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From "Sylvain Lebresne (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-11871) Allow to aggregate by time intervals
Date Wed, 25 May 2016 08:57:13 GMT


Sylvain Lebresne commented on CASSANDRA-11871:

I would truly hate to limit this to a single time-related function, so I *really* think we
should re-phrase this ticket as adding support for functions (as generally as possible, see
below) in {{GROUP BY}}.

In practice, there is obviously limitations to what functions we can support in {{GROUP BY}}.
Informally, we need the {{GROUP BY}} to "respect" the clustering order even when functions
are applied. And I believe that for that, we can use the following sufficient and reasonably
simple to check conditions:
# We only allow functions on a single column. That is, {{GROUP BY a, f(b)}} might be ok (if
the other conditions hold), but {{GROUP BY f(a, b)}} won’t be.
# If you ignore the function calls, the {{GROUP BY}} should still be valid (in the sense of
CASSANDRA-10707). That is, {{GROUP BY f(a), g(b)}} would only be valid if {{GROUP BY a, b}}
# We only allow functions in {{GROUP BY}} that are monotonic.

I'll note that those conditions are _sufficient_ (if I'm not missing something) but they may
not be _necessary_ (in other words, there is cases that will be rejected by those conditions
but coudl still "respect" the cluster order). This is ok though because those rules are general
enough to be useful imo. The main point is that they are simple to check.

On the 3rd point though, validating that a function is monotic automatically is not at all
easy and I'm not suggesting we do so. What I am suggesting is that we simply manually mark
functions that are know to be monotonic (very concretely, we'll add a {{isMonotonic}} method
to the {{Function}} class). And initially, we'll only mark "native" functions as such, UDF
will also have this method return {{false}}, though we could in the future allow users to
declare their UDF as monotonic if we so wish.

Of course, I fully agree that we also want to add a {{floor}}-like function as suggested above.

bq. An important aspect to keep in mind with a function like floor is the starting point.

I'm not exactly sure how that would work to be honest. First because, if I understand what
you're saying, it means the result of the floor method depends on what is in the {{WHERE}}
clause, and not only does that sound like a mess to implement, but how would that work if
I do:
SELECT floor(time, 3h), count() FROM foo GROUP BY floor(time, 3h);
When is my "starting point" then? Unless you mean that the {{floor}} function has some per-partition
state and the starting point is based on the first value it receives, but that's even worth
because that would mean the returning intervals are not predictable.

In fact, even if have a {{WHERE}}, I could have (assuming CASSANDRA-11873):
SELECT floor(time, 3h), count() FROM foo WHERE time > now() - 10h GROUP BY floor(time,
in which case I probably don't want my intervals to be based on the value of {{now()}}.

Am I missing something obvious?

Anyway, I really think the {{floor}} function should be a normal function, that only depends
on its arguments. I do like the flexibility of being able to define precise intervals, and
I suppose we should provide the starting point as argument of the method. Meaning that general
form of the function would be {{floor(value, range, starting_point)}} and would return the
biggest time {{t}} such that {{t < value}} and such that {{t = starting_point + n * range}}
for some integer {{n}}. Assuming we can provide that, we could just also have {{floor(value,
range)}} as just a shortcut using a hard-coded {{starting_point}} (some Jan 1 at 0:00). But
that said, I'm not sure such method can be computed efficiently, so not sure how viable that

> Allow to aggregate by time intervals
> ------------------------------------
>                 Key: CASSANDRA-11871
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: CQL
>            Reporter: Benjamin Lerer
>            Assignee: Benjamin Lerer
>             Fix For: 3.x
> For time series data it can be usefull to aggregate by time intervals.
> The idea would be to add support for one or several functions in the {{GROUP BY}} clause.
> Regarding the implementation, even if in general I also prefer to follow the SQL syntax,
I do not believe it will be a good fit for Cassandra.
> If we have a table like:
> {code}
> {
>     symbol text,
>     date date,
>     time time,
>     priceMantissa int,
>     priceExponent tinyint,
>     volume int,
>     PRIMARY KEY ((symbol, date), time)
> };
> {code}
> The trades will be inserted with an increasing time and sorted in the same order. As
we can have to process a large amount of data, we want to try to limit ourself to the cases
where we can build the groups on the flight (which is not a requirement in the SQL world).
> If we want to get the number of trades per minutes with the SQL syntax we will have to
> {{SELECT hour(time), minute(time), count() FROM Trades WHERE symbol = 'AAPL' AND date
= '2016-01-11' GROUP BY hour(time), minute(time);}}
> which is fine. The problem is that if the user invert by mistake the functions like that:
> {{SELECT hour(time), minute(time), count() FROM Trades WHERE symbol = 'AAPL' AND date
= '2016-01-11' GROUP BY minute(time), hour(time);}}
> the query will return weird results.
> The only way to prevent that would be to check the function order and make sure that
we do not allow to skip functions (e.g. {{GROUP BY hour(time), second(time)}}).
> In my opinion a function like {{floor(<columnName>, <time range>)}} will
be much better as it does not allow for this type of mistakes and is much more flexible (you
can create 5 minutes buckets if you want to).
> {{SELECT floor(time, m), count() FROM Trades WHERE symbol = 'AAPL' AND date = '2016-01-11'
GROUP BY floor(time, m);}}
> An important aspect to keep in mind with a function like {{floor}} is the starting point.
For a query like:  {{SELECT floor(time, m), count() FROM Trades WHERE symbol = 'AAPL' AND
date = '2016-01-11' AND time >= '01:30:00' AND time =< '07:30:00' GROUP BY floor(time,
2h);}}, I think that ideally the result should return 3 groups: {{01:30:00}}, {{03:30:00}}
and {{05:30:00}}.  

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