Horton

Re: psql's FETCH_COUNT (cursor) is not being respected for CTEs

Daniel Verite <daniel@manitou-mail.org>

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From: "Daniel Verite" <daniel@manitou-mail.org>
To: "Tom Lane" <tgl@sss.pgh.pa.us>
Cc: Laurenz Albe <laurenz.albe@cybertec.at>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>, Robert Haas <robertmhaas@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>
Date: 2024-04-08T14:25:36Z
Lists: pgsql-hackers 
	Tom Lane wrote:

> I've reconsidered after realizing that implementing FETCH_COUNT
> atop traditional single-row mode would require either merging
> single-row results into a bigger PGresult or persuading psql's
> results-printing code to accept an array of PGresults not just
> one.  Either of those would be expensive and ugly, not to mention
> needing chunks of code we don't have today.

Yes, we must accumulate results because the aligned format needs to
know the columns widths for a entire "page", and the row-by-row logic
does not fit that well in that case.
One of the posted patches implemented this with an array of PGresult
in single-row mode [1] but I'm confident that the newer version you
pushed with the libpq changes is a better approach.

> So I whacked the patch around till I liked it better, and pushed it.

Thanks for taking care of this!


[1]
https://www.postgresql.org/message-id/092583fb-97c5-428f-8d99-fd31be4a5290@manitou-mail.org


Best regards,
-- 
Daniel Vérité
https://postgresql.verite.pro/
Twitter: @DanielVerite

Commits

  1. Further review for re-implementation of psql's FETCH_COUNT feature.

  2. Re-implement psql's FETCH_COUNT feature atop libpq's chunked mode.

  3. Support retrieval of results in chunks with libpq.

  4. Attempt to fix newly added Memoize regression test