Re: Parallel Aggregates for string_agg and array_agg - Mailing list pgsql-novice
| From | Tomer Praizler |
|---|---|
| Subject | Re: Parallel Aggregates for string_agg and array_agg |
| Date | |
| Msg-id | CAD=kdR8qQehNXLCXj5tM92ZWh8cB4OFVNNQjC-T6ZRgZiOghtg@mail.gmail.com Whole thread Raw |
| In response to | Re: Parallel Aggregates for string_agg and array_agg (David Rowley <david.rowley@2ndquadrant.com>) |
| Responses |
Re: Parallel Aggregates for string_agg and array_agg
(David Rowley <david.rowley@2ndquadrant.com>)
|
| List | pgsql-novice |
Thank you, David!
I guess I wasn't 100% clear, an example will do the trick.
So let's take for example the following records:
timestamp port super_name name ids count
2019-03-06 10:00:00 22 ssh abc 1,2 10
2019-03-06 10:00:00 22 ssh xyz 3,4 20
2019-03-06 10:00:00 22 ssh abc 5,6 30
2019-03-06 10:00:00 22 ssh abc 7,8 40
2019-03-06 10:00:00 22 ssh foo 9,10 50
The primary key is combined of 6 columns in this example: (timestamp, port, super_name), I have around 8 values for every key.
My table is partitioned by day, and I have around 2-3M rows in each partition. I am trying to aggregate on the last 7 days, which is around 23M rows.
my query looks something like this:
SELECT x.timestamp, x.port, x.super_name, max(x.timestamp) AS last_seen, coalesce(array_length(array_merge_agg(x.ids), 1), 0) AS my_count, coalesce(array_length(array_agg(DISTINCT x.name), 1), 0) AS names, sum(x.count) AS final_count
FROM x
GROUP BY x.timestamp, x.port, x.super_name
ORDER BY sum(x.count)
This result in a plan without parallel execution because of the array_agg on a string field. when I remove it the query planner spawns a parallel execution plan.
It reduces the time from 5 minutes to around 1 minute which is also a lot. (if there is any idea on how to optimize farther please help:) btw, hardware resources is not a problem)
Thanks!
On Thu, 14 Mar 2019 at 01:02 David Rowley <david.rowley@2ndquadrant.com> wrote:
On Thu, 14 Mar 2019 at 09:53, Tomer Praizler <tomer.praizler@gmail.com> wrote:
> Just wanted to check if there is any magic that can be done to make this happen.
> The closest thing I ran into was this guy patch - https://www.postgresql.org/message-id/CAKJS1f8LV7AT%3DAAhdYGKtGrGkSkEgO6C_SW2Ztz1sR3encisqw%40mail.gmail.com
If you're able to change the SQLs and point them at some other
aggregate function, then you could create an extension with the code
from that patch and CREATE AGGREGATE your own version of those
functions using the combine and [de]serial functions. I know that
creating your own extension is pretty out there for the novice mailing
list, but if I thought of another easier way if have told you that
instead.
> I didn't try it, but wanted to check if there is any way to deal with the need to aggregate on a string, by creating an array while doing a group by? Should I manipulate my data to be able to do it, maybe by generating an int out of those strings? Any other idea?
Well, array_agg is non-parallel too, so don't see how having an array
and converting that into a string later would help. The other
aggregates that are parallel aware don't really let you get individual
values back out of the aggregated state, so there's not really a way
to turn that into an array or a string containing all the values that
were aggregated.
Does your use-case really need parallel versions of these aggregates?
I imagined that these would perform best when the underlying scan had
to skip lots of values, or when the aggregate had a FILTER (WHERE ...)
clause. Maybe if the filtering can be done by using an index then
performance would up to the level you need?
If you could share a simplified version of your use case perhaps
someone can suggest a way to speed it up another way.
--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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