Re: Reuse the dead item on unique index.

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> Atsushi Ogawa <atsushi(dot)ogawa(at)gmail(dot)com> writes:
> > When _bt_check_unique finds a dead item that has same data as new
> > item, LP_DEAD is set to the item. Can we reuse this dead item instead
> > of inserting new item?
>
> This strikes me as a pretty bad idea for the same reason pointed out
> recently in other threads: the notion of equality embodied in a btree
> opclass' equals function may have little or nothing to do with true
> identity. So your assumption that it's the "same" data is faulty.

Thanks, I understand the problem.
When the size of new item and dead item is the equal, the new item can
be overwrited at the position of the dead item.

> Also, I'm dubious about the assumption that "can be marked LP_DELETED"
> is the same as "can be physically removed right now". The side-effects
> on indexscans happening concurrently with yours could be bad. At the
> very least you'd need to obtain super-exclusive lock (cf btbulkdelete)
> before doing the replacement.

I agree. I will add code that checks the refcount of buffer. If refcount
is 1, current process has super-exclusive lock, and we can overwrite the
dead item. If refcount > 1, I use _bt_insertonpg.

regards,

--- Atsushi Ogawa