在DB2 / AS400上使用Views和CTE

Question

a Generic question..

I have an employee Table( EMPMAST ) which has the New as well old Employee data. There is a flag called Current? which is 'Y' if he/she is a current employee. Now I have to select records in my SQLRPGLE with only the current records and also some other criteria's(For Example EMPNAME = 'SAM') .What is the best way to deal with it. (in terms of performance and system usage)

1. To create a View over the EMPMAST with Current? = 'Y'. Then use it in the program with other conditions.
2. Use a CTE(With AS) in the Program which would have the Condition Current = 'Y' and use it.
3. use the table directly without CTE and View
4. any other option

Answer 1

Options 1, 2 and 3 would all perform the same. They would likely all have the same optimized query and access plan.

Answer 2

A CTE vs. a View are two different things. A View would be appropriate for a query that is going to be used in multiple locations, A CTE is only available in the query in which it is defined. I usually don't use the CTE except to replace a complex subquery. In your case the condition is simple enough to be contained in the where clause so I don't see the need to introduce additional complexity.

Some folks will tell you not to query the table directly in the program, but to always use a view. That way you add an extra layer of insulation between the program and the database, and you can still define record structures with ExtName , and not have to worry about changes to the table unless they affect the view itself. In this case you would likely have a dedicated view for each program that uses the table.

I tend to just use a hybrid of these techniques. I query tables, CTE's, or Views depending on the situation, and define my record structures explicitly in the program. I prefer to just query the table, but if I have some complex query logic which is unique to the program, I will use a CTE. I do have a few Views, but these are limited to those queries that happen in multiple programs where I want to ensure the same logic is applied consistently.