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

524
CHAPTER 12 ■ DATATYPES
To truncate that date down to the year, all the database had to do was put 1s in the last
5 bytes—a very fast operation. We now have a sortable, comparable DATE field that is truncated
to the year level, and we got it as efficiently as possible. What many people do instead of using
TRUNC, however, is use a date format in the TO_CHAR function. For example, they will use
Where to_char(date_column,'yyyy') = '2005'
instead of
Where trunc(date_column,'y') = to_date('01-jan-2005','dd-mon-yyyy')
The latter is a far more performant and less resource-intensive approach. If we make a
copy of ALL_OBJECTS and save out just the CREATED column
ops$tkyte@ORA10G> create table t
2 as
3 select created from all_objects;
Table created.
ops$tkyte@ORA10G> exec dbms_stats.gather_table_stats( user, 'T' );
PL/SQL procedure successfully completed.
and then, with SQL_TRACE enabled, we repeatedly query this table using both techniques, we
see the following results:
select count(*)
from
t where to_char(created,'yyyy') = '2005'
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 4 0.01 0.05 0 0 0 0
Execute 4 0.00 0.00 0 0 0 0
Fetch 8 0.41 0.59 0 372 0 4
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 16 0.42 0.64 0 372 0 4
select count(*)
from
t where trunc(created,'y') = to_date('01-jan-2005','dd-mon-yyyy')
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 4 0.00 0.00 0 0 0 0
Execute 4 0.00 0.00 0 0 0 0
Fetch 8 0.04 0.16 0 372 0 4
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 16 0.04 0.16 0 372 0 4