A Systematic Approach to View Creation

Historically, one of the most fertile grounds for disagreement between database professionals was the appropriate usage of views. Some analysts promote the liberal creation and usage of views, whereas others preach a more conservative approach.

When properly implemented and managed, views can be fantastic tools that help to ease data access and simplify development. Although views are simple to create and implement, few organizations take a systematic and logical approach to view creation. And therein lies the problem. A strategic and reasonable policy guiding the creation and maintenance of views is required to avoid a muddled and confused mish-mash of view usage. Basically, views are very useful when implemented wisely, but can be an administrative burden if implemented without planning.

Let's back up a minute and define what a view is.  First, remember that all SQL access to relational tables results in another table. This is called relational closure and it is a requirement of the relational model. A view can be considered to be a logical table. That is, a view is a "logical" representation of data that is "physically" stored in other tables (and perhaps other views as well). Views are defined using SQL and are represented internally to the DBMS by SELECT statements, not by stored data (unless it is a materialized view, which is another thing altogether and beyond the scope of this particular post). Almost any SQL that can be issued natively can be coded into a view.

View Implementation Rules

After you understand the basics of views, you should develop guidelines for view creation in order to limit administrative burden. The following rules can be used to ensure that views are created in a responsible and useful manner at your shop. These rules were developed over a number of years as a result of reviewing implementations and working with views in many different environments. There are three basic view implementation rules:

1. The View Usage Rule

2. The Proliferation Avoidance Rule

3. The View Synchronization Rule

The View Usage Rule: The first rule is simple: your view creation strategy should be goal-oriented. Views should be created only when they satisfy a specific application or business requirement. That requirement should be documented somewhere, preferably in a dictionary or repository. Although this rule may seem to be obvious, views are implemented at some shops without much thought as to how they will be used. This can cause the number of views that must be supported and maintained to continually expand until so many views exist that it is impossible to categorize their uses. And the time needed to maintain and administer the system increases as the number of views increases.

There are five basic uses for which views excel. These are: 

1) to provide row and column level security, 

2) to ensure efficient access paths, 

3) to mask complexity from the user, 

4) to ensure proper data derivation, and 

5) to rename tables and/or columns.

The second rule is the Proliferation Avoidance Rule. This rule is simple and to the point: do not needlessly create views (or, indeed, any database objects) that are not absolutely required. Whenever a database object is created additional entries are placed in the system catalog. Creating needless views causes "catalog clutter"…that is, entries in the catalog for objects which are not needed and/or are not used.

The proliferation avoidance rule is based on common sense. Why create something that is not needed? It just takes up space that could be used for something that is needed.

The third, and final view implementation rule is the View Synchronization Rule. The basic intention of this rule is to ensure that views are kept in sync with the base tables upon which they are based.

Whenever a change is made to a base table, all views that are dependent upon that base table should be analyzed to determine if the change impacts them. All views should remain logically pure. The view was created for a specific reason (if we followed the View Usage Rule above). The view should therefore remain useful for that specific reason. This can only be accomplished by ensuring that all subsequent changes that are pertinent to a specified usage are made to all views that satisfy that usage.

For example, say a view was created to satisfy an access usage, such as a join between the employee table and the department table. If a column is added to the employee table specifying the employee’s location, it should also be added to the EMP_DEPT view if it is pertinent to that view’s specific use. Of course, the column can be added to the table immediately and to the view at the earliest convenience of the development team.

The synchronization rule requires that strict change impact analysis procedures be in place. Every change to a base table should trigger the usage of these procedures. Simple SQL queries can be created to assist in the change impact analysis. These queries should pinpoint any SQL in QMF queries, application packages, dynamic packages, and so on. Policies for informing users of the views to be impacted also need to be established before view changes can be implemented.

View synchronization is needed to support the view usage rule. By keeping views in sync with table changes the original purpose of the view is maintained.

Synopsis

Views are practical and helpful when implemented in a systematic and thoughtful manner. Hopefully, this post has provided you with some food for thought pertaining to how views are implemented at your shop. 

And if you follow the guidelines contained in this article, in the end, all that will remain is a beautiful view!

The Most Misunderstood Features of DB2 – Part 4: Base Table Views

Welcome to my continuing series focusing on The Most Misunderstood Features of DB2. Today’s topic is an oldie but a goodie: base table views.

What is a Base Table View?

I am going to assume that regular readers of this blog know what a view is (if not, go here). So what is a base table view? It is a view that contains all of the columns and all of the rows of the base table and nothing else. In other words, it is like a SELECT * against the base table with no WHERE clauses.

Now why would somebody create such a beast? This gets to the heart of the misunderstanding here. There is a continuing notion “out there” that it is a good idea to create a base table view for every table and to give programmers access to the base table view only, not the base table itself.

