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From: Vitaliy Garnashevich <vgarnashevich@gmail.com>
To: Claudio Freire <klaussfreire@gmail.com>
Cc: hzzhangjiazhi <hzzhangjiazhi@corp.netease.com>
Cc: Gary Doades <gpd@gpdnet.co.uk>
Cc: Rick Otten <rottenwindfish@gmail.com>
Cc: pgsql-performance@lists.postgresql.org <pgsql-performance@lists.postgresql.org>
Subject: Re: effective_io_concurrency on EBS/gp2
Date: Mon, 5 Feb 2018 13:26:43 +0200
Message-ID: <7895b3f0-0871-2b04-d3a3-3ade175e7212@gmail.com> (raw)
In-Reply-To: <CAGTBQpZyXSxPuCFLBR3eg95=Lk3MexzaQdCGkuvH+vdKnN6gHg@mail.gmail.com>
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> I mean, that the issue is indeed affected by the order of rows in the
> table. Random heap access patterns result in sparse bitmap heap scans,
> whereas less random heap access patterns result in denser bitmap heap
> scans. Dense scans have large portions of contiguous fetches, a
> pattern that is quite adversely affected by the current prefetch
> mechanism in linux.
>

Thanks for your input.

How can I test a sparse bitmap scan? Can you think of any SQL commands 
which would generate data and run such scans?

Would a bitmap scan over expression index ((aid%1000)=0) do a sparse 
bitmap scan?

Regards,
Vitaliy




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