> > > Hi, > > so let's suppose that the WAL is: > > LSN 10: start transaction 123 > > LSN 11: update tuple 100 > > checkpoint position here (not a record but just for understanding) > > LSN 12: update tuple 100 > > LSN 13: update tuple 100 > > LSN 14: checkpoint record ( postion=11) > > LSN 15: update tuple 100 > > and that the system crashes now, before ending to write all the > > transaction's recs to the WAL (other updates and commit record missing). > > > > At the replay, starting from LSN 12, the entire page we had at LSN 11 is > > written to the disk, though carrying inconsistent data. > > Then we can even replay up to the end of WAL but always getting > > inconsistent data. > > BUT, you say, as the tuple is not commited in the WAL, only the old > version > > of the tuple will be visible? Right? > > Yes -- all the updated tuples are invisible because the commit record > for transaction 123 does not appear in wal. A future VACUUM will remove > all those tuples. Note that precisely for this reason, the original > version of the tuple had not been removed yet. > > -- > Álvaro Herrera https://www.2ndQuadrant.com/ > PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services >
Ok! Now many things are clear to me.... Thank you very much Pupillo
