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 <luckyjack...@gmail.com> > 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! >
