Select for update returns no rows even though there is no locking thread

Question

I'm using Ibatis library over oracle sql for my query. The select for update statement returns no rows. This happens intermittently. When this was happening last time, I executed the select statement on sqldeveloper (but without the 'for update') and got rows. This situation is not easily reproducible so I've not yet been able to ascertain whether rows are returned on sqldeveloper with the 'for update' clause. But I know for sure that there was no other thread locking the rows. How could this be happening?

Answer 1

The select for update statement returns no rowsWhy do you think that a select for update will always return rows?
the for update clause if there not to garantee the presence of rows but to lock the row when it is present
sql> select * from t;
         A          B C
         1          1 step1
         2          2 step2
         3          3 step3Then session 1 issues the following select
SELECT     *
      FROM t
     WHERE a = 1
FOR UPDATE NOWAIT;If session 2 issues the same select before session 1 commits or rolls back
SELECT     *
      FROM t
     WHERE a = 1
FOR UPDATE NOWAIT;It will get the following error
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specifiedBut if session 2 issue such a kind of select
sql> SELECT     *
  2        FROM t
  3       WHERE a = 99
  4  FOR UPDATE NOWAIT;
no rows selectedYou see then that a select for update can return no rows
Best Regards
Mohamed Houri