Why does a SELECT query cause "dirtied" buffers?

From: Thomas Kellerer <spam_eater(at)gmx(dot)net>
To: pgsql-general(at)postgresql(dot)org
Subject: Why does a SELECT query cause "dirtied" buffers?
Date: 2017-10-27 19:24:40
Message-ID: ot015g$ml7$1@blaine.gmane.org
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When using explain (analyze, buffers) things like this can show up:

Buffers: shared hit=137 read=6545 dirtied=46

Given the description in the manual:

The number of blocks dirtied indicates the number of previously unmodified blocks that were changed by this query

I am a bit surprised why a SELECT (read-only) query would cause buffers to be dirtied?

Under which situation does a SELECT query change a block?

Regards
Thomas

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