consideration and yes. But I get "snapshot too old" from the last query with the patched version. maybe I'm missing something. I'm going to investigate the case. regards. -- Kyotaro Horiguchi NTT Open Source Software Center