How do I sort a linked list in sql?

Question

I have implemented a linked list as a self-referencing database table:

CREATE TABLE LinkedList(
    Id bigint NOT NULL,
    ParentId bigint NULL,
    SomeDat         


        

Answer 1

PostgreSQL version.

Create table, indexes and data:

DROP TABLE IF EXISTS LinkedList;

CREATE TABLE LinkedList (
    Id BIGINT NOT NULL,
    ParentId BIGINT NULL,
    SomeData VARCHAR(50)
);

CREATE INDEX LinkedList_Id_idx on LinkedList (Id);
CREATE index LinkedList_ParentId_idx on LinkedList (ParentId);

INSERT INTO LinkedList
    (Id, ParentId, SomeData)
VALUES 
    (24971,   NULL,      0),
    (38324,   24971,     1),
    (60088,   60089,     3),
    (60089,   38324,     2),
    (61039,   61497,     5),
    (61497,   60088,     4),
    (109397,  109831,    7),
    (109831,  61039,     6);

Actual query:

WITH RECURSIVE SortedList AS (
    SELECT
        *,
        0 AS SortKey
    FROM LinkedList
    WHERE ParentId IS NULL
    UNION ALL (
        SELECT
            LinkedList.*,
            SortedList.SortKey + 1 AS SortKey
        FROM LinkedList
        INNER JOIN SortedList
            ON (LinkedList.ParentId = SortedList.Id)
    )
)
SELECT
    *
FROM SortedList
ORDER BY SortKey;

Results:

   id   | parentid | somedata | sortkey
--------+----------+----------+---------
  24971 |          | 0        |       0
  38324 |    24971 | 1        |       1
  60089 |    38324 | 2        |       2
  60088 |    60089 | 3        |       3
  61497 |    60088 | 4        |       4
  61039 |    61497 | 5        |       5
 109831 |    61039 | 6        |       6
 109397 |   109831 | 7        |       7

Also did some benchmarks:

\set N 10000

DELETE FROM LinkedList;
INSERT INTO LinkedList VALUES (1, NULL, 1);
INSERT INTO LinkedList (
    SELECT
        generate_series AS Id,
        (generate_series - 1) AS ParentId,
        generate_series AS SomeData
    FROM GENERATE_SERIES(2, :N)
);

EXPLAIN ANALYZE
WITH RECURSIVE SortedList AS (
    SELECT
        *,
        0 AS SortKey
    FROM LinkedList
    WHERE ParentId IS NULL
    UNION ALL (
        SELECT
            LinkedList.*,
            SortedList.SortKey + 1 AS SortKey
        FROM LinkedList
        INNER JOIN SortedList
            ON (LinkedList.ParentId = SortedList.Id)
    )
)
SELECT
    *
FROM SortedList
ORDER BY SortKey;

Results:

Sort  (cost=6236.12..6300.16 rows=25616 width=138) (actual time=17857.640..17858.207 rows=10000 loops=1)
  Sort Key: sortedlist.sortkey
  Sort Method: quicksort  Memory: 1166kB
  CTE sortedlist
    ->  Recursive Union  (cost=4.40..2007.10 rows=25616 width=138) (actual time=0.032..17844.139 rows=10000 loops=1)
          ->  Bitmap Heap Scan on linkedlist  (cost=4.40..42.78 rows=16 width=138) (actual time=0.031..0.032 rows=1 loops=1)
                Recheck Cond: (parentid IS NULL)
                Heap Blocks: exact=1
                ->  Bitmap Index Scan on linkedlist_parentid_idx  (cost=0.00..4.40 rows=16 width=0) (actual time=0.006..0.006 rows=2 loops=1)
                      Index Cond: (parentid IS NULL)
          ->  Hash Join  (cost=5.20..145.20 rows=2560 width=138) (actual time=0.896..1.780 rows=1 loops=10000)
                Hash Cond: (linkedlist_1.parentid = sortedlist_1.id)
                ->  Seq Scan on linkedlist linkedlist_1  (cost=0.00..96.00 rows=3200 width=134) (actual time=0.002..0.784 rows=10000 loops=10000)
                ->  Hash  (cost=3.20..3.20 rows=160 width=12) (actual time=0.001..0.001 rows=1 loops=10000)
                      Buckets: 1024  Batches: 1  Memory Usage: 9kB
                      ->  WorkTable Scan on sortedlist sortedlist_1  (cost=0.00..3.20 rows=160 width=12) (actual time=0.000..0.001 rows=1 loops=10000)
  ->  CTE Scan on sortedlist  (cost=0.00..512.32 rows=25616 width=138) (actual time=0.034..17851.344 rows=10000 loops=1)
Planning Time: 0.163 ms
Execution Time: 17858.957 ms

So this query is pretty slow.