On Fri, Oct 14, 2016 at 1:40 PM, Masahiko Sawada <sawada.m...@gmail.com> wrote: > Hi all, > > I have a question about behavior of snapshot too old feature. > > For example, I set old_snapshot_threshold = 1min and prepare a table > and two terminals. > And I did the followings steps. > > 1. [Terminal 1] Begin transaction and get snapshot data and wait. > BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ; > SELECT * FROM test; > > 2. [Terminal 2] Another session updates test table in order to make > snapshot dirty. > BEGIN; > UPDATE test SET c = c + 100; > COMMIT; > > 3. [Terminal 1] 1 minute after, read the test table again in same > transaction opened at #1. I got no error. > SELECT * FROM test; > > 4. [Terminal 2] Another session reads the test table. > BEGIN; > SELECT * FROM test; > COMMIT; > > 5. [Terminal 1] 1 minute after, read the test table again, and got > "snapshot error" error. > SELECT * FROM test; > > Since #2 makes a snapshot I got at #1 dirty, I expected to get > "snapshot too old" error at #3 where I read test table again after > enough time. But I could never get "snapshot too old" error at #3. >
Here, the basic idea is that till the time corresponding page is not pruned or table vacuuming hasn't triggered, this error won't occur. So, I think what is happening here that during step #4 or step #3, it has pruned the table, after which you started getting error. -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
