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

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CHAPTER 11 ■ INDEXES
ops$tkyte@ORA10G> set autotrace traceonly explain
ops$tkyte@ORA10G> select *
2 from emp
3 where upper(ename) = 'KING';
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=2 Card=2 Bytes=92)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'EMP' (TABLE) (Cost=2 Card=2 Bytes=92)
2 1 INDEX (RANGE SCAN) OF 'EMP_UPPER_IDX' (INDEX) (Cost=1 Card=2)
will make use of this index, gaining the performance boost an index can deliver. Before this
feature was available, every row in the EMP table would have been scanned, uppercased, and
compared. In contrast, with the index on UPPER(ENAME), the query takes the constant KING to
the index, range scans a little data, and accesses the table by rowid to get the data. This is
very fast.
This performance boost is most visible when indexing user-written functions on
columns. Oracle 7.1 added the ability to use user-written functions in SQL, so we could
do something like this:
SQL> select my_function(ename)
2 from emp
3 where some_other_function(empno) > 10
4 /
This was great because we could now effectively extend the SQL language to include
application-specific functions. Unfortunately, however, the performance of the preceding
query was a bit disappointing at times. Say the EMP table had 1,000 rows in it. The function
SOME_OTHER_FUNCTION would be executed 1,000 times during the query, once per row. In addition,
assuming the function took one-hundredth of a second to execute, this relatively simple
query now takes at least ten seconds.
Let’s look at a real example, where we’ll implement a modified SOUNDEX routine in PL/SQL.
Additionally, we’ll use a package global variable as a counter in our procedure, which will
allow us to execute queries that make use of the MY_SOUNDEX function and see exactly how
many times it was called:
ops$tkyte@ORA10G> create or replace package stats
2 as
3 cnt number default 0;
4 end;
5 /
Package created.
ops$tkyte@ORA10G> create or replace
2 function my_soundex( p_string in varchar2 ) return varchar2
3 deterministic
4 as
5 l_return_string varchar2(6) default substr( p_string, 1, 1 );
6 l_char varchar2(1);