System Columns in Postgresql

In postgresql ever table has several system columns that are implicitly defined by the system. Therefore, these names cannot be used as names of user-defined columns. 

oid

The object identifier (object ID) of a row. This column is only present if the table was created using WITH OIDS, or if the default_with_oids configuration variable was set at the time. This column is of type oid (same name as the column).

tableoid

The OID of the table containing this row. This column is particularly handy for queries that select from inheritance hierarchies, since without it, it's difficult to tell which individual table a row came from. The tableoid can be joined against the oid column of pg_class to obtain the table name.

xmin

The identity (transaction ID) of the inserting transaction for this row version. (A row version is an individual state of a row; each update of a row creates a new row version for the same logical row.)

cmin

The command identifier (starting at zero) within the inserting transaction.

xmax

The identity (transaction ID) of the deleting transaction, or zero for an undeleted row version. It is possible for this column to be nonzero in a visible row version. That usually indicates that the deleting transaction hasn't committed yet, or that an attempted deletion was rolled back.

cmax

The command identifier within the deleting transaction, or zero.

ctid

The physical location of the row version within its table. Note that although the ctid can be used to locate the row version very quickly, a row's ctid will change each time it is updated or moved by VACUUM FULL. Therefore ctid is useless as a long-term row identifier. The OID, or even better a user-defined serial number, should be used to identify logical rows.