SQL Server : slow query with 3 “OR” condition and order by Id

问题

My query:

SELECT TOP 30 a.*  
FROM Car 
WHERE (ModelId = 642 AND RegionId IN (23, 63, 1, 30, 5, 6, 7, 9, 15)) OR
      (ModelId = 242 AND RegionId IN (23, 63, 1, 30, 5, 6, 7, 9, 15, 26)) OR 
      (ModelId = 334 AND RegionId IN (23, 63, 1, 30, 5, 6, 7, 9, 15, 26)) 
ORDER BY Id DESC

It executes in 30 seconds.

If I remove order by Id section, it executes in 1 second.

Or if I remove one OR condition, it also executes in 1 second.

WHERE (ModelId = 642 AND RegionId IN (23, 63, 1, 30, 5, 6, 7, 9, 15)) OR    
      (ModelId = 242 AND RegionId IN (23, 63, 1, 30, 5, 6, 7, 9, 15, 26)) 
ORDER BY Id DESC

Columns ModelId and RegionId has indexes. Id is primary key.

Execution plan show that 97% clustered index searched for Id.

Please, help me. Why so slowly for 3 OR condition with ORDER BY?

回答1:

You could try to expand OR(from Oracle but idea is the same):

OR expansion is a transformation that can be used to optimize disjunctive queries (queries that contain OR clauses).

The basic idea in OR expansion is to transform a query containing disjunctions into the form of a UNION ALL query of two or more branches. This is done by splitting the disjunction into its components and associating each component with a branch of a UNION ALL query.

WITH cte AS (
   SELECT * 
   FROM Car
   WHERE ModelId = 642 AND RegionId IN (23,63,1,30,5,6,7,9,15)
   UNION ALL
   SELECT * 
   FROM Car
   WHERE ModelId = 242 AND RegionId IN (23,63,1,30,5,6,7,9,15,26)
   UNION ALL
   SELECT *
   FROM car
   WHERE ModelId = 334 AND RegionId IN (23,63,1,30,5,6,7,9,15,26)
)
SELECT TOP 30 *
FROM cte
ORDER BY id DESC;

回答2:

Might be worse

SELECT TOP 30 a.*  
FROM Car 
WHERE (ModelId in (642, 242, 334) AND RegionId IN (23, 63, 1, 30, 5, 6, 7, 9, 15)) 
   OR (ModelId in (242, 334) AND RegionId  = 26)
ORDER BY Id DESC

来源：https://stackoverflow.com/questions/49758239/sql-server-slow-query-with-3-or-condition-and-order-by-id

标签

sql

sql-server

performance

select

sql-order-by