From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <david.rowley@2ndquadrant.com>
cc: Jesper Pedersen <jesper.pedersen@redhat.com>,
        Andres Freund <andres@anarazel.de>,
        Robert Haas <robertmhaas@gmail.com>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: POC: converting Lists into arrays
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Comments: In-reply-to David Rowley <david.rowley@2ndquadrant.com>
	message dated "Mon, 22 Jul 2019 20:34:13 +1200"
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Date: Mon, 22 Jul 2019 10:50:20 -0400
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David Rowley <david.rowley@2ndquadrant.com> writes:
> I looked over this and only noted down one thing:

> In estimate_path_cost_size, can you explain why list_concat_copy() is
> needed here? I don't see remote_param_join_conds being used after
> this, so might it be better to just get rid of remote_param_join_conds
> and pass remote_conds to classifyConditions(), then just
> list_concat()?

Hm, you're right, remote_param_join_conds is not used after that,
so we could just drop the existing list_copy() and make it

        remote_conds = list_concat(remote_param_join_conds,
                                   fpinfo->remote_conds);

I'm disinclined to change the API of classifyConditions(),
if that's what you were suggesting.

>> It turns out there are a *lot* of places where list_concat() callers
>> are now leaking the second input list (where before they just leaked
>> that list's header).  So I've got mixed emotions about the choice not
>> to add a variant function that list_free's the second input.

> In some of these places, for example, the calls to
> generate_join_implied_equalities_normal() and
> generate_join_implied_equalities_broken(), I wonder, since these are
> static functions if we could just change the function signature to
> accept a List to append to.

I'm pretty disinclined to do that, too.  Complicating function APIs
for marginal performance gains isn't something that leads to
understandable or maintainable code.

> If we do end up with another function, it might be nice to stay away
> from using "concat" in the name. I think we might struggle if there
> are too many variations on concat and there's a risk we'll use the
> wrong one.  If we need this then perhaps something like
> list_append_all() might be a better choice... I'm struggling to build
> a strong opinion on this though. (I know that because I've deleted
> this paragraph 3 times and started again, each time with a different
> opinion.)

Yeah, the name is really the sticking point here; if we could think
of a name that was easy to understand then the whole thing would be
much easier to accept.  The best I've been able to come up with is
"list_join", by analogy to bms_join for bitmapsets ... but that's
not great.

			regards, tom lane