The reason given for doing so is to insulate programs from change. But this is a weak argument at best. I admit that this particular misunderstanding is less prevalent than it was in the earlier years of DB2. But I do still find shops adhering to this archaic, and poorly founded, idea.

Reasons Given for Base Table Views

One reason given by base table view proponents is that when you add a column to a table you do not have to change the program. But this is the case, too, if you code your programs appropriately, avoiding SELECT * and coding only the specific columns you need to access. Most shops do this even when using base table views.

Other reasons given revolve around removing columns or splitting tables. If you are interested in all of the reasons and rebuttals I urge you to read my article (PDF) on this topic from 1991 titled One View Per Base Table? Don’tDo It! (Wow! This is a quarter of a Century old now!)

Another View Fallacy

I recall yet another misunderstanding about views from many years ago that I have not heard in a long time. Basically, the argument was that accessing a base table view would out-perform accessing the base table.

This was never proven to me – and I do not believe it was ever true. How could it be? Static SQL against a base table view would use view merge so it would be the same as going against the base table. And dynamic SQL would have the added step of having to resolve the view to the base table. Sooo…

Summary

Base table views are not worth the time and effort because the bring no added value of any type whatsoever. Do not use them.

Top Ten Most Pervasive Myths About DB2 for z/OS

Today's blog offers up yet another Top Ten list for DB2 users, perusers, and abusers... This time counting down the most common myths that are perpetrated "out there" regarding DB2 and how it works (or doesn't work)...


1.Use Views to Insulate Programs from Change

              This lie has been told for almost as long as DB2 has been around. I first wrote about this way back in 1991 for Database Programming & Design. Check that article out here if you don't understand why this is a bad idea, in general. 

2.Locking Problems Indicate a Database Problem

              Locking problems are generally caused by bad program design. You should write code to reduce the duration of locks and to COMMIT regularly... and then locking won't be a problem, for the most part.

3.Primary Key is Usually a Good Choice for Clustering

              Actually, the foreign key is likely to be a better choice. When you join PK-->FK there will be one PK to multiple FK. Wouldn't it be best if the FKs were clustered on the same page (or pages)?

4.Just Using the Defaults Should Work Out Well

              Don't rely on defaults. Many of them are outdated or wrong... and even if they aren't it will be better if you review and investigate all options before explicitly specifying the parameter value you want. 

5.Programmers Don’t Need to Know How to Tune SQL

              Programming performance-oriented SQL into your programs from the beginning would go a long way toward improving performance overall... and reducing the length of the application development lifecycle. 

6.Black Boxes Work Well for Performance

              No they don't!!!

7.Using NULLs Can Save Space

              No they can't!!!

8.RUNSTATS Aren’t That Important

              If you don't work with up-to-date RUNSTATS then you are hobbling the DB2 Optimizer and almost assuredly getting sub-optimal access paths for your DB2 SQL. 

9.DB2 is a Hog

              If you don't use it properly, every piece of software can become a resource hog. If you acquire the knowledge on how to work properly with DB2 then it will hum along like a well-oiled machine!

10.It Depends!

              This is the answer that can be used for every DB2 question. But if that answer is not followed up with what "it" depends upon, then it is a useless answer... and whoever gave you that answer is probably just trying to get rid of you instead of helping you with your problems.

View Naming Conventions

Naming conventions sometimes instigate conflict within the world of DB2, especially when it comes to views. But, really, it should be very easy. Just always remember, that a view is a logical table. It consists of rows and columns, exactly the same as a DB2 table. A DB2 view can (syntactically) be used in SQL SELECT, UPDATE, DELETE, and INSERT statements in the same way that a DB2 table can. Furthermore, a view can be used functionally the same as a DB2 table (with certain limitations on updating as outlined in my article).

Therefore, shouldn't it stand to reason that views should be held to the same naming conventions as are used for tables? (As an aside, the same can be said for DB2 aliases and synonyms).

End users querying views don't need to know whether they are accessing a view or a table. That is the whole purpose of views. Why then, should we enforce an arbitrary naming standard, such as putting a V in the first or last position of a view name, on views?

DBAs and technical analysts, those individuals who have a need to differentiate between tables and views, can utilize the DB2 Catalog to determine which objects are views and which objects are tables.

Most users don't care whether they are using a table, view, synonym, or alias. They simply want to access the data. And, in a relational database, tables, views, synonyms, and aliases all logically appear to be identical to the end user: collections of rows and columns.

There are certain operations that cannot be performed on certain types of views, but the end users who need to know this will generally be sophisticated users. For example, very few shops allow end users to update any table they want using a report writer or query tool (e.g. QMF, SPUFI, etc.). Updates, deletions, and insertions (the operations which are not available to some views) are generally coded into application programs and executed in batch or via online transactions. Most end users need to query tables dynamically.

Now you tell me, which name will your typical end user remember more readily when he needs to access his marketing contacts: MKT_CONTACT or VMKTCT01?