Re: POC: converting Lists into arrays

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
Cc: Jesper Pedersen <jesper(dot)pedersen(at)redhat(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Robert Haas <robertmhaas(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: POC: converting Lists into arrays
Date: 2019-07-15 19:49:50
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I wrote:
> BTW, further on the subject of performance --- I'm aware of at least
> these topics for follow-on patches:
> ...
> * Adjust API for list_qsort(), as discussed, to save indirections and
> avoid constructing an intermediate pointer array. I also seem to recall
> one place in the planner that's avoiding using list_qsort by manually
> flattening a list into an array, qsort'ing, and rebuilding the list :-(

Here's a proposed patch for that. There are only two existing calls
of list_qsort(), it turns out. I didn't find the described spot in
the planner (I believe I was thinking of choose_bitmap_and(), but its
usage isn't quite as described). However, I found about four other
places that were doing pretty much exactly that, so the attached
also simplifies those places to use list_qsort().

(There are some other places that could perhaps be changed also,
but it would require more invasive hacking than I wanted to do here,
with less-clear benefits.)

A possibly controversial point is that I made list_qsort() sort the
given list in-place, rather than building a new list as it has
historically. For every single one of the existing and new callers,
copying the input list is wasteful, because they were just leaking
the input list anyway. But perhaps somebody feels that we should
preserve the "const input" property? I thought that changing the
function to return void, as done here, might be a good idea to
ensure that callers notice its API changed --- otherwise they'd
only get a warning about incompatible signature of the passed
function pointer, which they might not notice; in fact I'm not
totally sure all compilers would even give such a warning.

If there's not complaints about that, I'm just going to go ahead
and push this --- it seems simple enough to not need much review.

regards, tom lane

Attachment Content-Type Size
list_qsort-API-change.patch text/x-diff 16.1 KB

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