PostgreSQL: select nearest rows according to sort order

Question

I have a table like this:

     a    |  user_id
----------+-------------
  0.1133  |  2312882332
  4.3293  |  7876123213
  3.1133  |  2312332332
  1.3293  |  7876543213
  0.0033  |  2312222332
  5.3293  |  5344343213
  3.2133  |  4122331112
  2.3293  |  9999942333

And I want to locate a particular row - 1.3293 | 7876543213 for example - and select the nearest 4 rows. 2 above, 2 below if possible.
Sort order is ORDER BY a ASC.

In this case I will get:

  0.0033  |  2312222332
  0.1133  |  2312882332
  2.3293  |  9999942333
  3.1133  |  2312332332

How can I achieve this using PostgreSQL? (BTW, I'm using PHP.)

P.S.: For the last or first row the nearest rows would be 4 above or 4 below.

Answer 1

Test case:

CREATE TEMP TABLE tbl(a float, user_id bigint);
INSERT INTO tbl VALUES
 (0.1133, 2312882332)
,(4.3293, 7876123213)
,(3.1133, 2312332332)
,(1.3293, 7876543213)
,(0.0033, 2312222332)
,(5.3293, 5344343213)
,(3.2133, 4122331112)
,(2.3293, 9999942333);

Query:

WITH x AS (
    SELECT a
          ,user_id
          ,row_number() OVER (ORDER BY a, user_id) AS rn
    FROM   tbl
    ), y AS (
    SELECT rn, LEAST(rn - 3, (SELECT max(rn) - 5 FROM x)) AS min_rn
    FROM   x
    WHERE  (a, user_id) = (1.3293, 7876543213)
    )
SELECT *
FROM   x, y
WHERE  x.rn  > y.min_rn
AND    x.rn <> y.rn
ORDER  BY x.a, x.user_id
LIMIT  4;

Returns result as depicted in the question. Assuming that (a, user_id) is unique.

It is not clear whether a is supposed to unique. That's why I sort by user_id additionally to break ties. That's also why I use the window function row_number(), an not rank() for this. row_number() is the correct tool in any case. We want 4 rows. rank() would give an undefined number of rows if there were peers in the sort order.

This always returns 4 rows as long as there are at least 5 rows in the table. Close to first / last row, the first / last 4 rows are returned. The two rows before / after in all other cases. The criteria row itself is excluded.

---

Improved performance

This is an improved version of what @Tim Landscheidt posted. Vote for his answer if you like the idea with the index. Don't bother with small tables. But will boost performance for big tables - provided you have a fitting index in place. Best choice would be a multicolumn index on (a, user_id).

WITH params(_a, _user_id) AS (SELECT 5.3293, 5344343213) -- enter params once
    ,x AS  (
    (
    SELECT a
          ,user_id
          ,row_number() OVER (ORDER BY a DESC, user_id DESC) AS rn
    FROM   tbl, params p
    WHERE  a < p._a
       OR  a = p._a AND user_id < p._user_id -- a is not defined unique
    ORDER  BY a DESC, user_id DESC
    LIMIT  5  -- 4 + 1: including central row
    )
    UNION ALL -- UNION right away, trim one query level
    (
    SELECT a
          ,user_id
          ,row_number() OVER (ORDER BY a ASC, user_id ASC) AS rn
    FROM   tbl, params p
    WHERE  a > p._a
       OR  a = p._a AND user_id > p._user_id
    ORDER  BY a ASC, user_id ASC
    LIMIT  5
    )
    )
    , y AS (
    SELECT a, user_id
    FROM   x, params p
    WHERE (a, user_id) <> (p._a, p._user_id) -- exclude central row
    ORDER  BY rn  -- no need to ORDER BY a
    LIMIT  4
    )
SELECT *
FROM   y
ORDER  BY a, user_id   -- ORDER result as requested

Major differences to @Tim's version:

• According to the question (a, user_id) form the search criteria, not just a. That changes window frame, ORDER BY and WHERE clause in subtly different ways.

• UNION right away, no need for an extra query level. You need parenthesis around the two UNION-queries to allow for individual ORDER BY.

• Sort result as requested. Requires another query level (at hardly any cost).

• As parameters are used in multiple places I centralized the input in a leading CTE.
  For repeated use you can wrap this query almost 'as is' into an SQL or plpgsql function.

