I think that since COPY FREEZE can be executed only when the table is
created or truncated within the transaction other users cannot insert
any rows during COPY FREEZE.
Right. But the truncating transaction can insert unfrozen rows into the table before inserting more rows via COPY FREEZE.
postgres=# CREATE EXTENSION pageinspect ;
CREATE EXTENSION
postgres=# BEGIN;
BEGIN
postgres=# TRUNCATE testtab ;
TRUNCATE TABLE
postgres=# INSERT INTO testtab VALUES (100, 200);
INSERT 0 1
postgres=# COPY testtab FROM STDIN WITH (FREEZE);
Enter data to be copied followed by a newline.
End with a backslash and a period on a line by itself, or an EOF signal.
>> 1 2
>> 2 3
>> \.
COPY 2
postgres=# COMMIT;
postgres=# SELECT lp, to_hex(t_infomask) FROM heap_page_items(get_raw_page('testtab', 0));
lp | to_hex
----+--------
1 | 800
2 | b00
3 | b00
(3 rows)
The first row in inserted by regular insert and it's not frozen. The next 2 are frozen. We can't mark such as page all-visible, all-frozen.
I'd suggest to measure performance overhead. I can imagine one use
case of COPY FREEZE is the loading a very large table. Since in
addition to set visibility map bits this patch could scan a very large
table I'm concerned that how much performance is degraded by this
patch.
Ok. I will run some tests. But please note that this patch is a bug fix to address the performance issue that is caused by having to rewrite the entire table when all-visible bit is set on the page during first vacuum. So while we may do some more work during COPY FREEZE, we're saving a lot of page writes during next vacuum. Also, since the scan that we are doing in this patch is done on a page that should be in the buffer cache, we will pay a bit in terms of CPU cost, but not anything in terms of IO cost.
Thanks,
Pavan