pgsql: Don't count zero-filled buffers as 'read' in EXPLAIN.

Don't count zero-filled buffers as 'read' in EXPLAIN.

If you extend a relation, it should count as a block written, not
read (we write a zero-filled block). If you ask for a zero-filled
buffer, it shouldn't be counted as read or written.

Later we might consider counting zero-filled buffers with a separate
counter, if they become more common due to future work.

Author: Thomas Munro
Reviewed-by: Haribabu Kommi, Kyotaro Horiguchi, David Rowley
Discussion: https://postgr.es/m/CAEepm%3D3JytB3KPpvSwXzkY%2Bdwc5zC8P8Lk7Nedkoci81_0E9rA%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d67dae036bd71afd2be7a2fafd7bc1ef38bfcdd1

Modified Files
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src/backend/storage/buffer/bufmgr.c | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)