| From: | Marko Kreen <markokr(at)gmail(dot)com> |
|---|---|
| To: | Simon Riggs <simon(at)2ndquadrant(dot)com> |
| Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Greg Stark <gsstark(at)mit(dot)edu>, Stephen Frost <sfrost(at)snowman(dot)net>, Robert Haas <robertmhaas(at)gmail(dot)com>, Jeremy Kerr <jk(at)ozlabs(dot)org>, "<pgsql-hackers(at)postgresql(dot)org>" <pgsql-hackers(at)postgresql(dot)org>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>, Gurjeet Singh <singh(dot)gurjeet(at)gmail(dot)com> |
| Subject: | Re: [PATCH] backend: compare word-at-a-time in bcTruelen |
| Date: | 2009-06-18 12:14:41 |
| Message-ID: | e51f66da0906180514i1de3b6cdvc04e5134d416c200@mail.gmail.com |
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| Lists: | pgsql-hackers |
On 6/18/09, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On Tue, 2009-06-16 at 10:23 -0400, Tom Lane wrote:
> > Speaking of which, what about some performance numbers? Like Heikki,
> > I'm quite suspicious of whether there is any real-world gain to be had
> > from this approach.
>
>
> It has been "lore" for some time that VARCHAR is cheaper than
> VARCHAR(n), so I'm looking forward to this improvement as a real-world
> gain. (If done right).
>
> I've looked at the code and the thing that bothers me is that I can't
> see where or why bcTruelen would be called more often for VARCHAR(n)
> than it would be for VARCHAR, on a Select/Sort only workload.
I'd guess plain VARCHAR simply does not have blanks at the end,
so Truelen is cheap.
> Are we tuning the right thing? Is there some code we can completely
> avoid?
>
> If not, does this mean it is a generic effect? Does this imply that
> NUMERIC(n) is somehow worse than NUMERIC? etc..
Probably not. For numeric the (n) seems to be only checked input time.
--
marko
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