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

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CHAPTER 11 ■ INDEXES
column in the DEPT table: D.DNAME. We see a FROM clause, making this CREATE INDEX statement
resemble a query. We have a join condition between multiple tables. This CREATE INDEX statement
indexes the DEPT.DNAME column, but in the context of the EMP table. If we ask those
questions mentioned earlier, we would find the database never accesses the DEPT at all, and it
need not do so because the DNAME column now exists in the index pointing to rows in the EMP
table. For purposes of illustration, we will make the EMP and DEPT tables appear “large” (to
avoid having the CBO think they are small and full scanning them instead of using indexes):
ops$tkyte@ORA10G> begin
2 dbms_stats.set_table_stats( user, 'EMP',
3 numrows => 1000000, numblks => 300000 );
4 dbms_stats.set_table_stats( user, 'DEPT',
5 numrows => 100000, numblks => 30000 );
6 end;
7 /
PL/SQL procedure successfully completed.
and then we’ll perform our queries:
ops$tkyte@ORA10G> set autotrace traceonly explain
ops$tkyte@ORA10G> select count(*)
2 from emp, dept
3 where emp.deptno = dept.deptno
4 and dept.dname = 'SALES'
5 /
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=1 Card=1 Bytes=13)
1 0 SORT (AGGREGATE)
2 1 BITMAP CONVERSION (COUNT) (Cost=1 Card=10000 Bytes=130000)
3 2 BITMAP INDEX (SINGLE VALUE) OF 'EMP_BM_IDX' (INDEX (BITMAP))
As you can see, to answer this particular question, we did not have to actually access
either the EMP or DEPT table—the entire answer came from the index itself. All the information
needed to answer the question was available in the index structure.
Further, we were able to skip accessing the DEPT table and, using the index on EMP that
incorporated the data we needed from DEPT, gain direct access to the required rows:
ops$tkyte@ORA10G> select emp.*
2 from emp, dept
3 where emp.deptno = dept.deptno
4 and dept.dname = 'SALES'
5 /
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=6145 Card=10000 Bytes=870000)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'EMP' (TABLE) (Cost=6145 Card=10000 ...
2 1 BITMAP CONVERSION (TO ROWIDS)
3 2 BITMAP INDEX (SINGLE VALUE) OF 'EMP_BM_IDX' (INDEX (BITMAP))