When you issue a statement that uses a database link, Oracle creates a session for 
you on the remote database using that link. The connection remains open until you 
end your local session or until the number of database links for your session 
exceeds the value of the initialization parameter OPEN_LINKS. If you want to reduce 
the network overhead associated with keeping the link open, use this clause to 
close the link explicitly if you do not plan to use it again in your session. 


-- Window 1
SQL> select sid,username from v$session;

       SID USERNAME
---------- ------------------------------
         1
         2
         3
         4
         5
         6
         7
         8
         9
        10
        11 SCOTT

       SID USERNAME
---------- ------------------------------
        14

12 rows selected.

-- Window 2
SQL> desc user_tables@phoenix_scott
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 TABLE_NAME                                NOT NULL VARCHAR2(30)
 TABLESPACE_NAME                                    VARCHAR2(30)
 .
 .
 .

-- Window 1
SQL> /

       SID USERNAME
---------- ------------------------------
         1
         2
         3
         4
         5
         6
         7
         8
         9
        10
        11 SCOTT

       SID USERNAME
---------- ------------------------------
        14
        18 SCOTT

13 rows selected.

-- Window 2
SQL> alter session close database link phoenix_scott;

Session altered.

-- Window 1
SQL> /

       SID USERNAME
---------- ------------------------------
         1
         2
         3
         4
         5
         6
         7
         8
         9
        10
        11 SCOTT

       SID USERNAME
---------- ------------------------------
        14

12 rows selected.

SQL>