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@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@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!