Postgres 13

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CREATE TABLE data (
  data_id serial PRIMARY KEY
, day     date   NOT NULL  -- helps a bit
, values  numeric[] 
);

-- random dates over 15 years. Roughly 30 dates per day, 160k rows
INSERT INTO data (day, values)
SELECT '2000-01-01'::date + (random() * 5500)::int
     , ('{123' || repeat(',456', (random() * 10)::int) || '}')::numeric[]  -- dummy of varying size
FROM   generate_series(1, 160000) g;

CREATE INDEX data_day_idx ON data (day);

CREATE TABLE



INSERT 0 160000



CREATE INDEX


-- rCTE
WITH RECURSIVE cte AS (
   (
   SELECT day
   FROM   data
   WHERE  day >= '2010-01-01'
   ORDER  BY 1
   LIMIT  1
   )
  
   UNION ALL
   SELECT (SELECT day FROM data
           WHERE  day > c.day
           AND    day < '2011-01-01'
           ORDER  BY 1
           LIMIT  1)
   FROM   cte c
   WHERE  day IS NOT NULL
   )
SELECT count(*) AS ct
FROM   cte
WHERE  day IS NOT NULL;


ct


365



SELECT 1


-- EXISTS
SELECT count(*) AS ct
FROM   generate_series(timestamp '2010-01-01'
                     , timestamp '2010-12-31'
                     , interval  '1 day') AS d(day)
WHERE  EXISTS (SELECT FROM data WHERE day = d.day::date);


ct


365



SELECT 1