Why calculated fields using scalar functions are slow - sql

Question

I have below table:
CREATE TABLE [dbo].[Client](
[Id] [int] IDENTITY(1,1) NOT NULL,
[Name] [nvarchar](150) NOT NULL,
[InternalSiteId] AS (isnull(CONVERT([int],[dbo].[GetCurrentTemporalValue]([Id],'Client_InternalSite')),(0))),
[BudgetingStatusId] AS (isnull(CONVERT([int],[dbo].[GetCurrentTemporalValue]([Id],'Client_BudgetingStatus')),(1))),
[BusinessUnitId] AS (isnull(CONVERT([int],[dbo].[GetCurrentTemporalValue]([Id],'Client_BusinessUnit')),(0))),
CONSTRAINT [PK_dbo.Client] PRIMARY KEY CLUSTERED
(
[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, OPTIMIZE_FOR_SEQUENTIAL_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
which has 3 calculated fields using scalar function:
CREATE FUNCTION [dbo].[GetCurrentTemporalValue]
(
#clientId INT,
#temporalType NVARCHAR(128)
)
RETURNS NVARCHAR(255)
AS
BEGIN
DECLARE #retVal INT
DECLARE #at DATETIME
SET #at = GETUTCDATE()
SELECT #retVal = CAST(Value AS INT) FROM dbo.Temporal
WHERE 1 = 1
AND ClientId = #clientId
AND TemporalType = #temporalType
AND ( ValidFrom <= #at OR ValidFrom IS NULL )
AND ( ValidTo >= #at OR ValidTo IS NULL)
RETURN #retVal
END
GO
and this function uses below table:
CREATE TABLE [dbo].[Temporal](
[Id] [int] IDENTITY(1,1) NOT NULL,
[ClientId] [int] NOT NULL,
[Value] [nvarchar](255) NULL,
[ValidFrom] [datetime2](7) NULL,
[ValidTo] [datetime2](7) NULL,
[TemporalType] [nvarchar](128) NOT NULL,
CONSTRAINT [PK_dbo.Temporal] PRIMARY KEY CLUSTERED
(
[Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, OPTIMIZE_FOR_SEQUENTIAL_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY]
GO
The Client table has around 2500 records but the select * from [Client] takes about 14 seconds!
When I comment these 3 calculated fields, it gets back to normal value below one second. So the scalar function seems to cause the issue.
I tried to make condition easier ( left only 1 = 1 ) but it didn't change the speed.
What I want to achieve is:
historical values (kept in Temporal table)
current value to be pointed out by Client.InternalSiteId, Client.BudgetingStatusId and Client.BusinessUnitId
I cannot just simply make InternalSiteId a regular int fields with foreign key to, let say, InternalSiteHistoryTable as there are ValidFrom and ValidTo fields pointing out the period in which given value is valid and current and e.g. ValidFrom can be set to future value. That's why I need such calculations in function to find out current value.
What should I do / change to achieve above goals, but keep the reasonable fetching data speed?

Answer 1

Use a view to join to your Temporal table instead of embedding function calls
-- Table without those functions to slow things down
CREATE TABLE [dbo].[ClientName](
[Id] [int] IDENTITY(1,1) NOT NULL,
[Name] [nvarchar](150) NOT NULL,
CONSTRAINT [PK_dbo.Client] PRIMARY KEY CLUSTERED
(
[Id] ASC
) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF,
ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, OPTIMIZE_FOR_SEQUENTIAL_KEY = OFF)
ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
--instead, use a view to do the same thing
CREATE VIEW [dbo].[Client] as
SELECT
ClientName.Id,
ClientName.Name,
isnull(CONVERT([int], Client_InternalSite.Value), 0) AS InternalSiteId,
isnull(CONVERT([int], Client_BudgetingStatus.Value, 1) AS BudgetingStatusId,
isnull(CONVERT([int], Client_BusinessUnit.Value, 0) AS BusinessUnitId
FROM ClientName INNER JOIN
( SELECT Value, ClientId
FROM Temporal
WHERE TemporalType='Client_InternalSite'
AND ( ValidFrom <= GETUTCDATE() OR ValidFrom IS NULL )
AND ( ValidTo >= GETUTCDATE() OR ValidTo IS NULL)
) AS Client_InternalSite ON ClientName.ID = Client_InternalSite.ClientID
INNER JOIN
( SELECT Value, ClientId
FROM Temporal
WHERE TemporalType='Client_BudgetingStatus'
AND ( ValidFrom <= GETUTCDATE() OR ValidFrom IS NULL )
AND ( ValidTo >= GETUTCDATE() OR ValidTo IS NULL)
) AS Client_BudgetingStatus ON ClientName.ID = Client_BudgetingStatus.ClientID
INNER JOIN
( SELECT Value, ClientId
FROM Temporal
WHERE TemporalType='Client_BusinessUnit'
AND ( ValidFrom <= GETUTCDATE() OR ValidFrom IS NULL )
AND ( ValidTo >= GETUTCDATE() OR ValidTo IS NULL)
) AS Client_BusinessUnit ON ClientName.ID = Client_BusinessUnit.ClientID
GO
I can't test this against your DB, so I don't know about your indexes on your Temporal table (ID, ValidFrom, ValidTo columns), but typically a VIEW like this is going to run quicker because the tables are queried only once.