Answer 2

set search_path='tmp';

DROP TABLE lutser;
CREATE TABLE lutser
        ( val float
        , num bigint
        );
INSERT INTO lutser(val, num)
VALUES ( 0.1133  ,  2312882332  )
      ,( 4.3293  ,  7876123213  )
      ,( 3.1133  ,  2312332332  )
      ,( 1.3293  ,  7876543213  )
      ,( 0.0033  ,  2312222332  )
      ,( 5.3293  ,  5344343213  )
      ,( 3.2133  ,  4122331112  )
      ,( 2.3293  ,  9999942333  )
        ;

WITH ranked_lutsers AS (
        SELECT val, num
        ,rank() OVER (ORDER BY val) AS rnk
        FROM lutser
        )
SELECT that.val, that.num
        , (that.rnk-this.rnk) AS relrnk
FROM ranked_lutsers that
JOIN ranked_lutsers this ON (that.rnk BETWEEN this.rnk-2 AND this.rnk+2)
WHERE this.val = 1.3293
        ;

Results:

DROP TABLE
CREATE TABLE
INSERT 0 8
  val   |    num     | relrnk 
--------+------------+--------
 0.0033 | 2312222332 |     -2
 0.1133 | 2312882332 |     -1
 1.3293 | 7876543213 |      0
 2.3293 | 9999942333 |      1
 3.1133 | 2312332332 |      2
(5 rows)

As Erwin pointed out, the center row is not wanted in the output. Also, the row_number() should be used instead of rank().

WITH ranked_lutsers AS (
        SELECT val, num
        -- ,rank() OVER (ORDER BY val) AS rnk
        , row_number() OVER (ORDER BY val, num) AS rnk
        FROM lutser
) SELECT that.val, that.num
        , (that.rnk-this.rnk) AS relrnk
FROM ranked_lutsers that
JOIN ranked_lutsers this ON (that.rnk BETWEEN this.rnk-2 AND this.rnk+2 )
WHERE this.val = 1.3293
AND that.rnk <> this.rnk
        ;

Result2:

  val   |    num     | relrnk 
--------+------------+--------
 0.0033 | 2312222332 |     -2
 0.1133 | 2312882332 |     -1
 2.3293 | 9999942333 |      1
 3.1133 | 2312332332 |      2
(4 rows)

UPDATE2: to always select four, even if we are at the top or bottom of the list. This makes the query a bit uglier. (but not as ugly as Erwin's ;-)

WITH ranked_lutsers AS (
        SELECT val, num
        -- ,rank() OVER (ORDER BY val) AS rnk
        , row_number() OVER (ORDER BY val, num) AS rnk
        FROM lutser
) SELECT that.val, that.num
        , ABS(that.rnk-this.rnk) AS srtrnk
        , (that.rnk-this.rnk) AS relrnk
FROM ranked_lutsers that
JOIN ranked_lutsers this ON (that.rnk BETWEEN this.rnk-4 AND this.rnk+4 )
-- WHERE this.val = 1.3293
WHERE this.val = 0.1133
AND that.rnk <> this.rnk
ORDER BY srtrnk ASC
LIMIT 4
        ;

Output:

  val   |    num     | srtrnk | relrnk 
--------+------------+--------+--------
 0.0033 | 2312222332 |      1 |     -1
 1.3293 | 7876543213 |      1 |      1
 2.3293 | 9999942333 |      2 |      2
 3.1133 | 2312332332 |      3 |      3
(4 rows)

UPDATE: A version with a nested CTE (featuring outer join!!!). For conveniance, I added a primary key to the table, which sounds like a good idea anyway IMHO.

WITH distance AS (
        WITH ranked_lutsers AS (
        SELECT id
        , row_number() OVER (ORDER BY val, num) AS rnk
        FROM lutser
        ) SELECT l0.id AS one
        ,l1.id AS two
        , ABS(l1.rnk-l0.rnk) AS dist
        -- Warning: Cartesian product below
        FROM ranked_lutsers l0
        , ranked_lutsers l1 WHERE l0.id <> l1.id

        )
SELECT lu.*
FROM lutser lu
JOIN distance di
ON lu.id = di.two
WHERE di.one= 1
ORDER by di.dist
LIMIT 4 
        ;

Answer 3

And another one:

WITH prec_rows AS
  (SELECT a,
          user_id,
          ROW_NUMBER() OVER (ORDER BY a DESC) AS rn
   FROM tbl
   WHERE a < 1.3293
   ORDER BY a DESC LIMIT 4),
     succ_rows AS
  (SELECT a,
          user_id,
          ROW_NUMBER() OVER (ORDER BY a ASC) AS rn
   FROM tbl
   WHERE a > 1.3293
   ORDER BY a ASC LIMIT 4)
SELECT a, user_id
FROM
  (SELECT a,
          user_id,
          rn
   FROM prec_rows
   UNION ALL SELECT a,
                    user_id,
                    rn
   FROM succ_rows) AS s
ORDER BY rn, a LIMIT 4;

AFAIR WITH will instantiate a memory table, so the focus of this solution is to limit its size as much as possible (in this case eight rows).