Dynamic query plan vs normal query plan

Question

I have a query with like operator.
DECLARE @query varchar(52)
SET @query='A12657'
IF @query IS NOT NULL
SET @query='%'+LTRIM(RTRIM(@query))+'%'SELECT eord_type_id FROM izdelek
WHERE (izd_naziv_ANG like @query OR izd_id like @query)
OPTION (RECOMPILE)
Query works 1 ms, 5 ms is for plan recompile. The execution plan is:
But if I run the same query as dynamic sql:
DECLARE @query varchar(52), @sql NVARCHAR(4000), @paramList NVARCHAR(500)
SET @query='A12657'
SELECT @paramlist = N'@query VARCHAR(52)'
if @query IS NOT NULL
SET @query='%'+LTRIM(RTRIM(@query))+'%'
SET @sql=N'SELECT eord_type_id FROM izdelek
WHERE (izd_naziv_ANG like @query OR izd_id like @query)
OPTION (RECOMPILE)'
EXEC sp_executesql @sql, @paramlist, @query
The execution plan is different and execution time is much slower - 2877 ms.
Where is the catch? What should I change that dynamic query would work the same.
What I also don't understand is key lookup at the end of the plan. Since we have clustered index seek at the beginning of the plan, the optimizer could read additional column(eord_type_id) from this index seek.
And why it is using merge since it has both columns from WHERE clause (izd_naziv_ang and izd_id) included inside index IX_izdelek. izd_id is in fact clustered key.
How can i improve this query?
But I would like to know at most, why dynamic sql is so much slower, since the query is totally the same in both cases and how to achieve that both queries have the similar execution time?

Answer 1

First of all , why do you use LIKE operator instead of EQUAL (=). I see you are looking for exact value..
--SQL Server creates 3 execution plan rather only one
DBCC FREEPROCCACHE
GO
SELECT *
FROM Sales.SalesOrderHeader
WHERE SalesOrderID = 56000
GO
SELECT * FROM
AdventureWorks.Sales.SalesOrderHeader WHERE
SalesOrderID = 56001
GO
declare @i int
set @i = 56004
SELECT *
FROM Sales.SalesOrderHeader
WHERE SalesOrderID = @i
GO
select  stats.execution_count AS exec_count, 
p.size_in_bytes as [size], 
[sql].[text] as [plan_text]
from sys.dm_exec_cached_plans p
outer apply sys.dm_exec_sql_text (p.plan_handle) sql
join sys.dm_exec_query_stats stats ON stats.plan_handle = p.plan_handle
GO
----This time only (we get parameterization)
DBCC FREEPROCCACHE
GO
EXEC sp_executesql N'SELECT  SUM(LineTotal) AS LineTotal
FROM Sales.SalesOrderHeader H
JOIN Sales.SalesOrderDetail D ON D.SalesOrderID = H.SalesOrderID
WHERE H.SalesOrderID = @SalesOrderID', N'@SalesOrderID INT', 56000
GO
EXEC sp_executesql N'SELECT  SUM(LineTotal) AS LineTotal
FROM Sales.SalesOrderHeader H
JOIN Sales.SalesOrderDetail D ON D.SalesOrderID = H.SalesOrderID
WHERE H.SalesOrderID = @SalesOrderID', N'@SalesOrderID INT', 56005
GO
select  stats.execution_count AS exec_count, 
LEFT([sql].[text], 80) as [plan_text]
from sys.dm_exec_cached_plans p
outer apply sys.dm_exec_sql_text (p.plan_handle) sql
join sys.dm_exec_query_stats stats ON stats.plan_handle = p.plan_handle
GO
Best Regards,Uri Dimant SQL Server MVP,
http://sqlblog.com/blogs/uri_dimant/
MS SQL optimization: MS SQL Development and Optimization
MS SQL Consulting:
Large scale of database and data cleansing
Remote DBA Services:
Improves MS SQL Database Performance
SQL Server Integration Services:
Business Intelligence