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Re: xlog location arithmetic

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Euler Taveira de Oliveira <euler(at)timbira(dot)com>, Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: xlog location arithmetic
Date: 2012-03-09 15:31:26
Message-ID: CA+TgmoacKyNb-omk=csGESfZDEqL+=5am9JsPh7+G568NpZNbA@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Fri, Mar 9, 2012 at 9:55 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> I wrote:
>> Fujii Masao <masao(dot)fujii(at)gmail(dot)com> writes:
>>> Euler proposed one more patch upthread, which replaces pg_size_pretty(bigint)
>>> with pg_size_pretty(numeric) so that pg_size_pretty(pg_xlog_location_diff())
>>> succeeds. It's also worth committing this patch?
>
>> Why would it be useful to use pg_size_pretty on xlog locations?
>> -1 because of the large expense of bigint->numeric->whatever conversion
>> that would be added to existing uses.
>
> Actually ... now that I look at it, isn't it completely bogus to be
> using numeric for the result of pg_xlog_location_diff?  There's no way
> for the difference of two xlog locations to be anywhere near as wide as
> 64 bits.  That'd only be possible if XLogFileSize exceeded 1GB, which we
> don't let it get anywhere near.

rhaas=# select pg_xlog_location_diff('ffffffff/0', '0/0');
 pg_xlog_location_diff
-----------------------
  18374686475393433600
(1 row)

rhaas=# select pg_xlog_location_diff('ffffffff/0', '0/0')::int8;
ERROR:  bigint out of range
STATEMENT:  select pg_xlog_location_diff('ffffffff/0', '0/0')::int8;

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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