BufferUsage counters' values have changed

Hi hackers,

I noticed that BufferUsage counters' values are strangely different for the
same queries on REL_15_STABLE and REL_16_STABLE. For example, I run

CREATE EXTENSION pg_stat_statements;
CREATE TEMP TABLE test(b int);
INSERT INTO test(b) SELECT generate_series(1,1000);
SELECT query, local_blks_hit, local_blks_read, local_blks_written,
       local_blks_dirtied, temp_blks_written FROM pg_stat_statements;

and output on REL_15_STABLE contains

query              | INSERT INTO test(b) SELECT generate_series($1,$2)
local_blks_hit     | 1005
local_blks_read    | 2
local_blks_written | 5
local_blks_dirtied | 5
temp_blks_written  | 0

while output on REL_16_STABLE contains

query              | INSERT INTO test(b) SELECT generate_series($1,$2)
local_blks_hit     | 1006
local_blks_read    | 0
local_blks_written | 0
local_blks_dirtied | 5
temp_blks_written  | 8


I found a bug that causes one of the differences. Wrong counter is incremented
in ExtendBufferedRelLocal(). The attached patch fixes it and should be applied
to REL_16_STABLE and master. With the patch applied output contains

query              | INSERT INTO test(b) SELECT generate_series($1,$2)
local_blks_hit     | 1006
local_blks_read    | 0
local_blks_written | 8
local_blks_dirtied | 5
temp_blks_written  | 0


I still wonder why local_blks_written is greater than it was on REL_15_STABLE,
and why local_blks_read became zero. These changes are caused by fcdda1e4b5.
This code is new to me, and I'm still trying to understand whether it's a bug
in computing the counters or just changes in how many blocks are read/written
during the query execution. If anyone can help me, I would be grateful.

Best regards,
Karina Litskevich
Postgres Professional: http://postgrespro.com/