从表中的开始和结束日期在 Postgres 中生成_series

Question

I have been trying to generate a series of dates (YYYY-MM-DD HH) from the first until the last date in a timestamp field. I've got the generate_series() I need, however running into an issue when trying to grab the start and end dates from a table. I have the following to give a rough idea:

with date1 as
(
SELECT start_timestamp as first_date
FROM header_table
ORDER BY start_timestamp DESC
LIMIT 1
),
date2 as
(
SELECT start_timestamp as first_date
FROM header_table
ORDER BY start_timestamp ASC    
LIMIT 1
)
    select generate_series(date1.first_date, date2.first_date
                         , '1 hour'::interval)::timestamp as date_hour

from
(   select * from date1
    union
    select * from date2) as foo

Postgres 9.3

Answer 1

You don't need a CTE for this, that would be more expensive than necessary.
And you don't need to cast to timestamp , the result already is of data type timestamp when you feed timestamp types to generate_series() . Details here:

• Generating time series between two dates in PostgreSQL

In Postgres 9.3 or later you can use a LATERAL join:

SELECT to_char(ts, 'YYYY-MM-DD HH24') AS formatted_ts
FROM  (
   SELECT min(start_timestamp) as first_date
        , max(start_timestamp) as last_date
   FROM   header_table
   ) h
  , generate_series(h.first_date, h.last_date, interval '1 hour') g(ts);

Optionally with to_char() to get the result as text in the format you mentioned.

This works in any Postgres version:

SELECT generate_series(min(start_timestamp)
                     , max(start_timestamp)
                     , interval '1 hour') AS ts
FROM   header_table;

Typically a bit faster.
Calling set-returning functions in the SELECT list is a non-standard-SQL feature and frowned upon by some. Also, there were behavioral oddities (though not for this simple case) that were eventually fixed in Postgres 10. See:

• What is the expected behaviour for multiple set-returning functions in SELECT clause?

Note a subtle difference in NULL handling:

The equivalent of

max(start_timestamp)

is obtained with

ORDER BY start_timestamp DESC 
LIMIT 1

Without NULLS LAST NULL values come first in descending order (if there can be NULL values in start_timestamp ). You would get NULL for last_date and your query would come up empty.

Details:

• Why do NULL values come first when ordering DESC in a PostgreSQL query?

Answer 2

How about using aggregation functions instead?

with dates as (
      SELECT min(start_timestamp) as first_date, max(start_timestamp) as last_date
      FROM header_table
     )
select generate_series(first_date, last_date, '1 hour'::interval)::timestamp as date_hour
from dates;

Or even:

select generate_series(min(start_timestamp),
                       max(start_timestamp),
                       '1 hour'::interval
                      )::timestamp as date_hour
from header_table;

Answer 3

try this:

with dateRange as
  (
  SELECT min(start_timestamp) as first_date, max(start_timestamp) as last_date
  FROM header_table
  )
select 
    generate_series(first_date, last_date, '1 hour'::interval)::timestamp as date_hour
from dateRange

NB: You want the 2 dates in a row, not on separate rows.

see this sqlfiddle demo