Re: Why to index a "Recently DEAD" tuple when creating index

On Mon, Jun 10, 2019 at 12:15 PM Alex <zhihui(dot)fan1213(at)gmail(dot)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 & Regards,
Kuntal Ghosh
EnterpriseDB: http://www.enterprisedb.com