Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

CHAPTER 12 ■ DATATYPES 525
You can see the obvious difference. Using TO_CHAR consumed an order of magnitude more
CPU than using TRUNC. That is because TO_CHAR must convert the date to a string, using a much
larger code path, taking all of the NLS we have in place to do so. Then we had to compare a
string to a string. The TRUNC, on the other hand, just had to set the last 5 bytes to 1. Then it
compared 7 binary bytes to 7 binary bytes, and it was done. So, you should never use TO_CHAR
on a DATE column simply to truncate it.
Additionally, avoid applying a function at all to the DATE column when possible. Taking the
preceding example one step further, we can see that the goal was to retrieve all data in the year
2005. Well, what if CREATED had an index on it and a very small fraction of the values in that
table were in the year 2005? We would like to be able to use that index by avoiding a function
on the database and column using a simple predicate:
select count(*) from t
where created >= to_date('01-jan-2005','dd-mon-yyyy')
and created < to_date('01-jan-2006','dd-mon-yyyy');
We would achieve two things:
• An index on CREATED could be considered.
• The TRUNC function would not have to be invoked at all, and that overhead would go
away entirely.
This technique of using a range comparison instead of TRUNC or TO_CHAR applies equally to
the TIMESTAMP type discussed shortly. When you can avoid applying a function to a database
column in a query, you should. In general, avoiding the function will be more performant and
allow the optimizer to choose from a wider variety of access paths.
Adding Time to or Subtracting Time from a DATE
A question I am frequently asked is, “How do I add time to or subtract time from a DATE type?”
For example, how do you add one day to a DATE, or eight hours, or one year, or one month, and
so on? There are three techniques you’ll commonly use:
• Simply add a NUMBER to the DATE. Adding 1 to a DATE is a method to add 1 day. Adding
1/24 to a DATE therefore adds 1 hour, and so on.
• You may use the INTERVAL type, as described shortly, to add units of time. INTERVAL
types support two levels of granularity, years and months, or days/hours/minutes/seconds.
That is, you may have an interval of so many years and months or an interval of
so many days, hours, minutes, and seconds.
• Add months using the built-in ADD_MONTHS function. Since adding a month is generally
not as simple as adding 28 to 31 days, a special-purpose function was implemented to
facilitate this.
Table 12-3 demonstrates the techniques you would use to add N units of time to a date (or
subtract N units of time from a date, of course).