Using sql function generate_series() in redshift - sql

Question

I'd like to use the generate series function in redshift, but have not been successful.
The redshift documentation says it's not supported. The following code does work:
select *
from generate_series(1,10,1)
outputs:
1
2
3
...
10
I'd like to do the same with dates. I've tried a number of variations, including:
select *
from generate_series(date('2008-10-01'),date('2008-10-10 00:00:00'),1)
kicks out:
ERROR: function generate_series(date, date, integer) does not exist
Hint: No function matches the given name and argument types.
You may need to add explicit type casts. [SQL State=42883]
Also tried:
select *
from generate_series('2008-10-01 00:00:00'::timestamp,
'2008-10-10 00:00:00'::timestamp,'1 day')
And tried:
select *
from generate_series(cast('2008-10-01 00:00:00' as datetime),
cast('2008-10-10 00:00:00' as datetime),'1 day')
both kick out:
ERROR: function generate_series(timestamp without time zone, timestamp without time zone, "unknown") does not exist
Hint: No function matches the given name and argument types.
You may need to add explicit type casts. [SQL State=42883]
If not looks like I'll use this code from another post:
SELECT to_char(DATE '2008-01-01'
+ (interval '1 month' * generate_series(0,57)), 'YYYY-MM-DD') AS ym
PostgreSQL generate_series() with SQL function as arguments

Answer 1

Amazon Redshift seems to be based on PostgreSQL 8.0.2. The timestamp arguments to generate_series() were added in 8.4.
Something like this, which sidesteps that problem, might work in Redshift.
SELECT current_date + (n || ' days')::interval
from generate_series (1, 30) n
It works in PostgreSQL 8.3, which is the earliest version I can test. It's documented in 8.0.26.
Later . . .
It seems that generate_series() is unsupported in Redshift. But given that you've verified that select * from generate_series(1,10,1) does work, the syntax above at least gives you a fighting chance. (Although the interval data type is also documented as being unsupported on Redshift.)
Still later . . .
You could also create a table of integers.
create table integers (
n integer primary key
);
Populate it however you like. You might be able to use generate_series() locally, dump the table, and load it on Redshift. (I don't know; I don't use Redshift.)
Anyway, you can do simple date arithmetic with that table without referring directly to generate_series() or to interval data types.
select (current_date + n)
from integers
where n < 31;
That works in 8.3, at least.

Answer 2

Using Redshift today, you can generate a range of dates by using datetime functions and feeding in a number table.
select (getdate()::date - generate_series)::date from generate_series(1,30,1)
Generates this for me
date
2015-11-06
2015-11-05
2015-11-04
2015-11-03
2015-11-02
2015-11-01
2015-10-31
2015-10-30
2015-10-29
2015-10-28
2015-10-27
2015-10-26
2015-10-25
2015-10-24
2015-10-23
2015-10-22
2015-10-21
2015-10-20
2015-10-19
2015-10-18
2015-10-17
2015-10-16
2015-10-15
2015-10-14
2015-10-13
2015-10-12
2015-10-11
2015-10-10
2015-10-09
2015-10-08

Answer 3

The generate_series() function is not fully supported by Redshift. See the Unsupported PostgreSQL functions section of the developer guide.
UPDATE
generate_series is working with Redshift now.
SELECT CURRENT_DATE::TIMESTAMP - (i * interval '1 day') as date_datetime
FROM generate_series(1,31) i
ORDER BY 1
This will generate last 30 days date
Ref: generate_series function in Amazon Redshift

Answer 4

As of writing this, generate_series() on our instance of Redshift (1.0.33426) could not be used to, for example, create a table:
# select generate_series(1,100,1);
1
2
...
# create table normal_series as select generate_series(1,100,1);
INFO: Function "generate_series(integer, integer, integer) not supported.
ERROR: Specified types or functions (one per INFO message) not supported on Redshift tables.
However, with recursive works:
# create table recursive_series as with recursive t(n) as (select 1::integer union all select n+1 from t where n < 100) select n from t;
SELECT
-- modify as desired, here is a date series:
# select getdate()::date + n from recursive_series;
2021-12-18
2021-12-19
...

