Re: pgsql: Add deduplication to nbtree.

On Sun, Mar 1, 2020 at 2:14 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Attached patch shows how this could work. I prefer my original
> > approach, but I can see the argument for doing it this way.
>
> This does seem a bit duplicative ... and shouldn't both code paths
> include a final "Assert(d == vacposting->ndeletedtids)"?

No, because the "nhtids == 1" loop has a "break" when the first and
only TID that we need to keep around is hit.

> Another thing that maybe bears closer scrutiny is the size calculation.
> It seems a bit confused as to whether the offset of the posting list
> within the tuple, or the total tuple size, or both, needs to be
> MAXALIGN'd.

That's not quite true in my opinion. BTreeTupleSetPosting() has an
Assert() of its own about alignment. ISTM that it's reasonable for
_bt_update_posting() to assume that BTreeTupleGetPostingOffset() will
return a MAXALIGN()'d offset. Note also that _bt_form_posting() is
explicit about what it expects around alignment. This is
_bt_update_posting()'s sibling function. The _bt_update_posting()
calculation is explicitly documented as being derived from the one in
_bt_form_posting().

I am happy to add parallel-to-_bt_form_posting() assertions about
alignment to _bt_form_posting(), to nail it down completely. Plus I'll
add the assertion I suggested already. Once I commit a patch with
these two new assertions, I think that we can consider the matter
resolved. Does that seem reasonable to you?

-- 
Peter Geoghegan