2018 Archives - Page 2 of 2 - Duhallow Grey Geek

Indexes, Analysts and SQL Smells

SQL Indexes - A Goldilocks problem for analysts or an SQL Smell — SQL Indexes – A Goldilocks problem for analysts or an SQL Smell

Database Indexes are something which a lot of analysts ignore as being “too technical”. This is a pity.

Several SQL Smells in Phil Factor’s article point at possible bad decisions. Thinking in the Logical Model can improve these decisions.

Choosing the correct indexes is a typical “Goldilocks Problem”: not too few, not too many, just the right number! Bad or inadequate requirements will contribute to designers making bad decisions. Phil Factor describes having the wrong indexes as an SQL Smell!

What is an Index and what does it do for us?

If you are an analyst, you may not know exactly what an index is. In non-technical terms, an index provides a quick way for the database manager to find the rows it needs in a table. There are several sorts of index.

An index is a “thing” in its own right. An index takes up space in the database. Updating an index costs effort. The main benefit of an index is that it makes select or read operations faster.

Types: Unique, Non-Unique, Clustered

There are three main kinds of index: Unique, Non-Unique and Clustered.

Imagine that we have a very simply database consisting of 3 tables:

Customer (not shown in the diagram)
Order
OrderLine

Two tables: Order and OrderLine - Where to put the indexes? — Two tables: Order and OrderLine – Where to put the indexes?

Unique Indexes:

In the Order table we can see three columns which might be used to identify the order

If you are not familiar with GUIDs, they are a way of assigning identifiers or “keys”. They are worth finding out about. It would be unusual to expect a human being to type in a GUID. An “OrderNum” (which paradoxically might contain letters!) would be more convenient for the users.

We expect all three: OrderId, GUID and OrderNum, to be unique. Therefore, all three are candidates for Unique Indexes. If an application attempts to create a duplicate value in a column which has a unique index, then the database manager will raise an error and reject the transaction.

Non-Unique Indexes for Foreign Keys

Candidate Indexes on an "OrderLine" SQL table — Candidate Indexes on an “OrderLine” SQL table

In the OrderLine table you can see two columns which identify things in other tables: OrderId and ProductId. These are Foreign Keys. In this case we cannot say they are unique, but they are candidates for non-unique indexes.

We could also have used OrderNum or OrderGUID as Foreign Keys into Order.

It is good practice for the rows in the OrderLine table to have a unique identifier. There are two common ways of doing this.

We can assign an OrderLineId (which is unique across the whole table) or
we can use the combination of OrderId and OrderLineNum which together would identify a row.

In this example, both OrderLineId and the OrderId and OrderLineNum combination are (seperate) candidate unique indexes.

Clustered Indexes

The order of the rows in an SQL table is specified by the Clustered Index. Each table can have only one clustered index. The clustered index must be unique.

People often make the “primary key” the clustered index, but it is worth considering other options. In the example, OrderLines can be added to an order after it has been created.

Using the OrderId, OrderLineNum index as the clustered index would make the database store all the “lines” for one order together (whenever they were added to the order). That may be more efficient for retrieval. Phil Factor identifies two smells with the choice of clustered indexes.

Non-Unique Indexes for Searching

Candidate Indexes on a "Customer" SQL table - A Non-Unique index on Name would help searches — Candidate Indexes on a “Customer” SQL table – A Non-Unique index on Name would help searches

Columns which will be used for searching should be considered candidates for a non-unique index.

The role of Analysts in choosing Indexes and Index types

Indexes are usually specified in the “Physical Model”. The analyst can help the database designer make the right decisions, by applying a little thought. The analyst should not try to pre-empt the designers decisions. They should aim to assist by identifying relevant “candidates”.

"<yoastmark

Summary

Indexes enforce business rules like uniqueness in an SQL database. They influence database performance. Considering candidate indexes in the Logical Model and even the Conceptual Model will help database designers make better decisions.

Where next?

The next article is about the smell of nothing, or “Nulls”. Nulls present problems for developers and database designers.

Check Constraint to limit values – SQL Smell?

Do Check Constraints Cause an SQL Smell?

In his article on SQL Smells, Phil Factor is not in favour of using Check Constraint to limit the values in columns.

In my previous post I explained why Phil Factor recommends referential integrity. I am going to explain this apparent contradiction.

What are CHECK CONSTRAINTs and what do they do for us?

Using CHECK CONSTRAINT to Restrict Allowed Values

Like REFERENCES, the CHECK CONSTRAINT also says what are values can be stored in a column. The logical expression must be true for the value to be allowed.

