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I have a simple select distinct on some time series data:

SELECT DISTINCT user_id
FROM events
WHERE project_id = 6
AND time > '2015-01-11 8:00:00'
AND time < '2015-02-10 8:00:00';

And it takes 112 seconds. Here's the query plan:

http://explain.depesz.com/s/NTyA

My application has to preform a lot of distinct operations and counts like this. Is there a faster way to get this kind of data?

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Sam
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3 Answers3

39

You probably don't want to hear this, but the best option to speed up SELECT DISTINCT is to avoid DISTINCT to begin with. In many cases (not all!) it can be avoided with better database-design or better queries.

Sometimes, GROUP BY is faster, because it takes a different code path.

In your particular case, it doesn't seem like you can get rid of DISTINCT (well, see below). But you can support the query with a special index if you have many queries of that kind:

CREATE INDEX foo ON events (project_id, "time", user_id);

In Postgres 11 or later, you can use an actual "covering" index like:

CREATE INDEX foo ON events (project_id, "time") INCLUDE (user_id);

Adding user_id is only useful if you get index-only scans out of this. See:

Would remove the expensive Bitmap Heap Scan from your query plan, which consumes 90% of the query time.

Your EXPLAIN shows 2,491 distinct users out of half a million qualifying rows. This won't become super-fast, no matter what you do, but it can be substantially faster. With around 200 rows per user, emulating an index skip scan on above index might pay. The range condition on time complicates matters, and 200 rows per user is still a moderate number. So not sure. See:

Either way, if time intervals in your queries are always the same, a MATERIALIZED VIEW folding user_id per (project_id, <fixed time interval>) would go a long way. No chance there with varying time intervals, though. Maybe you could at least fold users per hour or some other minimum time unit, and that would buy enough performance to warrant the considerable overhead. Can be combined with either query style.

Nitpick:
Most probably, the predicates on "time" should really be:

AND "time" >= '2015-01-11 8:00:00'
AND "time" <  '2015-02-10 8:00:00';

Aside:
Don't use time as identifier. It's a reserved word in standard SQL and a basic type in Postgres.

Erwin Brandstetter
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5

Here's my test on Sam's case and Erwin's answer

drop table t1
create table t1 (id int, user_id int, project_id int, date_time timestamp without time zone) ;

insert into t1 -- 10 million row - size="498 MB"
select row_number() over(), round(row_number() over()/1000), round(row_number() over()/100000) , date
from generate_series('2015-01-01'::date, '2016-12-01'::date,'6 seconds'::interval
) date 
limit 10000000

-- before indexing - 10000000 row - output=100 row - time=2900ms
SELECT DISTINCT user_id
FROM t1
WHERE project_id = 1
AND date_time > '2015-01-01 8:00:00'
AND date_time < '2016-12-01 8:00:00' ;

CREATE INDEX foo ON t1 (project_id, date_time, user_id); -- time process=51.2 secs -- size="387 MB"         

-- after indexing - 10000000 row - output=100 row - time= 75ms (reduce ~ 38 times)
SELECT DISTINCT user_id
FROM t1
WHERE project_id = 1
AND date_time > '2015-01-01 00:00:00'
AND date_time < '2016-12-01 00:00:00' ;

Erwin said "You probably don't want to hear this, but the best option to speed up SELECT DISTINCT is to avoid DISTINCT to begin with. In many cases (not all!) it can be avoided with better database-design or better queries" . I think he's right, we should avoid using "distinct, group by, order by" (if any).

I met a situation as Sam's case and I think Sam can use partition on event table by month. It'll reduce your data size when you query, but you need a function (pl/pgsql) to execute instead of query above. The function will find appropriate partitions (depend on conditions) to execute query .

Marcello Miorelli
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Luan Huynh
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1

You could try creating a spatial index such as an "rtree" index on all your columns (time, project_id, user_id). I think this could speed up the query in theory, but I am not sure.

For others seeking speeding up SELECT DISTINCT without WHERE: Some database engines implement a special algorithm ("index skip scan", "loose indexscan", "jump scan") just to select distinct values from the leading columns of a b-tree index. PostgreSQL does not have it yet but has it on the roadmap as of 2020. See Loose indexscan on the Postgres Wiki. It does not help in this particular case, because you have a range filter on another column, which also has to use the leading columns of a b-tree index. You have to choose only one.

Tamas Hegedus
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