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IDS Forum

Re: RE: Record Locking and Update Statistics

Posted By: Fernando Nunes
Date: Tuesday, 20 September 2016, at 5:49 p.m.

In Response To: Re: RE: Record Locking and Update Statistics (RAY BURNS)

As Art wrote, for better and for worse Informix does not do lock escalation.
But apparently your situation is very common, seems very obvious ans is
terribly easy to solve.

You're probably doing daily(?) update stattistics... and as you say, by the
end of the week the table has very few records. So the engine thinks doing
a sequential scan is a very good plan (the query plans consider efficiency,
not concurrency).
If the table has in fact very few records and if your sessions have LOCK
MODE WAIT N (N being a very small number) it may work. But if the table
becomes bigger (more records) the sequentail plan is a bad thing... (in
fact, in most cases it's always a bad thing considering concurrency).
This is solved by running update statistics when the table has more
records, because by then a sequential plan doesn't seem good.

The solution? At leas a couple:
1- Use an hint on the query. I personally don't like this. It tend to
"stick around". Developers tend to think it's a "normal thing" (which may
be true in other RDBMS who invented the hints, because they needed to,
considering their optimizer is/was very bad), and it may become useless if
you change something, like the index names etc.

2- Stop running update statistics on those tables... Run it once when they
have a "medium" number of records and then include them in an exclusion
list used by your update statistics process. Art's dostats and my
dbs_updstats/tbl_updstats allow this. I'd need to re-check the AUTO UPDATE
statistics in their latest versions to understand how to do this (a dirty
trick would be to delete the entries created by the evaluator that refer to
those tables before statistics are effectively calculated)

So it seems everyting is working as "planned", but not as desired :)

Hope this helps.

On Tue, Sep 20, 2016 at 9:03 PM, RAY BURNS <ray.burns@velocityglobal.co.nz>
wrote:

> In this particular instance the application is adding and updating and
> deleting records to a file which is defined with row level locking. The
> record
> count is going up and down constantly. It will peek at about 5000 records
> and
> by the end of the week there will be a very small number. On Monday
> mornings
> the application is constantly getting deadlock errors (143) even though we
> have confirmed that the sessions receiving the errors are locking different
> records. We do an update statistics and the lock errors disappear.
>
> There is a suggestion that when there is a small number of records, the
> engine
> is deciding to issue a table lock rather than a row lock in order to
> improve
> performance.
>
> I have never heard of this, but it is certainly consistent with symptoms.
> Have
> you heard of such a behaviour and if so, do you know of any way of
> stopping it
> doing this?
>
> Ray
>
>
> ************************************************************
> *******************
> Forum Note: Use "Reply" to post a response in the discussion forum.
>
>

--
Fernando Nunes
Portugal

http://informix-technology.blogspot.com
My email works... but I don't check it frequently...

--001a11426122d7eaea053cf765bd

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