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problem in select max when executing the transaction in para


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Renato Zangerolami

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Joined: 06 Dec 2019
Posts: 28
Location: Brazil

PostPosted: Tue Mar 02, 2021 2:42 am
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I have a problem when executing a select max when the transaction (IMS) is executed, that same transaction can be executed at the same time. after the select is made an insert.
Example:
Transaction 1
Code:
EXEC SQL MAX (FIELD)
            INTO: XXXXX
            FROM TABLE
           WHERE ......

Transaction 2
Code:
EXEC SQL MAX (FIELD)
            INTO: XXXXX
            FROM TABLE
            WHERE ......

both transactions are returning the same select MAX (FIELD) because they are running at the same time, so I'm having a problem with the insert because I have a duplicate key
Does anyone have any idea how to solve this?
Coded for you
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sergeyken
Warnings : 2

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Joined: 29 Apr 2008
Posts: 1140

PostPosted: Tue Mar 02, 2021 3:16 am
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I believe this is a general logical problem with ANY DATABASE SYSTEM, not only IMS DB/DC.

You need to provide any type of blocking all other conflicting transactions from running while the first of them has not finished. The method of blocking may be different for various databases, but the idea must be logically the same for any of them.
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Renato Zangerolami

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Joined: 06 Dec 2019
Posts: 28
Location: Brazil

PostPosted: Tue Mar 02, 2021 5:34 pm
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I tried to use some options like
- WITH RR USE AND KEEP EXCLUSIVE LOCKS (WITH RR AND RS)
- WITH RR USE AND KEEP UPDATE LOCKS (WITH RR AND RS)
- WITH RR USE AND KEEP SHARE LOCKS (WITH RR AND RS)
But it didn't work very well

the best option that worked was using
EXEC SQL LOCK TABLE XXXX IN EXCLUSIVE MODE END-EXEC.
this is the first time I use this command, my only concern is with the delay this may cause, as this transaction processes thousands of accounts per day ...
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Rohit Umarjikar

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Joined: 21 Sep 2010
Posts: 2723
Location: NYC,USA

PostPosted: Tue Mar 02, 2021 7:48 pm
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Looks like a logical error.
Once you get the MAX( field) , you can simply check in DB if it exists and if yes then redo that MAX again to get latest value and do INSERT again.
OR ( This one is most preferred)
Explore if it supports transaction sequencing like we have in CICS by using ENQ and DEQ method which will work perfect and designed for such situations.
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sergeyken
Warnings : 2

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Joined: 29 Apr 2008
Posts: 1140

PostPosted: Tue Mar 02, 2021 9:33 pm
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AFAIK, in IMS DB/DC there is a command to perform such locking, at different levels:
LOCK DB
LOCK TRAN
UNLOCK DB
UNLOCK TRAN
etc.
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sergeyken
Warnings : 2

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Joined: 29 Apr 2008
Posts: 1140

PostPosted: Tue Mar 02, 2021 9:49 pm
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Renato Zangerolami wrote:
I tried to use some options like
- WITH RR USE AND KEEP EXCLUSIVE LOCKS (WITH RR AND RS)
- WITH RR USE AND KEEP UPDATE LOCKS (WITH RR AND RS)
- WITH RR USE AND KEEP SHARE LOCKS (WITH RR AND RS)
But it didn't work very well

the best option that worked was using
EXEC SQL LOCK TABLE XXXX IN EXCLUSIVE MODE END-EXEC.
this is the first time I use this command, my only concern is with the delay this may cause, as this transaction processes thousands of accounts per day ...

This looks to be one of acceptable methods. Some delay is possible, but this is inevitable (to avoid failures, or wrong results). You can minimize the delay by making the “distance” between LOCK TABLE, and UNLOCK TABLE as short as possible: LOCK it just before SELECT MAX, and UNLOCK it immediately after INSERT new_timestamp
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enrico-sorichetti

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Joined: 14 Mar 2007
Posts: 10796
Location: italy

PostPosted: Tue Mar 02, 2021 10:58 pm
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no forum advice will fix a bad application design icon_cool.gif
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