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!