Answer 5

I needed to do something similar, but with 5 minutes intervals over 7 days. So here's a CTE based hack (ugly but not too verbose)
INSERT INTO five_min_periods
WITH
periods AS (select 0 as num UNION select 1 as num UNION select 2 UNION select 3 UNION select 4 UNION select 5 UNION select 6 UNION select 7 UNION select 8 UNION select 9 UNION select 10 UNION select 11),
hours AS (select num from periods UNION ALL select num + 12 from periods),
days AS (select num from periods where num <= 6),
rightnow AS (select CAST( TO_CHAR(GETDATE(), 'yyyy-mm-dd hh24') || ':' || trim(TO_CHAR((ROUND((DATEPART (MINUTE, GETDATE()) / 5), 1) * 5 ),'09')) AS TIMESTAMP) as start)
select
ROW_NUMBER() OVER(ORDER BY d.num DESC, h.num DESC, p.num DESC) as idx
, DATEADD(minutes, -p.num * 5, DATEADD( hours, -h.num, DATEADD( days, -d.num, n.start ) ) ) AS period_date
from days d, hours h, periods p, rightnow n
Should be able to extend this to other generation schemes. The trick here is using the Cartesian product join (i.e. no JOIN/WHERE clause) to multiply the hand-crafted CTE's to produce the necessary increments and apply to an anchor date.

Answer 6

Redshift's generate_series() function is a leader node only function and as such you cannot use it for downstream processing on the compute nodes. This can be replace by a recursive CTE (or keep a "dates" table on your database). I have an example of such in a recent answer:
Cross join Redshift with sequence of dates
One caution I like to give in answers like this is to be careful with inequality joins (or cross joins or any under-qualified joins) when working with VERY LARGE tables which can happen often in Redshift. If you are joining with a moderate Redshift table of say 1M rows then things will be fine. But if you are doing this on a table of 1B rows then the data explosion will likely cause massive performance issues as the query spills to disk.
I've written a couple of white papers on how to write this type of query in a data space sensitive way. This issue of massive intermediate results is not unique to Redshift and I first developed my approach solving a client's HIVE query issue. "First rule of writing SQL for Big Data - don't make more"

Answer 7

Per the comments of #Ryan Tuck and #Slobodan Pejic generate_series() does not work on Redshift when joining to another table.
The workaround I used was to write out every value in the series in the query:
SELECT
'2019-01-01'::date AS date_month
UNION ALL
SELECT
'2019-02-01'::date AS date_month
Using a Python function like this:
import arrow
def generate_date_series(start, end):
start = arrow.get(start)
end = arrow.get(end)
months = list(
f"SELECT '{month.format('YYYY-MM-DD')}'::date AS date_month"
for month in arrow.Arrow.range('month', start, end)
)
return "\nUNION ALL\n".join(months)

Answer 8

perhaps not as elegant as other solutions, but here's how I did it:
drop table if exists #dates;
create temporary table #dates as
with recursive cte(val_date) as
(select
cast('2020-07-01' as date) as val_date
union all
select
cast(dateadd(day, 1, val_date) as date) as val_date
from
cte
where
val_date <= getdate()
)
select
val_date as yyyymmdd
from
cte
order by
val_date
;

Answer 9

For five minute buckets i would do the following:
select date_trunc('minute', getdate()) - (i || ' minutes')::interval
from generate_series(0, 60*5-1, 5) as i
You could replace 5 by any given interval, and 60 by the number of rows you want.

Answer 10

SELECT CURRENT_DATE::TIMESTAMP - (i * interval '1 day') as date_datetime
FROM generate_series(1,(select datediff(day,'01-Jan-2021',now()::date))) i
ORDER BY 1