Search Postgresql Archives

Re: What is the difference between cmin and cmax

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Hello:
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>

 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!



[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Index of Archives]     [Postgresql Jobs]     [Postgresql Admin]     [Postgresql Performance]     [Linux Clusters]     [PHP Home]     [PHP on Windows]     [Kernel Newbies]     [PHP Classes]     [PHP Books]     [PHP Databases]     [Postgresql & PHP]     [Yosemite]
  Powered by Linux