Обсуждение: BUG #16803: create a table only one text/varchar column, storage options toast_tuple_target doesn't work

The following bug has been logged on the website:

Bug reference:      16803
Logged by:          osdba
Email address:      mailtch@163.com
PostgreSQL version: 11.10
Operating system:   CentOS7.6
Description:

create a table only one text/varchar colume,  storage options
toast_tuple_target doesn't work, see:

[pg03@pgtrain pgdata]$ psql
psql (11.10)
Type "help" for help.

postgres=# create table test01(id int, t text);
CREATE TABLE
postgres=# alter table test01 set (toast_tuple_target=1024);
ALTER TABLE
postgres=# insert into test01 select seq, repeat('a', 1800) from
generate_series(1, 1000) as seq;
INSERT 0 1000
postgres=# select pg_total_relation_size('test01');
 pg_total_relation_size
------------------------
                2080768
(1 row)

postgres=# truncate table test01;
TRUNCATE TABLE
postgres=# insert into test01 select seq, repeat('a', 2800) from
generate_series(1, 1000) as seq;
INSERT 0 1000
postgres=# select pg_total_relation_size('test01');
 pg_total_relation_size
------------------------
                 114688
(1 row)

because I set toast_tuple_target to 1024，so trigger toast compress when
insert data that length is 1800 or 2800, but only trigger toast compress
when data length is 2800.

when I create table have more than one text column， then storage options
toast_tuple_target can be work correctly: 

postgres=# create table test01(id int, t1 text, t2 text);
CREATE TABLE
postgres=# alter table test01 set (toast_tuple_target=1024);
ALTER TABLE
postgres=# insert into test01 select seq, repeat('a', 1800), repeat('b',
1800) from generate_series(1, 1000) as seq;
INSERT 0 1000
postgres=# select pg_total_relation_size('test01');
 pg_total_relation_size
------------------------
                 139264
(1 row)

postgres=# truncate table test01;
TRUNCATE TABLE
postgres=# insert into test01 select seq, repeat('a', 2800), repeat('b',
2800) from generate_series(1, 1000) as seq;
INSERT 0 1000
postgres=# select pg_total_relation_size('test01');
 pg_total_relation_size
------------------------
                 163840
(1 row)

you can see, when table have two text column, then storage options
toast_tuple_target can be work correctly.

this is a bug?

PG Bug reporting form <noreply@postgresql.org> writes:
> create a table only one text/varchar colume,  storage options
> toast_tuple_target doesn't work, see:

The reason your first example isn't doing anything is that the tuples are
shorter than TOAST_TUPLE_THRESHOLD, which is the threshold row length for
applying the toasting code at all.  The reloption doesn't affect that;
it only changes the target length once we've gotten into the toaster.

It's not real clear to me why whoever provided a knob for
TOAST_TUPLE_TARGET didn't see fit to also provide one for
TOAST_TUPLE_THRESHOLD, but I suppose that was a deliberate choice.
The documentation is certainly clear enough that you can only
adjust the target.

            regards, tom lane