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

CHAPTER 11 ■ INDEXES 473
select * from T where x is null;
This query cannot use the index we just created—the row (NULL, NULL) simply is not in
the index, hence the use of the index would in fact return the wrong answer. Only if at least
one of the columns is defined as NOT NULL can the query use an index. For example, the following
shows Oracle will use an index for an X IS NULL predicate if there is an index with X on the
leading edge and at least one other column in the index is NOT NULL:
ops$tkyte@ORA10GR1> create table t ( x int, y int NOT NULL );
Table created.
ops$tkyte@ORA10GR1> create unique index t_idx on t(x,y);
Index created.
ops$tkyte@ORA10GR1> insert into t values ( 1, 1 );
1 row created.
ops$tkyte@ORA10GR1> insert into t values ( NULL, 1 );
1 row created.
ops$tkyte@ORA10GR1> begin
2 dbms_stats.gather_table_stats(user,'T');
3 end;
4 /
PL/SQL procedure successfully completed.
When we go to query that table this time, we’ll discover this:
ops$tkyte@ORA10GR1> set autotrace on
ops$tkyte@ORA10GR1> select * from t where x is null;
X
Y
---------- ----------
1
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=ALL_ROWS (Cost=1 Card=1 Bytes=5)
1 0 INDEX (RANGE SCAN) OF 'T_IDX' (INDEX (UNIQUE)) (Cost=1 Card=1 Bytes=5)
Previously, I said that you can use to your advantage the fact that totally null entries are
not stored in a B*Tree index—here is how. Say you have a table with a column that takes
exactly two values. The values are very skewed; say, 90 percent or more of the rows take on one
value and 10 percent or less take on the other value. You can index this column efficiently to
gain quick access to the minority rows. This comes in handy when you would like to use an
index to get to the minority rows, but you want to full scan to get to the majority rows, and you
want to conserve space. The solution is to use a null for majority rows and whatever value
you want for minority rows or, as demonstrated earlier, use a function-based index to index
only the non-null return values from a function.