On Tue, Mar 20, 2018 at 11:41 PM, Masahiko Sawada <sawada.m...@gmail.com> wrote: > Hi, Amit > > On Tue, Mar 20, 2018 at 8:02 PM, Amit Kapila <amit.kapil...@gmail.com> wrote: >> On Tue, Mar 20, 2018 at 12:22 PM, Masahiko Sawada <sawada.m...@gmail.com> >> wrote: >>> Hi, >>> >>> Long transactions often annoy users because if a long transaction >>> exists on a database vacuum cannot reclaim efficiently. There are >>> several reason why they exist on a database but it's a common case >>> where users or applications forget to commit/rollback transactions. >>> That is, transaction is not executing SQL and its state is 'idle in >>> transaction' on pg_stat_activity. In this case, such transactions >>> don't affect vacuum's job either if they aren't assigned transaction >>> id or if they don't have a snapshot. However if they have xid it will >>> affect vacuum's job even if they don't have a snapshot. >>> >>> I think that to decide which deleted tuples must be preserved we don't >>> need to care about backend PGXACT.xid but must care about PGXACT.xmin. >>> But current GetOldestXmin considers both of them. I guess one reason >>> why GetOldestXmin does so is that it's also used to determine where to >>> truncate pg_subtrans. Is there anything else reason? >>> >> >> I think the main reason is that while computing snapshots, we also >> rely on PGXACT.xid. Basically, it can be present in some other >> snapshots xmin. Now, if you ignore it in vacuum (GetOldestXmin), then >> it is quite possible that the xid we have ignored will be part of some >> other snapshot's xmin which I think in turn can lead to wrong results. >> > > Sorry I'm still confusing. You meant that it's possible that an xmin > of a snapshot can be older than the oldest PGXACT.xmin? If it's > possible I'm sure the problem happens but I'm not sure it can happen > because PGXACT.xmin is the oldest xid when taking a snapshot. I think > that the oldest PGXACT.xmin can be either the same as or younger than > the oldest PGXACT.xid. >
I sent the previous mail before checking the mail by Tom, and now I understood that the necessity of including PGXACT.xid. Sorry for the stupid question. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center
