From: Tom Lane <tgl@sss.pgh.pa.us>
To: Gurjeet Singh <gurjeet@singh.im>
cc: PostgreSQL Docs <pgsql-docs@postgresql.org>,
	PGSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: [DOCS] synchronize_seqscans' description is a bit misleading
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Comments: In-reply-to Gurjeet Singh <gurjeet@singh.im>
	message dated "Wed, 10 Apr 2013 23:39:38 -0400"
Date: Wed, 10 Apr 2013 23:56:44 -0400
Message-ID: <19284.1365652604@sss.pgh.pa.us>
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Gurjeet Singh <gurjeet@singh.im> writes:
> On Wed, Apr 10, 2013 at 11:10 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> The point you're missing is that the synchronization is self-enforcing:

> Let's consider a pathological case where a scan is performed by a user
> controlled cursor, whose scan speed depends on how fast the user presses
> the "Next" button, then this scan is quickly going to fall out of sync with
> other scans. Moreover, if a new scan happens to pick up the block reported
> by this slow scan, then that new scan may have to read blocks off the disk
> afresh.

Sure --- if a backend stalls completely, it will fall out of the
synchronized group.  And that's a good thing; we'd surely not want to
block the other queries while waiting for a user who just went to lunch.

> So, again, it is not guaranteed that all the scans on a relation will
> synchronize with each other. Hence my proposal to include the term
> 'probability' in the definition.

Yeah, it's definitely not "guaranteed" in any sense.  But I don't really
think your proposed wording is an improvement.  The existing wording
isn't promising guaranteed sync either, to my eyes.

Perhaps we could compromise on, say, changing "so that concurrent scans
read the same block at about the same time" to "so that concurrent scans
tend to read the same block at about the same time", or something like
that.  I don't mind making it sound a bit more uncertain, but I don't
think that we need to emphasize the probability of failure.

			regards, tom lane


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