Problem with "SELECT...FOR UPDATE OF..." and "POST command" combination

Question

Problem with "SELECT...FOR UPDATE OF..." and "POST command" combination
Problem in committing transactions in Multiple Forms (Oracle Forms) with POST built-in command:
Consider that the following statements are written in WHEN-WINDOW-CLOSED trigger of a called form.
Statements in called form (Form name: FORM_CHILD):
go_block('display_block') ;
do_key('execute_query') ;
-- Data from table_b will be populated in this block, based on the value of COLUMN_1 obtained
-- from TABLE_A.
-- Example: If the value of COLUMN_1 is 10, then all the matching records from TABLE_B, which
-- are inserted with value 10 in TABLE_B.COLUMN_1 will be fetched and shown here.
if user_choice = 'YES' then
commit ;
else
rollback ;
end if ;
Statements in calling forms:
There are two calling forms having following statements and it is going to call the above said called form.
CALLING FORM 1
Statements in KEY-COMMIT trigger:
post;
call_form(form_child, no_activate) ;
Statements in ON-INSERT trigger:
select column_1
from table_a
for update of column_1
where column_2 = 'X' ;
update table_a
set column_1 = column_1 + 1
where column_2 = 'X' ;
insert into table_b ...;
insert into table_b ...; Statements in KEY-COMMIT trigger:
post;
call_form(form_child, no_activate) ;
CALLING FORM 2:
Statements in ON-INSERT trigger:
select column_1
from table_a
for update of column_1
where column_2 = 'X' ;
update table_a
set column_1 = column_1 + 1
where column_2 = 'X' ;
insert into table_b ...;
insert into table_b ...;
insert into table_b ...;
Our understanding:
Assume that both the forms are running from two different machines/instances, issuing commit at the same time. In this case, forms will start executing the statements written in ON-INSERT trigger, the moment POST command is executed. Though the commit is issued at the same time, according to oracle, only one of the request will be taken for processing first. Assume that calling form 1 is getting processed first.
So, it fetches the value available in COLUMN_1 of TABLE_A and locks the row from further select, update, etc. as SELECT...FOR UPDATE command is used (note that NOWAIT is not given, hence the lock will be released only when COMMIT or ROLLBACK happens) and proceed executing further INSERT statements. Because of the lock provided by the SELECT...FOR UPDATE command, the statements in calling form 2 will wait for the resource.
After executing the INSERT statements, the FORM_CHILD is called. The rows inserted in to TABLE_A will be queried and shown. The database changes will be committed when user closes the window (as COMMIT is issued in its WHEN-WINDOW-CLOSED trigger). Then the SELECT...FOR UPDATE lock will be released and calling form 2's statements will be executed.
Actual happenings or Mis-behavior:
Calling form 2 starts executing INSERT statements instead of waiting for SELECT...FOR UPDATE lock. Also, the value selected from TABLE_A.COLUMN_1 is same in both the calling forms, which is wrong.
The rows inserted into TABLE_B are having similar COLUMN_1 values in calling form 2 and they are fetched and shown in the called form FORM_CHILD.
Note that in calling form 2 also POST only is issued, but the changes posted there are accessible in calling form 1 also, which is wrong.
Kindly suggest us as to how to fix above problem. It will be much use, if you can send us the information regarding the behavior of Oracle Forms POST built-in also.
Our mail ID: [email protected]
Thanks a lot in advance.

Answer 1

You have several problems:
1. On-Insert will ONLY run if you have created a new record in a base-table block. If you haven't done that, then the POST command will not cause it to run.
2. Select for update without a "no wait" will lock records for the first form, but when the second form tries this, it will hit the ORA-00054 exception, and will NOT wait. The only way you could make it wait is to issue an UPDATE sql command, which is not such a good way to go.
All POST does is issues SQL insert or update commands for any changes the user has made to records in a form's base-table blocks, without following with a Commit command.
Also understand that Commit is the same as Commit_Form, and Rollback is the same as Clear_Form. You should read up on these in the Forms help topics.