I am running
EXPLAIN (ANALYZE, BUFFERS) SELECT ... in my Postgres 9.3 server. I end up seeing something like
Buffers: shared hit=166416 dirtied=2 in the output.
From the documentation, "dirtied" indicates:
The number of blocks dirtied indicates the number of previously
unmodified blocks that were changed by this query; while the number of
blocks written indicates the number of previously-dirtied blocks
evicted from cache by this backend during query processing.
This sounds to me like the process of marking a block dirty should only happen when updating data though. My query is a
SELECT, however, and only reads data. I would imagine it would only report hits or reads. I am obviously mistaken. What exactly is happening in this situation, though?