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!
I looked into the source code, and I think I now understand it:
cmin and cmax are same! The documentation is too old now.
I made another test:
In terminal A:
pgsql=# begin;
BEGIN
pgsql=# select * from tab01;
id | cd
----+----
(0 rows)
pgsql=# select xmin,xmax,cmin,cmax,* from tab01;
xmin | xmax | cmin | cmax | id | cd
------+------+------+------+----+----
(0 rows)
pgsql=# insert into tab01 values(1,'1'),(2,'2'),(3,'3');
INSERT 0 3
pgsql=# insert into tab01 values(4,'4'),(5,'5'),(6,'6');
INSERT 0 3
pgsql=# select xmin,xmax,cmin,cmax,* from tab01;
xmin | xmax | cmin | cmax | id | cd
------+------+------+------+----+----
1897 | 0 | 0 | 0 | 1 | 1
1897 | 0 | 0 | 0 | 2 | 2
1897 | 0 | 0 | 0 | 3 | 3
1897 | 0 | 1 | 1 | 4 | 4
1897 | 0 | 1 | 1 | 5 | 5
1897 | 0 | 1 | 1 | 6 | 6
(6 rows)
pgsql=# commit;
Then I begin to delete record:
pgsql=# begin;
BEGIN
pgsql=# delete from tab01 where id=1 or id=2;
DELETE 2
pgsql=# delete from tab01 where id=3;
DELETE 1
pgsql=# delete from tab01 where id=4;
DELETE 1
pgsql=# delete from tab01 where id=5;
DELETE 1
pgsql=#
But I have not commit my deleting action.
Then in terminal B, I can see:
pgsql=# select xmin,xmax,cmin,cmax,* from tab01;
xmin | xmax | cmin | cmax | id | cd
------+------+------+------+----+----
1897 | 1898 | 0 | 0 | 1 | 1
1897 | 1898 | 0 | 0 | 2 | 2
1897 | 1898 | 1 | 1 | 3 | 3
1897 | 1898 | 2 | 2 | 4 | 4
1897 | 1898 | 3 | 3 | 5 | 5
1897 | 0 | 1 | 1 | 6 | 6
(6 rows)
pgsql=#
---------------
In fact , in the source code of PG,I can find it---heap_getsysattr function in heaptuple.c,here is it:
--------code begin----
case MinCommandIdAttributeNumber:
case MaxCommandIdAttributeNumber:
/*
* cmin and cmax are now both aliases for the same field, which
* can in fact also be a combo command id. XXX perhaps we should
* return the "real" cmin or cmax if possible, that is if we are
* inside the originating transaction?
*/
result = CommandIdGetDatum(HeapTupleHeaderGetRawCommandId(tup->t_data));
break;
--------code end-------
2013/7/2 高健 <luckyjackgao@xxxxxxxxx>