The logical expression can be limits or even a list of permitted values (as in the illustration).

What is the problem with Check Constraints?

This sounds like a marvellous idea! The benefit is clear. Constraints will exclude invalid data from the database, even when it is loaded using a utility (eg BULK INSERT). We can define constraints which will protect the database from bad data. So what is the problem?

Why does Phil Factor regard this as an SQL Smell? The answer is that using Check Constraints in this way embeds constant values in the structure of the SQL database!

The database structure is “locked down” in commercial systems. Only authorised people are allowed to make changes to the structure. Phil Factor wants to avoid these changes.

When should we use Check Constraints?

This criticism does not mean that we should never use Check Constraints. The ideal constraint will not change in the lifetime of the database.

Possible candidates for using CHECK CONSTRAINTs to Restrict Values

For attributes which represent “classifications” and “types” we should note how many different values we are expecting, and how frequently the allowed values change. Short lists which change very rarely may be acceptable.
On the other hand, consider re-designing a CHECK CONSTRAINT as a FOREIGN KEY by adding an additional table to contain the valid values. This has the benefit of making adding a new value a simple data change!
Do not use Check Constraints to enforce arbitrary limits.

Check Constraint and Requirements

We can identify candidates for Check Constraint when we construct the Conceptual Model. We should note:

The number of options
The expected frequency of change.
That information will enable us to make an informed decision about how to validate the values of that column.

Unfortunately, some of the examples using Check Constraints perform the checks against arbitrary values. These examples will work technically but copying them may cause the problems Phil Factor wants us to avoid.

Summary

Check Constraints provide a way of validating data values. They are appropriate for checking against values which do not change.

For lists, lookup-tables with Foreign Key Constraints may be better.

Do not use Check Constraints against arbitrary values or values which change frequently.

Where next?

The next article covers “Indexes”. I will explain how an Analyst can influence some design decisions.

Not using Referential Integrity – An SQL Smell!

Not Using Referential Integrity - SQL Smell? — Not Using Referential Integrity – SQL Smell?

In his article on SQL Smells, Phil Factor describes identifies two “smells” associated with Constraints in SQL. He regards not using Referential Integrity as an SQL Smell.

In this post I will explain what a Constraint is, and how one kind of Constraint provides Referential Integrity.

What are CONSTRAINTs and what do they do for us?

A Constraint limits the values which are allowed in a column. The SQL database manager will use constraints to validate data. There are several types of constraint.

Phil Factor thinks that not enforcing Referential Integrity is usually bad (and I agree).

Referential Integrity and the FOREIGN KEY REFERENCES Constraint

The FOREIGN KEY REFERENCES constraint on a column makes it a Foreign Key to another table. The Foreign Key must be present as a Primary Key in the referenced table. This is called Referential Integrity.

In the illustration, it is not possible for the Order table to contain a CustomerID which does not exist in the table Customer.

Enforcing Referential Integrity with REFERENCES

This is very powerful. The constraint makes the database manager check every change. Statements which would violate the Foreign Key constraint are rejected.

Performing the validation costs effort.

Phil Factor points out an additional benefit. The database manager may be able to use the Foreign Keys to improve performance of some queries.

Why wouldn’t you use Referential Integrity?

The arguments most frequently used for not implementing referential integrity are:

Performance: I have already mentioned the “cost” of checking the constraints. This argument:
- Prejudice against logic in the database
- A desire to minimise the “load” on the database manager.
Dirty Data: An “upstream” system may provide data which violates the constraint. Accepting this argument may mean importing faulty data, and the associated problems into your system!

“Staging tables” may be a better solution to this problem.

Both of these arguments are valid in some circumstances. There are times alternatives are better. Ask the following questions:

How are you going to perform the validation?
Which system components are going to perform the validation?
Do you expect the alternative solution to perform better?
Will the alternative solution be better in some way?

Not having answers to these questions is not really acceptable. Try and make a rational decision, based on numbers.

How Referential Integrity and Requirements interact

The Conceptual Data Model identifies business entities and relationships. Those relationships define the referential integrity requirements.

Not using referential integrity implies that:

The system is going to allow invalid data, or
The system is going to validate the data in some other way.

Summary

Foreign Key Constraints exclude some invalid data from the system. Orphaned records (eg non-existent Customers) are impossible.

Exceptions to this rule require a rational justification.

Where next?

In the next article I will look at “Check Constraints” which are a different way of ensuring valid data.