SQL Server - Multiple select queries hit performance - sql-server

Question

Recently I ran into an issue where we have multiple concurrent client requests causing performance issue in db. I tried the test scenario and as it turned out, when I run SELECT queries (same query) 6 to 7 times (gets worse with more), It degrades the performance and execution takes a lot of time. However I tried this one
SELECT TOP (100) COUNT(DISTINCT([Doc_Number])) AS "Expression"
FROM (
SELECT *
FROM "dbo"."Dummy_Table" "table_alias"
WHERE ((CAST("table_alias"."ID" AS NVARCHAR)) NOT IN
(
SELECT "PrimaryKey" AS ExceptionKey
FROM dbo.exceptions inner_exceptionStatus
LEFT JOIN dbo.Workflow inner_workflowStates ON
(inner_exceptionStatus."Status"= inner_workflowStates."UUID" AND
inner_exceptionStatus."UUID"= 'CA1662D6-73A2-4692-A765-E7E3EDB66062')
WHERE ("inner_workflowStates"."RemoveFromRecordSet" = 1 AND
"inner_workflowStates"."IsDeleted" = 0) AND
("inner_exceptionStatus"."IsArchived" IS NULL OR
"inner_exceptionStatus"."IsArchived" = 0)))) wrapperQuery
The query when runs alone takes around 1sec execution time. But If we runs it in parallel, for each query it takes up a wried amount of time of leads to timeout.
The only thing bothers me here is that SELECT query should be non-blocking and even with shared lock, then need to get along easily.
I am not sure if there is anything wrong in the query that adds up the situation.
Any help is deeply appreciated !!

Answer 1

Try this way
SELECT Count(DISTINCT( [Doc_Number] )) AS Expression
FROM dbo.Dummy_Table table_alias
WHERE NOT EXISTS (SELECT 1
FROM dbo.exceptions inner_exceptionStatus
INNER JOIN dbo.Workflow inner_workflowStates
ON ( inner_exceptionStatus.Status = inner_workflowStates.UUID
AND inner_exceptionStatus.UUID = 'CA1662D6-73A2-4692-A765-E7E3EDB66062' )
WHERE inner_workflowStates.RemoveFromRecordSet = 1
AND inner_workflowStates.IsDeleted = 0
AND ( inner_exceptionStatus.IsArchived IS NULL
OR inner_exceptionStatus.IsArchived = 0 )
AND table_alias.ID = PrimaryKey)
Made couple of changes.
Changed NOT IN to NOT EXISTS
Removed the convert in "table_alias"."ID" because it will avoid using any index present in "table_alias"."ID" column. If the conversion is really required then add it.
Removed Top (100) since there is no Group By it will return a single record as result.
Still if the query is running slow then you need to post the execution plan and make sure the statistics are up-to-date

Answer 2

You can simplyfy your query like this :
SELECT COUNT(DISTINCT(Doc_Number)) AS Expression
FROM dbo.Dummy_Table dmy
WHERE not exists
(
SELECT *
FROM dbo.exceptions ies
INNER JOIN dbo.Workflow iws ON ies.Status= iws.UUID AND ies.UUID= 'CA1662D6-73A2-4692-A765-E7E3EDB66062'
WHERE iws.RemoveFromRecordSet = 1 AND iws.IsDeleted = 0 AND (ies.IsArchived IS NULL OR ies.IsArchived = 0)
and dmy.ID=PrimaryKey
)
Like prdp say :
Changed NOT IN to NOT EXISTS
Removed the convert in "table_alias"."ID" because it will avoid using any index present in "table_alias"."ID" column. If the conversion is really required then add it.
Removed Top (100) since there is no Group By it will return a single record as result.
I add :
Remove you temporary table wrapperQuery
You can use INNER JOIN because into where you test RemoveFromRecordSet = 1 then you remove null values.
Remove not utils quotes ,brackets and parenthèses into where clause