SQL Server: stored procedure using recursive CTE finding values matching a total - sql

Question

I need to find within a stored procedure which values match a wanted total following valex's solution recursive query in SQL Server
The following works pretty well assuming the CTE anchor recordset is very small
CREATE TABLE #t ([id] INT, [num] FLOAT);
DECLARE #wanted FLOAT = 100000
INSERT INTO #t ([id], [num])
VALUES (1, 17000), (2, 33000), (3, 53000), (4, 47000), (5, 10000),
(6, 53000), (7, 7000), (8, 10000), (9, 20000), (10, 5000),
(11, 40000), (12, 30000), (13, 10000), (14, 8000), (15, 8000),
(16, 10000), (17, 74000)
/* when you add more records the query becomes too slow, remove this comment
to test*/
/*,(18,10000),(19,78000),(20,10000),(21,10000),(22,80000),(23,19000),
(24,8000),(25,5000),(26,10000),(27,4000),(28,46000),(29,48000),(30,20000),
(31,10000),(32,25000),(33,10000),(34,13000),(35,16000),(36,10000),
(37,5000), 38,5000),(39,30000),(40,15000),(41,10000)*/
;
CREATE NONCLUSTERED INDEX [idx_id] ON #t ([id]);
WITH CTE AS
(
SELECT
id, num AS CSum,
CAST(id AS VARCHAR(MAX)) AS path
FROM
#t
WHERE num <= #wanted
UNION ALL
SELECT
#t.id, #t.num + CTE.CSum AS CSum,
CTE.path + ',' + CAST(#t.id AS VARCHAR(MAX)) AS path
FROM
#T
INNER JOIN
CTE ON #T.num + CTE.CSum <= #wanted AND CTE.id < #T.id
WHERE
#T.num + CTE.CSum <= #wanted
)
SELECT TOP 1 Path
FROM CTE
WHERE CTE.CSum = #wanted
ORDER BY id
DROP TABLE #t
It will return 3,4 which are the first 2 rows whose [num] values gives the #wanted total.
This works reasonably fast when there are just a few records in the temp table #t but when you remove the comment and all remaining records (from id 17 to id 41) the query just takes forever because the CTE grows exponentially.
Is there a way to speed up the code? i just need the first matching total (the list anchor dataset is ordered so a result like 3,4 is better than 8,20,22)

Answer 1

What if you took an iterative approach? This would be pretty simple to give the ability to stop as soon as a solution is found.
This was put together quickly, so you may can optimize further. I tested for your example (ran in less than 1 second) and several other combinations and levels of depth.
Result Depth Total IdList NumList
------ ----------- ----------- ---------- -------------
Found 1 100000 3,4 53000,47000
Full Code:
-- Configuration
DECLARE #wanted FLOAT = 100000
DECLARE #MaxDepth INT = 10 -- Customize how many levels you want to look
SET NOCOUNT ON
IF OBJECT_ID('tempdb..#T') IS NOT NULL DROP TABLE #T
IF OBJECT_ID('tempdb..#T') IS NULL BEGIN
CREATE TABLE #T (Id INT, Num INT)
INSERT INTO #t ([id], [num])
VALUES (1, 17000), (2, 33000), (3, 53000), (4, 47000), (5, 10000),
(6, 53000), (7, 7000), (8, 10000), (9, 20000), (10, 5000),
(11, 40000), (12, 30000), (13, 10000), (14, 8000), (15, 8000),
(16, 10000), (17, 74000)
CREATE NONCLUSTERED INDEX [idx_id] ON #t ([id]);
END
-- Setup processing table
IF OBJECT_ID('tempdb..#U') IS NOT NULL DROP TABLE #U
CREATE TABLE #U (
MaxId INT,
Total INT,
IdList VARCHAR(MAX),
NumList VARCHAR(MAX)
)
-- Initial population from source table
INSERT #U
SELECT Id, Num,
CONVERT(VARCHAR(10), Id),
CONVERT(VARCHAR(10), Num)
FROM #T
-- Iterative approach
DECLARE #Depth INT = 0
WHILE NOT EXISTS (SELECT * FROM #U WHERE Total = #wanted) BEGIN
-- Increment depth
SET #Depth = #Depth + 1
IF #Depth >= #MaxDepth BEGIN
PRINT 'Max depth reached'
RETURN -- Stop processing further
END
-- Calculate sum for this depth
IF OBJECT_ID('tempdb..#V') IS NOT NULL
DROP TABLE #V
SELECT
T.Id AS MaxId,
U.Total + T.Num AS Total,
U.IdList + ',' + CONVERT(VARCHAR(10), T.Id) AS IdList,
U.NumList + ',' + CONVERT(VARCHAR(10), T.Num) AS NumList
INTO #V
FROM #U U
INNER JOIN #T T
ON U.MaxId < T.Id
-- Replace data for next iteration
TRUNCATE TABLE #U
INSERT #U
SELECT * FROM #V
-- Check if no more combinations available
IF ##ROWCOUNT = 0 BEGIN
PRINT 'All combinations tested'
RETURN -- Stop processing further
END
END
-- Return result
SELECT TOP 1 'Found' AS [Result], #Depth AS Depth, Total, IdList, NumList FROM #U WHERE Total = #wanted