Asktom: Lock

You Asked

In my PL/SQL code, when I select for update, if anybody else is working & has been locked 
what I want to update, I want my program to wait for n seconds for the row(s) to be 
released, if released then continue updating, if still not released after n seconds, 
detects the user who is locking the resource and informs me that person. I am writing 
pseudo code like this: 

    select xxx for update yyy 
       wait n seconds 
           message (the user USER is locking the record RECORD)

Thanks.

Tom:

Your wrong.  select * from update nowait will raise an ORA-54, resource busy.

That is what NOWAIT does.  Try it and see.

Why use execute immediate?  It is static sql.  sigh....


In one session I execute:

ops$tkyte@ORA817DEV.US.ORACLE.COM> lock table emp in exclusive mode;

Table(s) Locked.

In another, I run:


ops$tkyte@ORA817DEV.US.ORACLE.COM> set echo on
ops$tkyte@ORA817DEV.US.ORACLE.COM> set timing on
ops$tkyte@ORA817DEV.US.ORACLE.COM> @test
ops$tkyte@ORA817DEV.US.ORACLE.COM> declare
  2      resource_busy   exception;
  3      pragma exception_init( resource_busy, -54 );
  4      success boolean := False;
  5      l_rec   emp%rowtype;
  6  begin
  7 
  8    for i in 1 .. 3
  9    loop
10       exit when (success);
11       begin
12             select * into l_rec from emp where rownum=1 for update NOWAIT;
13             success := true;
14       exception
15             when resource_busy then
16                 dbms_lock.sleep(1);
17       end;
18     end loop;
19 
20     if ( not success ) then
21           raise_application_error( -20001, 'row is locked by another session' );
22     end if;
23  end;
24  /
declare
*
ERROR at line 1:
ORA-20001: row is locked by another session
ORA-06512: at line 21



works as advertised.

Again, in one session issue:

ops$tkyte@ORA817DEV.US.ORACLE.COM> update emp set ename=ename where empno=7369;    
1 row updated.

Now, goto another session and:

ops$tkyte@ORA817DEV.US.ORACLE.COM> declare
  2      resource_busy   exception;
  3      pragma exception_init( resource_busy, -54 );
  4      success boolean := False;
  5      l_rec   emp%rowtype;
  6  begin
  7 
  8    for i in 1 .. 3
  9    loop
10       exit when (success);
11       begin
12             select * into l_rec from emp where empno=7369 for update NOWAIT;
13             success := true;
14       exception
15             when resource_busy then
16                 dbms_lock.sleep(1);
17       end;
18     end loop;
19 
20     if ( not success ) then
21           raise_application_error( -20001, 'row is locked by another session' );
22     end if;
23  end;
24  /
declare
*
ERROR at line 1:
ORA-20001: row is locked by another session
ORA-06512: at line 21


again works as advertised. 
If you can prove otherwise, lay out the steps bit by bit and show us -- just like I do.