How to improve Clustered Index scan to Clustered Index seek?

Question

I have two tables

[dbo].[Employee](
    [EmployeeID] [int] IDENTITY(1,1) NOT NULL,
    [Title] [varchar](50) NULL,
    [Role] [int] NULL,
    [Status] [varchar](1) NULL)

[dbo].[Roles](
    [id] [int] IDENTITY(1,1) NOT NULL,
    [Role] [varchar](25) NULL,
    [Description] [varchar](25) NULL)

with primary clustered keys by id. Also I have stored procedure

SELECT
   [EmployeeID]
  ,[Title]
  ,employee.[Role]
  ,roles.Role AS RoleName
FROM [dbo].Employee AS employee
INNER JOIN [dbo].Roles AS roles ON roles.id = employee.Role
WHERE [Status] <> 'D'

The Execution plan shows me a 'Clustered Index Scan' that I want to avoid. Is there any way I can convert it to a 'Clustered Index Seek'? Execution Plan screen

< db fiddle >

Answer 1

As I mentioned in the comments, the CLUSTERED INDEX on dbo.Employees isn't going to help you here. The reason for this is because you are filtering on the column status , which is simply included in the CLUSTERED INDEX , but isn't ordered on it. As a result, the RDBMS has no option but to scan the entire table to filter out the rows where Status has a value of D .

You could add an INDEX on the column Status and INCLUDE the other columns, which may result in an index seek (not a clustered index seek), however, due to you using <> D then the RDBMS may feel still chose to perform a scan; whether it does depends on your data's distribution:

CREATE INDEX IX_EmployeeStatus ON dbo.Employee (Status) INCLUDE (Title, Role);

Also add a FOREIGN KEY to your table:

ALTER TABLE dbo.Employee ADD CONSTRAINT FK_EmployeeRole FOREIGN KEY (Role) REFERENCES dbo.Roles (id);

db<>fiddle

Answer 2

Provided you had an index on your Status column then that code should result in two index seeks. One for < 'D' and one for > 'D'. You just need an index on Status.

You can convert it to one seek using a temp table to flip your "Not" into an "is", which is always a good idea in query tuning:

CREATE TABLE #StatusMatch (StatusCode varchar(1) NOT NULL PRIMARY KEY CLUSTERED)
 INSERT INTO #StatusMatch WITH(TABLOCKX)
      SELECT Status 
        FROM dbo.Employee WITH(NOLOCK)
       WHERE Status <> 'D'
    GROUP BY Status
    ORDER BY Status;

      SELECT a.EmployeeID, a.Title, a.Role, b.Description AS RoleName
        FROM dbo.Employee a WITH(NOLOCK)
  INNER JOIN dbo.Roles b WITH(NOLOCK) ON a.Role = b.id
  INNER JOIN #StatusMatch c ON a.Status = c.StatusCode