One problem with all relational databases is having the
optional ability to index on a NULL column. By default, relational
databases ignore NULL values (because the relational model says that
NULL means "not present"). Hence, Oracle indexes will not include
NULL values.
For example, this index
definition would not index on "open positions", new employee positions
that are stored with a NULL employee name:
create index
emp_ename_idx
on
emp
(ename)
;
Whenever a SQL query asks for the open position
employee slots "where ename is NULL", there will be no index entries
for NULLS in emp_name_idx and Oracle would perform an unnecessary
large-table full-table scan.
Execution Plan
---------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1 Card=1 Bytes=6)
1 0 TABLE ACCESS (FULL) OF 'EMP' (Cost=1 Card=1 Bytes=6)
To get around the optimization of SQL queries that
choose NULL column values, we can create a function-based
index using the null value built-in SQL function to index only on the
NULL columns.
Note that the "null value" (NVL) function replaces NULL values with the
character string
"null', a real value that can participate in an index:
-- create an FBI on ename column with
NULL values
create index
emp_null_ename_idx
on
emp
(nvl(ename,'null'));
analyze index emp_null_ename_idx
compute statistics;
You can also do this techniques with NULL numeric values. This
syntax replaces NULL values with a zero:
-- create an FBI on emp_nbr column with
NULL values
create index
emp_null_emp_nbr_idx
on
emp
(nvl(ename,o));
analyze index emp_null_ename_idx
compute statistics;
Now we can use the index and greatly improve the speed of any
queries that require access to the NULL columns. Note that we must
make one of two changes:
1- Add a hint to force the index
2 - Change the WHERE predicate to match the function
Here is an example of using an index on NULL column values:
-- insert a NULL row
insert into emp (empno) values (999);
set autotrace traceonly explain;
-- test the index access
(change predicate to use FBI)
select /*+ index(emp_null_ename_idx) */
ename
from
emp e
where
nvl(ename,'null') = 'null'
;
Reader Comments:
I was reading "Indexing NULL table column
values for fast SQL performance" article, where you have mentioned that if
the column is having null values (of very less amount) and you want to
select where column is null then to use the index, create function based
index and changed your query to use that index.
I think, it can be done without changing query
as well......
SQL> select count(1) from t where n is
null;
COUNT(1)
----------
334
Execution Plan
---------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
(Cost=3 Card=1 Bytes=3)
1 0 SORT (AGGREGATE)
2 1 TABLE ACCESS (FULL) OF 'T'
(Cost=3 Card=334 Bytes=1002)
SQL> create index tind on t(n, 1); ---->
here 1 is just any arbitary value.
Index created.
SQL> exec
dbms_stats.gather_table_stats(user,'t',cascade=>true);
PL/SQL procedure successfully completed.
SQL> select count(1) from t where n is
null;
COUNT(1)
----------
334
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE
(Cost=2 Card=1 Bytes=4)
1 0 SORT (AGGREGATE)
2 1 INDEX (RANGE SCAN) OF 'TIND'
(NON-UNIQUE) (Cost=2 Card=3
34 Bytes=1336)
Sandeep Redkar
