Odd off-by-one dirty buffers and checkpoint buffers written

view bc is just a joining wrapper around pg_buffercache.

regression=# select datname, relname, count(*), sum(count(*)) over () AS
total from bc where isdirty group by datname, relname;
datname | relname | count | total
---------+---------+-------+-------
(0 rows)

regression=# update tenk1 set stringu1 = stringu1 || '' where (unique1 %
384) = 3;
UPDATE 27
regression=# select datname, relname, count(*), sum(count(*)) over () AS
total from bc where isdirty group by datname, relname;
datname | relname | count | total
------------+---------+-------+-------
regression | tenk1 | 3 | 3
(1 row)

regression=# checkpoint;
CHECKPOINT

2022-04-19 23:17:08.256 UTC [161084] LOG: checkpoint starting: immediate
force wait
2022-04-19 23:17:08.264 UTC [161084] LOG: checkpoint complete: wrote 4
buffers (0.0%); 0 WAL file(s) added, 0 removed, 0 recycled; write=0.002 s,
sync=0.002 s, total=0.009 s; sync files=2, longest=0.002 s, average=0.001
s; distance=12 kB, estimate=72358 kB

I've done this four times in a row and while the number of dirty buffers
shown each time vary (see below) I see that "wrote N buffers" is always
exactly one more than the total count of dirty buffers. I'm just curious
if anyone has a quick answer for this unusual correspondence.

David J.

regression=# update tenk1 set stringu1 = stringu1 || '' where (unique1 %
384) = 3;
UPDATE 27
regression=# select datname, relname, count(*), sum(count(*)) over () AS
total from bc where isdirty group by datname, relname;
datname | relname | count | total
------------+----------------------+-------+-------
regression | tenk1 | 33 | 102
regression | tenk1_hundred | 9 | 102
regression | tenk1_thous_tenthous | 18 | 102
regression | tenk1_unique1 | 27 | 102
regression | tenk1_unique2 | 15 | 102
(5 rows)

2022-04-19 23:13:03.480 UTC [161084] LOG: checkpoint starting: immediate
force wait
2022-04-19 23:13:03.523 UTC [161084] LOG: checkpoint complete: wrote 103
buffers (0.6%); 0 WAL file(s) added, 0 removed, 0 recycled; write=0.004 s,
sync=0.014 s, total=0.044 s; sync files=8, longest=0.008 s, average=0.002
s; distance=721 kB, estimate=110165 kB