Re: POC: converting Lists into arrays

From: Andres Freund <andres(at)anarazel(dot)de>
To: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, 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: 2019-03-04 23:54:02
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On 2019-03-05 12:42:47 +1300, David Rowley wrote:
> So you think targetlists are the only case to benefit from an array
> based list? (Ignoring the fact that I already showed another case)

No, that's not what I'm trying to say at all. I think there's plenty
cases where it'd be beneficial. In this subthread we're just arguing
whether it's somewhat feasible to not change everything, and I'm still
fairly convinced that's possible; but I'm not arguing that that's the
best way.

> It's true that linked lists are certainly better for some stuff;
> list_concat() is going to get slower, lcons() too, but likely we can
> have a bonus lcons() elimination round at some point. I see quite a
> few of them that look like they could be changed to lappend(). I also
> just feel that if we insist on more here then we'll get about nothing.
> I'm also blocked on my partition performance improvement goals on
> list_nth() being O(N), so I'm keen to see progress here and do what I
> can to help with that. With list_concat() I find that pretty scary
> anyway. Using it means we can have a valid list that does not get it's
> length updated when someone appends a new item. Most users of that do
> list_copy() to sidestep that and other issues... which likely is
> something we'd want to rip out with Tom's patch.

Yes, I think you have a point that progress here would be good and that
it's worth some pain. But the names will make even less sense if we just
shunt in an array based approach under the already obscure list
API. Obviously the individual pain of that is fairly small, but over the
years and everybody reading PG code, it's also substantial. So I'm


Andres Freund

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