Re: BUG #14687: pg_xlogdump does only count "main data" for record length and leading to incorrect statistics

On 06/05/2017 05:25 AM, Andres Freund wrote:
> Hi Heikki,
>
> On 2017-06-04 16:10:12 -0700, Andres Freund wrote:
>> On 2017-06-03 16:59:39 +0000, chjischj(at)163(dot)com wrote:
>>> Currently, record length in pg_xlogdump/pg_waldump's output only count
>>> length of "main data",
>>
>> Good catch! Will fix.
>
> I notice that this seems to originate in
> https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=ebc2b681b8ffa597137aa7a19619e6a21176e928
> https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=2c03216d831160bedd72d45f712601b6f7d03f1c
> and that it's not just stats, but also plain xlogdump displaying wrong
> data.
>
> Do you see any better way to get at the !fpi len than to iterate over
> the blocks and sum up the image length? It's not really accurate to
> display len (rec/tot) as total-fpi, total, but I don't have a better
> backward-compatible idea? Actually, I don't even have a better idea for
> master, so maybe it's ok as proposed?

Your patch seems OK to me. Defining non-FPI size as (total_len -
fpi_len) seems correct to me, I'm not sure what other definition you
could have.

> + /*
> + * Calculate the length of the record as the total length - the length of
> + * all the block images. That's commonly more accurate than just using
> + * XLogRecGetDataLen(), because that's only the main record, not per-block
> + * data registered wity XLogRegisterBufData().
> + */

s/wity/with/. But I would leave out the whole sentence starting with
"That's commonly more accurate ...". Using XLogRecGetDataLen() would be
(and was) just plain wrong.

- Heikki