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IDS Forum

Re: SELECT while being UPDATEd?

Posted By: Art Kagel
Date: Thursday, 25 August 2016, at 8:08 p.m.

In Response To: SELECT while being UPDATEd? (DAVID GROVE)

David:

You are correct. Your colleague is dead set on making Informix look bad. It
is most definitely NOT broken since it is working exactly as it is designed
and documented to work. As far as why some the select she uses in the
trigger to fetch values to insert (the WRONG way to do it, as you noted)
sometimes returns "old" values and sometimes "new" values, I cannot
comment. I would want to see the DDL that creates the table and its
triggers and the transaction that is updating the "date" and "time" columns
(one wonders why it isn't just a DATETIME YEAR TO SECOND instead, but
that's a different issue).

The correct way to do this is to have the insert executed directly by the
update trigger using the REFERENCING NEW AS <alias> or the REFERENCING OLD
AS <alias> alias names as appropriate to the intent (or through a trigger
procedure).

Art

Art S. Kagel, President and Principal Consultant
ASK Database Management
www.askdbmgt.com

Blog: http://informix-myview.blogspot.com/

Disclaimer: Please keep in mind that my own opinions are my own opinions
and do not reflect on the IIUG, nor any other organization with which I am
associated either explicitly, implicitly, or by inference. Neither do
those opinions reflect those of other individuals affiliated with any
entity with which I am affiliated nor those of the entities themselves.

On Thu, Aug 25, 2016 at 6:19 PM, DAVID GROVE <david.grove@alaska.gov> wrote:

> A colleague created an update trigger on a table A. This trigger invokes an
> SPL that INSERTs a new row in table B. Some of the values being INSERTed
> into
> B are being SELECTed from table A, from the very row being UPDATed (and
> that
> fired the trigger).
>
> My colleague is experiencing difficulty in that some of the data selected
> from
> A is "old" and some is "new".
>
> I offered the observation that one shouldn't try to read the exact row of a
> table that is currently being updated, while it is being updated.
>
> I further offered two alternatives:
> 1) Explicitly pass the required values FROM THE TRIGGER (using "NEW" or
> "OLD",
> as required).
> 2) Use a Trigger Routine to accomplish the same thing.
>
> Both were rejected because "Informix is flawed", and shouldn't work like
> that.
>
> So, another error is injected into the system while we "are moving on", as
> she
> stated.
>
> But, perhaps I am in error-- or misunderstanding (which is, essentially,
> the
> same thing). Maybe it is just fine to invoke an SPL, from a trigger, and
> said
> SPL SELECTs values (unspecified whether "old"/pre or "new"/post) from the
> row
> being UPDATEd (which caused the trigger to fire).
>
> I always thought that was bad practice and such values were in an
> inconsistent
> state.
>
> Set me straight if I am wrong, please.
>
> Can one legally read a row being changed, while in the process of being
> changed?
>
> DG
>
> P.S. The problem arose because the value actually being changed (and being
> read) is a datetime value. When changing the date, Informix acts as if
> "new"
> were specified. That is, the revised date is returned. When changing the
> time,
> Informix acts as if "old" were specified. That is, the previous time is
> returned.
>
>
> ************************************************************
> *******************
> Forum Note: Use "Reply" to post a response in the discussion forum.
>
>

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