What is the difference between cmin and cmax

Hello:

I have question for cmin and cmax.

It is said:

cmin is: The command identifier (starting at zero) within the
inserting transaction.

cmax is: The command identifier within the deleting transaction, or
zero.

http://www.postgresql.org/docs/9.1/static/ddl-system-columns.html

But I wonder what is the difference between cmin and cmax ?

Because during my test, cmin and cmax changed together and be the same
value:

At first I have two records.

In my terminal A I did:

[postgres(at)server bin]$ ./psql

psql (9.1.2)

Type "help" for help.

postgres=# select version();

version

--------------------------------------------------------------------------------

-------------------------------

PostgreSQL 9.1.2 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.1.2
20080

704 (Red Hat 4.1.2-52), 64-bit

(1 row)

postgres=# begin;

BEGIN

postgres=# select xmin,xmax,cmin,cmax,* from tab01;

xmin | xmax | cmin | cmax | id | cd

------+------+------+------+----+----

1738 | 0 | 0 | 0 | 1 | 1

1739 | 0 | 0 | 0 | 2 | 2

(2 rows)

postgres=# insert into tab01 values(3,'3');

INSERT 0 1

postgres=# insert into tab01 values(4,'4');

INSERT 0 1

postgres=# insert into tab01 values(5,'5');

INSERT 0 1

postgres=# select xmin,xmax,cmin,cmax,* from tab01;

xmin | xmax | cmin | cmax | id | cd

------+------+------+------+----+----

1738 | 0 | 0 | 0 | 1 | 1

1739 | 0 | 0 | 0 | 2 | 2

1740 | 0 | 0 | 0 | 3 | 3

1740 | 0 | 1 | 1 | 4 | 4

1740 | 0 | 2 | 2 | 5 | 5

(5 rows)

postgres=# update tab01 set id=50 where cd = '5';

UPDATE 1

postgres=# select xmin,xmax,cmin,cmax,* from tab01;

xmin | xmax | cmin | cmax | id | cd

------+------+------+------+----+----

1738 | 0 | 0 | 0 | 1 | 1

1739 | 0 | 0 | 0 | 2 | 2

1740 | 0 | 0 | 0 | 3 | 3

1740 | 0 | 1 | 1 | 4 | 4

1740 | 0 | 3 | 3 | 50 | 5

(5 rows)

postgres=# delete from tab01 where id=4;

DELETE 1

postgres=# select xmin,xmax,cmin,cmax,* from tab01;

xmin | xmax | cmin | cmax | id | cd

------+------+------+------+----+----

1738 | 0 | 0 | 0 | 1 | 1

1739 | 0 | 0 | 0 | 2 | 2

1740 | 0 | 0 | 0 | 3 | 3

1740 | 0 | 3 | 3 | 50 | 5

(4 rows)

postgres=# delete from tab01 where id=2;

DELETE 1

postgres=# select xmin,xmax,cmin,cmax,* from tab01;

xmin | xmax | cmin | cmax | id | cd

------+------+------+------+----+----

1738 | 0 | 0 | 0 | 1 | 1

1740 | 0 | 0 | 0 | 3 | 3

1740 | 0 | 3 | 3 | 50 | 5

(3 rows)

postgres=#

In terminal B, I did:

[postgres(at)server bin]$ ./psql

psql (9.1.2)

Type "help" for help.

postgres=# begin;

BEGIN

postgres=# select xmin,xmax,cmin,cmax,* from tab01;

xmin | xmax | cmin | cmax | id | cd

------+------+------+------+----+----

1738 | 0 | 0 | 0 | 1 | 1

1739 | 1740 | 5 | 5 | 2 | 2

(2 rows)

postgres=#

Thanks!