Re: [POC] FETCH limited by bytes.

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: pgsql-hackers(at)postgresql(dot)org, mkellycs(at)gmail(dot)com, ashutosh(dot)bapat(at)enterprisedb(dot)com
Subject: Re: [POC] FETCH limited by bytes.
Date: 2015-01-22 16:17:52
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Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> writes:
> Hello, as the discuttion on async fetching on postgres_fdw, FETCH
> with data-size limitation would be useful to get memory usage
> stability of postgres_fdw.

> Is such a feature and syntax could be allowed to be added?

This seems like a lot of work, and frankly an incredibly ugly API,
for a benefit that is entirely hypothetical. Have you got numbers
showing any actual performance win for postgres_fdw?

Even if we wanted to do something like this, I strongly object to
measuring size by heap_compute_data_size. That's not a number that users
would normally have any direct knowledge of; nor does it have anything
at all to do with the claimed use-case, where what you'd really need to
measure is bytes transmitted down the wire. (The difference is not small:
for instance, toasted values would likely still be toasted at the point
where you're measuring.)

regards, tom lane

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