Select for update that doesn't return any rows

Question

Are there any odd side-effects that may occur if a select for update that returns no results is never committed? I wouldn't think there are, but I'm not sure if there would be some kind of overhead or unforeseen consequences. This isn't a terribly important question, but it's come up in some coding I've done and I've not been able to find any documentation addressing it.

Answer 1

A select for update only locks rows that meet the predicate specified in the where clause. So, if the query returns no rows, no rows are locked.
session1> SELECT * FROM t;
        ID DESCR
         1 Un
         5 One
         2 THIS IS WA
session1> SELECT * FROM t
  2  WHERE id = 11 FOR UPDATE;
no rows selectedA second session can update rows in the table
session2> UPDATE t
  2  SET descr = 'One'
  3  WHERE id = 1;
1 row updated.John
Edited by: John Spencer on Jan 7, 2009 1:36 PM
I just realized that, although you can do updates on the table after the select fo update that returns no rows, you cannot do DDL operations liike a truncate. Unless the session that does the select for update either ends the transaction (i.e. commit or rollback) or ends the session DDL operations will fail.