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PostgreSQL: select nearest rows according to sort order

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 search for a specific row - 1.3293 | 7876543213

for example - and select the next 4 rows. 2 above, 2 below, if possible.
Sort order . 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? (I'm using PHP by the way.)

PS: for the last or first line, the next lines will be 4 higher or lower.

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sql-order-by postgresql rows selection window-functions


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3 answers


Test version:

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 the result as shown in the question. Assuming that (a, user_id)

is the only one.

It is unclear if a

unique. This is why I am sorting user_id

extra to break ties. This is why I am using the window functionrow_number()

, not rank()

for this. row_number()

is the right tool in any case. We need 4 lines. rank()

will provide undefined number of lines if the sort order was equal.

This always returns 4 rows if the table has at least 5 rows. Close to the first / last line, the first / last 4 lines are returned. Two lines before / after in all other cases. The criteria string itself is excluded.



Improved performance

This is an improved version of what @Tim Landscheidt posted. Vote for his answer if you like the index idea. Do not interfere with small tables. But it will improve performance for large tables - with a suitable index. The best choice would be a multi-column 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

The main differences from @Tim's version:

  • In accordance with the question, (a, user_id)

    form the search criteria, not just a

    . This changes the window frame ORDER BY

    and the WHERE

    suggestion is different.

  • UNION

    immediately, there is no need for an additional level of queries. You need to copy around two UNION queries to resolve separate ones ORDER BY

    .

  • Sort the result by request. Requires a different level of query (with little or no cost).

  • Since parameters are used in multiple places, I centralized the input in the master CTE.
    For reuse, you can wrap this query almost as is in a SQL or plpgsql function.

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And one more:

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

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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 line is not needed in the output. Also, 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: always select four, even if we are at the top or bottom of the list. This makes the request a little ugly. (but not as ugly as Erwin;)

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: version with nested CTE (with outer join !!!). For convenience, I've 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 
        ;
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