24 February 2018

Misusing Views and Pseudo-Constants

   Views as virtual tables can be misused to replace tables in certain circumstances, either by storing values within one or multiple rows, like in the below examples:

-- parameters for a BI solution
CREATE VIEW dbo.vLoV_Parameters
AS
SELECT Cast('ABC' as nvarchar(20)) AS DataAreaId
 , Cast(GetDate() as Date) AS CurrentDate 
 , Cast(100 as int) AS BatchCount 

GO

SELECT *
FROM dbo.vLoV_Parameters

GO

-- values for a dropdown 
 CREATE VIEW dbo.vLoV_DataAreas
 AS
 SELECT Cast('ABC' as nvarchar(20)) AS DataAreaId
 , Cast('Company ABC' as nvarchar(50)) AS Description 
 UNION ALL
 SELECT 'XYZ' DataAreaId 
 , 'Company XYZ'

GO

SELECT *
FROM dbo.vLoV_DataAreas

GO

    These solutions aren’t elegant, and typically not recommended because they go against one of the principles of good database design, namely “data belong in tables”, though they do the trick when needed. Personally, I used them only in a handful of cases, e.g. when it wasn’t allowed to create tables, when it was needed testing something for a short period of time, or when there was some overhead of creating a table for 2-3 values. Because of their scarce use, I haven’t given them too much thought, not until I discovered Jared Ko’s blog posting on pseudo-constants. He considers the values from the first view as pseudo-constants, and advocates for their use especially for easier dependency tracking, easier code refactoring, avoiding implicit data conversion and easier maintenance of values.


   All these are good reasons to consider them, therefore I tried to take further the idea to see if it survives a reality check. For this I took Dynamics AX as testing environment, as it makes extensive use of enumerations (aka base enums) to store list of values needed allover through the application. Behind each table there are one or more enumerations, the tables storing master data abounding of them.  For exemplification let’s consider InventTrans, table that stores the inventory transactions, the logic that governs the receipt and issued transactions are governed by three enumerations: StatusIssue, StatusReceipt and Direction.

-- Status Issue Enumeration 
 CREATE VIEW dbo.vLoV_StatusIssue
 AS
 SELECT cast(0 as int) AS None
 , cast(1 as int) AS Sold
 , cast(2 as int) AS Deducted
 , cast(3 as int) AS Picked
 , cast(4 as int) AS ReservPhysical
 , cast(5 as int) AS ReservOrdered
 , cast(6 as int) AS OnOrder
 , cast(7 as int) AS QuotationIssue

GO

-- Status Receipt Enumeration 
 CREATE VIEW dbo.vLoV_StatusReceipt
 AS
SELECT cast(0 as int) AS None
 , cast(1 as int) AS Purchased
 , cast(2 as int) AS Received
 , cast(3 as int) AS Registered
 , cast(4 as int) AS Arrived
 , cast(5 as int) AS Ordered
 , cast(6 as int) AS QuotationReceipt

GO

-- Inventory Direction Enumeration 
 CREATE VIEW dbo.vLoV_InventDirection
 AS
 SELECT cast(0 as int) AS None
 , cast(1 as int) AS Receipt
 , cast(2 as int) AS Issue

   To see these views at work let’s construct the InventTrans table on the fly:

-- creating an ad-hoc table  
 SELECT *
 INTO  dbo.InventTrans
 FROM (VALUES (1, 1, 0, 2, -1, 'A0001')
 , (2, 1, 0, 2, -10, 'A0002')
 , (3, 2, 0, 2, -6, 'A0001')
 , (4, 2, 0, 2, -3, 'A0002')
 , (5, 3, 0, 2, -2, 'A0001')
 , (6, 1, 0, 1, 1, 'A0001')
 , (7, 0, 1, 1, 50, 'A0001')
 , (8, 0, 2, 1, 100, 'A0002')
 , (9, 0, 3, 1, 30, 'A0003')
 , (10, 0, 3, 1, 20, 'A0004')
 , (11, 0, 1, 2, 10, 'A0001')
 ) A(TransId, StatusIssue, StatusReceipt, Direction, Qty, ItemId)


    Here are two sets of examples using literals vs. pseudo-constants:

--example issued with literals 
SELECT top 100 ITR.*
 FROM dbo.InventTrans ITR
 WHERE ITR.StatusIssue = 1 
   AND ITR.Direction = 2

GO
 --example issued with pseudo-constants
 SELECT top 100 ITR.*
 FROM dbo.InventTrans ITR
      JOIN dbo.vLoV_StatusIssue SI
        ON ITR.StatusIssue = SI.Sold
      JOIN dbo.vLoV_InventDirection ID
        ON ITR.Direction = ID.Issue

GO

--example receipt with literals 
 SELECT top 100 ITR.*
 FROM dbo.InventTrans ITR
 WHERE ITR.StatusReceipt= 1
   AND ITR.Direction = 1

GO

--example receipt with pseudo-constants
 SELECT top 100 ITR.*
 FROM dbo.InventTrans ITR
      JOIN dbo.vLoV_StatusReceipt SR
        ON ITR.StatusReceipt= SR.Purchased
      JOIN dbo.vLoV_InventDirection ID
        ON ITR.Direction = ID.Receipt

 
  As can be seen the queries using pseudo-constants make the code somehow readable, though the gain is only relative, each enumeration implying an additional join. In addition, when further business tables are added to the logic (e.g. items, purchases or sales orders)  it complicates the logic, making it more difficult to separate the essential from nonessential. Imagine a translation of the following query:

-- complex query 
  SELECT top 100 ITR.*
  FROM dbo.InventTrans ITR
              <several tables here>
  WHERE ((ITR.StatusReceipt<=3 AND ITR.Direction = 1)
    OR (ITR.StatusIssue<=3 AND ITR.Direction = 2))
    AND (<more constraints here>)


   The more difficult the constraints in the WHERE clause, the more improbable is a translation of the literals into pseudo-constraints. Considering that an average query contains 5-10 tables, each of them with 1-3 enumerations, the queries would become impracticable by using pseudo-constants and quite difficult to troubleshoot their execution plans.

    The more I’m thinking about, an enumeration data type as global variable in SQL Server (like the ones available in VB) would be more than welcome, especially because values are used over and over again through the queries. Imagine, for example, the possibility of writing code as follows:

-- hypothetical query
SELECT top 100 ITR.*
FROM dbo.InventTrans ITR
WHERE ITR.StatusReceipt = @@StatusReceipt .Purchased
  AND ITR.Direction = @@InventDirection.Receipt

   From my point of view this would make the code more readable and easier to maintain. Instead, in order to make the code more readable, one’s usually forced to add some comments in the code. This works as well, though the code can become full of comments.

-- query with commented literals
SELECT top 100 ITR.*
FROM dbo.InventTrans ITR
WHERE ITR.StatusReceipt <=3  Purchased, Received, Registered 
   AND ITR.Direction = 1-- Receipt

   In conclusion, pseudo-constants’ usefulness is only limited, and their usage is  against developers’ common sense, however a data type in SQL Server with similar functionality would make code more readable and easier to maintain.


PS: It is possible to simulate an enumeration data type in tables’ definition by using a CHECK constraint.

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