MySQL/Postgres query 5 minutes interval data

问题

I need help with the query, let's say that this is the data in table.

timestamp           
------------------- 
2010-11-16 10:30:00
2010-11-16 10:37:00
2010-11-16 10:40:00 
2010-11-16 10:45:00
2010-11-16 10:48:00
2010-11-16 10:55:00
2010-11-16 10:56:00

I want to get every first row (timestamp) that is at least 5 minutes later than the last. In this case the query should return:

timestamp           
------------------- 
2010-11-16 10:30:00
2010-11-16 10:37:00
2010-11-16 10:45:00
2010-11-16 10:55:00

回答1:

Recursive CTE

Since each row depends on the one before, it is hard to solve with a set-based approach. Resorting to a recursive CTE (which is standard SQL):

WITH RECURSIVE cte AS (
   (SELECT ts FROM tbl
    ORDER  BY ts
    LIMIT  1)

   UNION ALL
   (SELECT t.ts
    FROM   cte c
    JOIN   tbl t ON t.ts >= c.ts + interval '5 min'
    ORDER  BY t.ts
    LIMIT  1)
   )
SELECT * FROM cte ORDER BY ts;

Note the update from my first draft:
Aggregate functions are not allowed in a recursive CTE. I substituted with ORDER BY / LIMIT 1, which should be fast when supported by an index on ts.

The parentheses around each leg of the UNION query are necessary to allow LIMIT, which would otherwise only be permitted once at the end of a UNION query.

PL/pgSQL function

A procedural solution (example with a plpgsql function) iterating through the sorted table would probably be a lot faster, since it can make do with a single table scan:

CREATE OR REPLACE FUNCTION f_rowgrid(i interval)
  RETURNS SETOF timestamp AS
$func$
DECLARE
   _this  timestamp;
   _last  timestamp := '-infinity';     -- init so that 1 row passes
BEGIN

FOR _this IN
    SELECT ts FROM tbl ORDER BY 1
LOOP
    IF _this >= _last + i THEN
       RETURN NEXT _this;
       _last := _this;
    END IF;
END LOOP;

END
$func$ LANGUAGE plpgsql;

Call: