Re: Accounting of zero-filled buffers in EXPLAIN (BUFFERS)

From: Andres Freund <andres(at)anarazel(dot)de>
To: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>
Cc: Pg Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Accounting of zero-filled buffers in EXPLAIN (BUFFERS)
Date: 2018-04-30 03:13:18
Message-ID: 20180430031318.k3skm4gfy7okcsnr@alap3.anarazel.de
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On 2018-04-30 14:59:31 +1200, Thomas Munro wrote:
> Hi,
>
> In EXPLAIN (BUFFERS), there are two kinds of cache misses that show up
> as "reads" when in fact they are not reads at all:
>
> 1. Relation extension, which in fact writes a zero-filled block.
> 2. The RBM_ZERO_* modes, which provoke neither read nor write.

Just for understanding: 2) can never happen for buffers showing up in
EXPLAIN, right?

I'm not saying you shouldn't fix the accounting...

Greetings,

Andres Freund

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