Most DBAs have grappled with the pros and cons of choosing one data type versus another. Sometimes the decision is easy, whereas sometimes the decision is a little bit more difficult. In today’s blog entry, we’ll discuss both situations.
Data type and length are the most fundamental integrity constraints applied to data in a database. Simply by specifying the data type for each column when a table is created, DB2 automatically ensures that only the correct type of data is stored in that column. Processes that attempt to insert or update the data to a non-conforming value will be rejected. Furthermore, a maximum length is assigned to the column to prohibit larger values from being stored in the table.
The DBA must choose the data type and length of each column wisely. The general rule of thumb for choosing a data type for the columns in your tables is to choose the data type that most closely matches the domain of correct values for the column. That means you should try to adhere to the following rules:
- If the data is numeric, favor SMALLINT, INTEGER, BIGINT, or DECIMAL data types. DECFLOAT and FLOAT are also options for very large numbers.
- If the data is character, use CHAR or VARCHAR data types.
- If the data is date and time, use DATE, TIME, and TIMESTAMP data types.
- If the data is multimedia, use GRAPHIC, VARGRAPHIC, BLOB, CLOB, or DBCLOB data types.
But, of course, there are always exceptions. For example, what about social security number? That is a numeric column, but you won’t be performing calculations using it. And, to format it correctly requires hyphens, which cannot be stored in a numeric data type. So, perhaps, SSN is an exception.
Let’s consider another typical situation: the desire to display leading zeroes. Maybe the application requires a four-byte code that is used to identify products, accounts, or some other business object, and all of the codes are numeric and will stay that way. But, for reporting purposes, the users want the codes to print out with leading zeroes. So, the users request that the column be defined as CHAR(4) to ensure that leading zeroes are always shown. But what are the drawbacks of doing this?
Well, there are drawbacks! Without proper edit checks, inserts and updates could place invalid alphabetic characters into the product code. This can be a very valid concern if ad hoc data modifications are permitted. This is rare in production databases, but data problems can still occur if the proper edit checks are not coded into every program that can modify the data. But let’s assume that proper edit checks are coded and will never be bypassed. This removes the data integrity question (yeah, right!).
There is another problem that is related to performance and filter factors. Consider the possible number of values that a CHAR(4) column and a SMALLINT column can assume. Even if edit checks are coded for each, DB2 is not aware of these and assumes that all combinations of characters are permitted. DB2 uses base 37 math when it determines access paths for character columns, under the assumption that 26 alphabetic letters, 10 numeric digits, and a space will be used. This adds up to 37 possible characters. For a four-byte character column there are 374 or 1,874,161 possible values.
A SMALLINT column can range from -32,768 to 32,767 producing 65,536 possible small integer values. The drawback here is that negative or 5-digit product codes could be entered. However, if we adhere to our proper edit check assumption, the data integrity problems will be avoided here, as well. And we can always code a simple CHECK constraint to ensure that the code is always greater than zero.
DB2 will use the HIGH2KEY and LOW2KEY values to calculate filter factors. For character columns, the range between HIGH2KEY and LOW2KEY is larger than numeric columns because there are more total values. The filter factor will be larger for the numeric data type than for the character data type which may influence DB2 to choose a different access path. For this reason, favor the SMALLINT over the CHAR(4) definition.
The leading zeroes problem might be able to be solved using other methods. If you are using a report writer, most of them have quick methods of displaying leading zeroes. When using QMF, you can ensure that leading zeroes are shown by using the “J” edit code. Report programs can be coded to display leading zeroes easily enough by moving the host variables to appropriate display fields. Ad hoc access through other reporting tools typically provide a parameter that can enable leading zeroes to be displayed.
In general, it is wise to choose a data type which is closest to the domain for the column. If the column is to store numeric data, favor choosing a numeric data type: SMALLINT, INTEGER, DECIMAL, or floating point. In addition, always be sure to code appropriate edit checks to ensure data integrity.
A DATE with DB2
What about dates? In my opinion, you should always use a DATE data type for date data. Why anyone would ever store a date or time in a DB2 table any other format than DATE, TIME, or TIMESTAMP is beyond me. But, oh, they do it all the time. And it causes all sorts of headaches. The benefits of using the proper DB2 data type are many, including:
- Ensuring data integrity because DB2 will ensure that only valid date and time values are stored
- The ability to use date and time arithmetic
- A vast array of built-in functions to operate on and transform date and time values
- Multiple formatting choices
Additionally, you have multiple built-in functions at your disposal for manipulating date and time data, but only when the data is stored as DATE, TIME, and TIMESTAMP data types. The time-related DB2 functions include CHAR, DATE, DAY, DAYOFMONTH, DAYOFWEEK, DAYOFYEAR, DAYS, HOUR, JULIAN_DAY, MICROSECOND, MIDNIGHT_SECONDS, MINUTE, MONTH, QUARTER, SECOND, TIME, TIMESTAMP, WEEK, WEEK_ISO, and YEAR.
I get questions all the time from folks who’ve stored dates using character data types; they ask all kinds of integrity and manipulation questions and I always, always, always start my reply with “You shouldn’t have done that!”… and then I try to help them out, if I can. Don’t fall into the trap of storing dates using anything but a DATE data type… you’ll thank me later.
There is a lot more that can be said about choosing appropriate data types, but I think we’ve covered enough for today. The bottom line on data types is to use them to protect the integrity of your DB2 data and to simplify your job by taking advantage of DB2’s built-in capabilities. By choosing that data type that most closely matches your data you will be doing yourself, your systems, and your users a big favor.
This blog post was originally published on Craig Mullins’ blog at: http://db2portal.blogspot.com/2009/10/on-importance-of-choosing-correct-data.html
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