From: | Greg Stark <gsstark(at)mit(dot)edu> |
---|---|
To: | Stephen Frost <sfrost(at)snowman(dot)net> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Pre-alloc ListCell's optimization |
Date: | 2011-05-27 03:58:58 |
Message-ID: | BANLkTimonxq4q2aVDohb5ww+MXZrRY8nmg@mail.gmail.com |
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On Thu, May 26, 2011 at 8:52 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
> list_concat() does explicitly say that cells will
> be shared afterwards and that you can't pfree() either list (note that
> there's actually a couple cases currently that I discovered which were
> also addressed in the original patch where I commented out those
> pfree()'s).
So in traditional list it would splice the second argument onto the
end of the first list. This has a few effects that it sounds like you
haven't preserved. For example if I insert an element anywhere in
list2 -- including in the first few elements -- it's also inserted
into list1.
I'm not really sure we care about these semantics with our lists
though. It's not like they're supposed to be a full-featured lisp
emulator and it's not like the C code pulls any particularly clever
tricks with lists. I suspect we may have already broken these
semantics long ago but I haven't looked to see if that's the case.
--
greg
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