Re: Polyphase merge is obsolete

On 19/11/2022 13:00, Peter Eisentraut wrote:
> On 18.10.21 14:15, Heikki Linnakangas wrote:
>> On 05/10/2021 20:24, John Naylor wrote:
>>> I've had a chance to review and test out the v5 patches.
>>
>> Thanks! I fixed the stray reference to PostgreSQL 14 that Zhihong
>> mentioned, and pushed.
>
> AFAICT, this thread updated the API of LogicalTapeSetCreate() in PG15,
> but did not adequately update the function header comment. The comment
> still mentions the "shared" argument, which has been removed. There is
> a new "preallocate" argument that is not mentioned at all. Also, it's
> not easy to match the actual callers to the call variants described in
> the comment. Could someone who remembers this work perhaps look this
> over and update the comment?

Is the attached more readable?

I'm not 100% sure of the "preallocate" argument. If I understand
correctly, you should pass true if you are writing multiple tapes at the
same time, and false otherwise. HashAgg passed true, tuplesort passes
false. However, it's not clear to me why we couldn't just always do the
preallocation. It seems pretty harmless even if it's not helpful. Or do
it when there are multiple writer tapes, and not otherwise. The
parameter was added in commit 0758964963 so presumably there was a
reason, but at a quick glance at the thread that led to that commit, I
couldn't see what it was.

- Heikki