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Look at the following example starting from the top row (id=9) and work your way down, selecting a limit of 4 rows that have sec's that we have not yet seen. We "select" id=9 because we don't yet have sec=1. We continue to work our way down like this, but when we get to id=7 we skip it because we already have sec=5 (from row with id=8). We continue in the same manner, and we finally stop at id=3 because we have accumulated 4 rows (our desired limit).

 id | sec
----+-----
  9 |   1  <- 1
  8 |   5  <- 2
  7 |   5  # skip, already have sec=5
  6 |   4  <- 3
  5 |   1  # skip, already have sec=1
  4 |   1  # skip, already have sec=1
  3 |   3  <- 4
  2 |   2
  1 |   1

Of course the SQL algorithm can (will!) be different than I described.

Desired result:

 id
----
  9
  8
  6
  3
(4 rows)

If I wanted to increase the limit to 5 rows, then the row with id=2 would be included in the results. However, if I increased the limit to 6 rows, the row with id=1 would not be added because sec=1 has already been seen.

Note: Though it shouldn't matter, I am on PostgreSQL 9.3.1.


In case you want to quickly build the table to test this out:

CREATE TABLE my_table (id serial primary key, sec integer DEFAULT 0 NOT NULL);
INSERT INTO my_table (sec) VALUES
  (1)
, (2)
, (3)
, (1)
, (1)
, (4)
, (5)
, (5)
, (1);
CREATE INDEX index_my_table_on_sec ON my_table (sec);
share|improve this question
    
I had a new idea and added a working recursive CTE to my answer (finally). Would you mind comparing performance on your big table? –  Erwin Brandstetter Apr 9 at 6:21
    
Sure, but please tell me how to use it - just as earlier you showed me how to call your f_first_uniq function. –  user664833 Apr 9 at 18:44
    
Oh, it's just a plain query. Use it just like the other query. I added a demo to the fiddle (which seems to be down atm). –  Erwin Brandstetter Apr 9 at 19:06
1  
On my computer, given a table of 1M rows, on average the PARTITION query takes 5.9s, the DISTINCT query takes 3.6s, the RECURSIVE query takes 1.5s, and the FUNCTION query takes less than 1ms. –  user664833 Apr 13 at 23:39
    
Thanks for the feedback! It's good to back the quick tests with some "real life" numbers. –  Erwin Brandstetter Apr 13 at 23:49

2 Answers 2

up vote 5 down vote accepted

In Postgres, this is simpler with DISTINCT ON:

SELECT *
FROM (
   SELECT DISTINCT ON (sec)
          id, sec
   FROM   tbl
   ORDER  BY sec, id DESC
   ) sub
ORDER  BY id DESC
LIMIT  4;

Detailed explanation in this related answer on SO:
Select first row in each GROUP BY group?

For a big table and small LIMIT, neither this nor @a_horse's solution are very efficient. The subquery will plough through the whole table, wasting a lot of time ...

Recursive CTE

I have tried and failed to solve similar problems with a recursive CTE in the past and resorted to a procedural solution with PL/pgSQL. Example:
Self-join unique pairs

Finally, here is a working recursive CTE (!):

WITH RECURSIVE cte AS (
  (SELECT id, '{}'::int[] AS last_arr, ARRAY[sec] AS arr
   FROM   tbl
   ORDER  BY id DESC
   LIMIT  1)

   UNION ALL
  (SELECT b.id, c.arr
        , CASE WHEN b.sec = ANY (c.arr) THEN c.arr ELSE b.sec  || c.arr END
   FROM   cte c
   JOIN   tbl b ON b.id < c.id
   WHERE  array_length(c.arr, 1) < 4
   ORDER  BY id DESC
   LIMIT  1)
   )
SELECT id, arr[1] AS sec
FROM   cte
WHERE  last_arr <> arr;

It's not as fast or elegant as I had hoped for, not nearly as fast as the function below, but faster than the query above in my tests.

PL/pgSQL function

Fastest by far:

CREATE OR REPLACE FUNCTION f_first_uniq(_rows int)
   RETURNS TABLE (id int, sec int) AS
$func$
DECLARE
   _arr int[];
BEGIN

FOR id, sec IN
   SELECT t.id, t.sec FROM tbl t ORDER BY t.id DESC
LOOP
   IF sec = ANY (_arr) THEN
   ELSE
      RETURN NEXT;
      _arr := _arr || sec;
      EXIT WHEN array_length(_arr, 1) >= _rows;
   END IF;
END LOOP;
END
$func$  LANGUAGE plpgsql;

Call:

SELECT * FROM f_first_uniq(4);

SQL Fiddle demonstrating all three.

Could be made out to work for any table with dynamic SQL and EXECUTE and table and column names as parameters ...

Why bother?

In a test table with only 30k rows the function ran 2000x faster than the above query (which already ran ~ 30% faster than a_horse's version). This difference grows with the size of the table. Performance of the function is about constant, while the query's performance gets progressively worse, since it tries to find distinct values in all of the table first. Try this in a table with a million rows ...

share|improve this answer
SELECT id,
       sec
FROM (
  SELECT id,
         sec,
         row_number() OVER (PARTITION BY sec ORDER BY id DESC) AS rn
  FROM my_table
) t
WHERE rn = 1
ORDER BY id DESC 
LIMIT 4;

SQLFiddle example: http://sqlfiddle.com/#!15/1ca01/1

share|improve this answer
    
I know we're supposed to avoid "thanks" comments, but I really want to let you know how much I appreciate this answer - it does exactly what I need, and I would not have been able to come up with it myself. So, thank you kindly! Out of curiosity, and as far as you know, is there more than one way to do it or is this pretty much it? –  user664833 Apr 5 at 20:05
    
@user664833: Feedback like this (including thanks) in a comment is welcome. Just keep the noise level in questions and answers low. –  Erwin Brandstetter Apr 5 at 21:51
    
@ErwinBrandstetter: Ok. Do you think my question had too much noise, and if so, what could have been avoided or improved? Thanks! –  user664833 Apr 5 at 22:00
    
@user664833: Your question is good. You might have started with what you want instead of a detailed algorithm how to get it. And I'll trim some noise from your code. But you made your case clear and your question is good. –  Erwin Brandstetter Apr 5 at 22:15
1  
@a-horse-with-no-name: I am very sorry, and with all due respect, I had to re-assign the answer mark to @ErwinBrandstetter because his DISTINCT ON solution is considerably faster than yours (28-43%) based on my benchmarks on a table with 100k rows (his query regularly returns in 450-500ms. Furthermore, the speed of his f_first_uniq function is remarkable (it regularly returns in 0.5-5ms. Nonetheless, I do sincerely appreciate your answer, and would have continued to use your solution had @ErwinBrandstetter not presented his. I thank you kindly once again. –  user664833 Apr 8 at 18:12

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