SQL - Find missing int values in mostly ordered sequential series

Question

I manage a message based system in which a sequence of unique integer ids will be entirely represented at the end of the day, though they will not necessarily arrive in orde

Answer 1

SET search_path='tmp';

DROP table tmp.table_name CASCADE;
CREATE table tmp.table_name ( num INTEGER NOT NULL PRIMARY KEY);
-- make some data
INSERT INTO tmp.table_name(num) SELECT generate_series(1,20);
-- create some gaps
DELETE FROM tmp.table_name WHERE random() < 0.3 ;

SELECT * FROM table_name;

-- EXPLAIN ANALYZE
WITH zbot AS (
    SELECT 1+tn.num  AS num
    FROM table_name tn
    WHERE NOT EXISTS (
        SELECT * FROM table_name nx
        WHERE nx.num = tn.num+1
        )
    )
, ztop AS (
    SELECT -1+tn.num  AS num
    FROM table_name tn
    WHERE NOT EXISTS (
        SELECT * FROM table_name nx
        WHERE nx.num = tn.num-1
        )
    )
SELECT zbot.num AS bot
    ,ztop.num AS top
FROM zbot, ztop
WHERE zbot.num <= ztop.num
AND NOT EXISTS ( SELECT *
    FROM table_name nx
    WHERE nx.num >= zbot.num
    AND nx.num <= ztop.num
    )
ORDER BY bot,top
    ;

Result:

CREATE TABLE
INSERT 0 20
DELETE 9
 num 
-----
   1
   2
   6
   7
  10
  11
  13
  14
  15
  18
  19
(11 rows)

 bot | top 
-----+-----
   3 |   5
   8 |   9
  12 |  12
  16 |  17
(4 rows)

Note: a recursive CTE is also possible (and probably shorter).

UPDATE: here comes the recursive CTE ...:

WITH RECURSIVE tree AS (
    SELECT 1+num AS num
    FROM table_name t0
    UNION
    SELECT 1+num FROM tree tt
    WHERE EXISTS ( SELECT *
        FROM table_name xt
        WHERE xt.num > tt.num
        )
    )
SELECT * FROM tree
WHERE NOT EXISTS (
    SELECT *
    FROM table_name nx
    WHERE nx.num = tree.num
    )
ORDER BY num
    ;

Results: (same data)

 num 
-----
   3
   4
   5
   8
   9
  12
  16
  17
  20
 (9 rows)