On Mon, Jun 10, 2019 at 1:30 PM Alex <zhihui.fan1...@gmail.com> wrote: > > > > On Mon, Jun 10, 2019 at 3:28 PM Kuntal Ghosh <kuntalghosh.2...@gmail.com> > wrote: >> >> On Mon, Jun 10, 2019 at 12:15 PM Alex <zhihui.fan1...@gmail.com> wrote: >>> >>> HEAPTUPLE_RECENTLY_DEAD, /* tuple is dead, but not deletable yet */ >>> >>> It is a tuple which has been deleted AND committed but before the delete >>> there is a transaction started but not committed. Let call this transaction >>> as Transaction A. >>> >>> if we create index on this time, Let's call this index as Index A， it still >>> index this record. my question is why need this. >>> >> In this case, the changes of the tuple is not visible yet. Now suppose, your >> transaction A is serializable and you've another serializable transaction B >> which can see the index A. It generates a plan that requires to fetch the >> deleted tuple through an index scan. If the tuple is not present in the >> index, how are you going to create a conflict edge between transaction A and >> transaction B? >> >> Basically, you need to identify the following clause to detect serializable >> conflicts: >> Transaction A precedes transaction B. (Because, transaction A has deleted a >> tuple and it's not visible to transaction B) >> > > thanks Ghosh. Looks your answer is similar with my previous point > (transaction is serializable). actually if the transaction B can't see the > “deleted" which has been committed, should it see the index A which is > created after the "delete" transaction? > I think what I'm trying to say is different.
For my case, the sequence is as following: 1. Transaction A has deleted a tuple, say t1 and got committed. 2. Index A has been created successfully. 3. Now, transaction B starts and use the index A to fetch the tuple t1. While doing visibility check, transaction B gets to know that t1 has been deleted by a committed transaction A, so it can't see the tuple. But, it creates a dependency edge that transaction A precedes transaction B. This edge is required to detect a serializable conflict failure. If you don't create the index entry, it'll not be able to create that edge. -- Thanks & Regards, Kuntal Ghosh EnterpriseDB: http://www.enterprisedb.com
