Re: embedded list

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
Cc: Andres Freund <andres(at)2ndquadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org, Peter Geoghegan <peter(at)2ndquadrant(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>
Subject: Re: embedded list
Date: 2012-10-18 20:15:47
Message-ID: 4201.1350591347@sss.pgh.pa.us
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Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> writes:
> Here's the final version. I think this is ready to go in.

I got around to reviewing this today. I'm pretty seriously annoyed at
the definition of dlist_delete: it should *not* require the list header.
The present coding simply throws away one of the primary advantages of
a doubly-linked list over a singly-linked list, namely that you don't
have to have your hands on the list header in order to unlink a node.
This isn't merely academic either, as I see that the patch to catcache
code actually added a field to struct catctup to support making the
list header available. That's a complete waste of 8 bytes (on a 64-bit
machine) per catalog cache entry. The only thing it buys for us is
the ability to run dlist_check, which is something that isn't even
compiled (not even in an Assert build), and which doesn't actually do
that much useful even if it is compiled --- for instance, there's no way
to verify that the nodes were actually in the list claimed.

I think we should remove the head argument at least from dlist_delete,
and probably also dlist_insert_after and dlist_insert_before.

regards, tom lane

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