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

CHAPTER 11 ■ INDEXES 461
ops$tkyte@ORA10G> create index emp_soundex_idx on
2 emp( my_soundex(ename) ) tablespace ts4k;
emp( my_soundex(ename) ) tablespace ts4k
*
ERROR at line 2:
ORA-01450: maximum key length (3118) exceeded
It is not that the index actually contains any keys that large, but that it could as far as the
database is concerned. But the database understands SUBSTR. It sees the inputs to SUBSTR of
1 and 6, and knows the biggest return value from this is six characters; hence, it permits the
index to be created. This size issue can get you, especially with concatenated indexes. Here is
an example on an 8KB blocksize tablespace:
ops$tkyte@ORA10G> create index emp_soundex_idx on
2 emp( my_soundex(ename), my_soundex(job) );
emp( my_soundex(ename), my_soundex(job) )
*
ERROR at line 2:
ORA-01450: maximum key length (6398) exceeded
Here, the database thinks the maximum key size is 8,000 bytes and fails the CREATE once
again. So, to index a user-written function that returns a string, we should constrain the return
type in the CREATE INDEX statement. In the example, knowing that MY_SOUNDEX returns at most
six characters, we are substringing the first six characters.
We are now ready to test the performance of the table with the index on it. We would like
to monitor the effect of the index on INSERTs as well as the speedup for SELECTs to see the
effect on each. In the unindexed test case, our queries take over one second, and if we were
to run SQL_TRACE and TKPROF during the inserts, we could observe that without the index, the
insert of 9,999 records took about .5 seconds:
insert into emp NO_INDEX
(empno,ename,job,mgr,hiredate,sal,comm,deptno)
select rownum empno,
initcap(substr(object_name,1,10)) ename,
substr(object_type,1,9) JOB,
rownum MGR,
created hiredate,
rownum SAL,
rownum COMM,
(mod(rownum,4)+1)*10 DEPTNO
from all_objects
where rownum < 10000
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.03 0.06 0 0 0 0
Execute 1 0.46 0.43 0 15439 948 9999