Re: POC: Cleaning up orphaned files using undo logs

On Fri, Jul 12, 2019 at 5:40 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> I am not sure but don't we need to retain the color of z as well?

I believe that would be very wrong. If you recolor an internal node,
you'll break the constant-black-height invariant.

> Apart from this, the duplicate key (ex. for size queues the size of
> two requests can be same) handling might need some work. Basically,
> either special combine function needs to be written (not sure yet what
> we should do there) or we always need to ensure that the key is unique
> like (size + start_urec_ptr). If the size is the same, then we can
> decide based on start_urec_ptr.

I think that this problem is somewhat independent of whether we use an
rbtree or a binaryheap or some other data structure. I would be
inclined to use XID as a tiebreak for the size queue, so that it's
more likely to match the ordering of the XID queue, but if that's
inconvenient, then some other arbitrary value like start_urec_ptr
should be fine.

> I think we can go by changing the implementation to rbtree by doing
> some enhancements instead of the binary heap or alternatively, we can
> use one of the two ideas suggested by you in the email above [1] to
> simplify the code and keep using the binary heap for now. Especially,
> I like the below one.
> "2. However, I don't think we should have a separate request object
> for each queue anyway. We should insert pointers to the same objects
> in all the relevant queue (either size + XID, or else error). So
> instead of having three sets of objects, one for each queue, we'd just
> have one set of objects and point to them with as many as two
> pointers.
> We'd therefore need LESS memory than we're using today, because we
> wouldn't have separate arrays for XID, size, and error queue
> elements."
>
> I think even if we currently go with a binary heap, it will be
> possible to change it to rbtree later, but I am fine either way.

Well, I don't see much point in revising all of this logic twice. We
should pick the way we want it to work and make it work that way.

--
Robert Haas
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