Thread

Re: Revisiting {CREATE INDEX, REINDEX} CONCURRENTLY improvements

Matthias van de Meent <boekewurm+postgres@gmail.com> — 2025-11-28T16:57:55Z

On Fri, 28 Nov 2025 at 15:50, Mihail Nikalayeu
<mihailnikalayeu@gmail.com> wrote:
>
> Hello!
>
> On Thu, Nov 27, 2025 at 9:07 PM Matthias van de Meent
> <boekewurm+postgres@gmail.com> wrote:
> > While it might not break, and might not hold back other tables'
> > visibility horizons, it'll still hold back pruning on the table we're
> > acting on, and that's likely one which already had bloat issues if
> > you're running RIC (or REPACK).
>
> Yes, a good point about REPACK, agreed.
>
> BTW, what is about using the same reset snapshot technique for REPACK also?
>
> I thought it is impossible, but what if we:
>
> * while reading the heap we "remember" our current page position into
> shared memory
> * preserve all xmin/max/cid into newly created repacked table (we need
> it for MVCC-safe approach anyway)
> * in logical decoding layer - we check TID of our tuple and looking at
> "current page" we may correctly decide what to do with at apply phase:
>
> - if it in "non-yet read pages" - ignore (we will read it later) - but
> signal scan to ensure it will reset snapshot before that page
> (reset_before = min(reset_before, tid))
> - if it in "already read pages" - remember the apply operation (with
> exact target xmin/xmax and resulting xmin/xmax)

Yes, exactly - keep track of which snapshot was used for which part of
the table, and all updates that add/remove tuples from the scanned
range after that snapshot are considered inserts/deletes, similar to
how it'd work if LR had a filter on `ctid BETWEEN '(0, 0)' AND
'(end-of-snapshot-scan)'` which then gets updated every so often.

I'm a bit worried, though, that LR may lose updates due to commit
order differences between WAL and PGPROC. I don't know how that's
handled in logical decoding, and can't find much literature about it
in the repo either.


Kind regards,

Matthias van de Meent
Databricks (https://www.databricks.com)