Date: Mon, 24 Sep 2018 21:38:52 -0500
From: Justin Pryzby <pryzby@telsasoft.com>
To: Vladimir Ryabtsev <greatvovan@gmail.com>
Cc: f.pardi@portavita.eu, pgsql-performance@lists.postgresql.org
Subject: Re: Why could different data in a table be processed with different
 performance?
Message-ID: <20180925023852.GX2471@telsasoft.com>
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On Mon, Sep 24, 2018 at 05:59:12PM -0700, Vladimir Ryabtsev wrote:
> > This seems significant..it means the heap was probably written in
> backwards
> order relative to the IDs, and the OS readahead is ineffective when index
> scanning across a range of IDs.
> But again, why is it different for one range and another? It was reversed
> for both ranges.

I don't have an explaination for it.. but I'd be curious to know
pg_stats.correlation for the id column:

SELECT schemaname, tablename, attname, correlation FROM pg_stats WHERE tablename='articles' AND column='article_id' LIMIT 1;

Justin