Re: Obvious data mismatch in View2 which basically SELECT * from View1

On Wed, Sep 16, 2020 at 9:26 AM Ben <bentenzha(at)outlook(dot)com> wrote:

> Dear list,
>
> Recently I am getting feedback, data in my analytic report is not
> repeatable. From time to time they get different data for the same time
> span.
> (but IIRC previously it was OK). Therefore I started debuging the View
> chain for that report, during which I bumped into this issue/phenomenon.
>
> In a over -simplified version:
>
> CREATE VIEW2 AS SELECT * FROM VIEW1;
> SELECT col1 FROM VIEW2 WHERE cond1=True;
> SELECT col1 FROM VIEW1 WHERE cond1=True;
>
> Now col1 from both views looks different. I don't know where to start to
> solve this problem.
>
> The actual situation is a bit more than that, the following is the
> actual query:
>
>
> -- trying to audit utlog weighed stat
> with t as (
> select '2020-07-01 00:00:00'::timestamp t0, '2020--07-02
> 0:0:0'::timestamp t1
> )
> --select * from t;
> select *
> -- from utlog.cache_stats_per_shift_per_reason_weighed_stats
> -- from utlog.stats_per_shift_filtered_per_reason
> from utlog.stats_per_shift_filtered (let's call
> it #View2 for short)
> -- from utlog.stats_per_shift_filtered_b0206 (let's call it
> #View1 for short)
> -- from utlog.stats_per_shift
> cross join t
> where wline = 'F02' and wts >= t.t0 and wts < t.t1 and wsft ='D'
> limit 100
> ;
>

Not sure if it might be something lost in your simplification here, but you
have a LIMIT with no ORDER BY there. That basically means "give me 100
random rows" (but not with a very good random level). It does not return
rows in a consistent/predictable order. So as long as that query is part of
what you're doing, you should not be surprised if you get the rows in an
inconsistent/unpredictable order, with whatever follow-on effects that
might have. (And it can lead to weird follow-on effects like the ones
you're talking about when used in larger query structures)

--
Magnus Hagander
Me: https://www.hagander.net/ <http://www.hagander.net/>
Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>