Using function based index (oracle) to speed up count(X) - oracle

Question

I've a table Film:
CREATE TABLE film (
film_id NUMBER(5) NOT NULL,
title varchar2(255));
And I wanted to make the query, which counts how many titles start with the same word and only displays ones with more than 20, faster using a function based index. The query:
SELECT FW_SEPARATOR.FIRST_WORD AS "First Word", COUNT(FW_SEPARATOR.FIRST_WORD) AS "Count"
FROM (SELECT regexp_replace(FILM.TITLE, '(\w+).*$','\1') AS FIRST_WORD FROM FILM) FW_SEPARATOR
GROUP BY FW_SEPARATOR.FIRST_WORD
HAVING COUNT(FW_SEPARATOR.FIRST_WORD) >= 20;
The thing is, I created this function based index:
CREATE INDEX FIRST_WORD_INDEX ON FILM(regexp_replace(TITLE, '(\w+).*$','\1'));
But it didn't speed anything up...
I was wondering if anyone could help me with this :)

Answer 1

Add a redundant predicate to the query to convince Oracle that the expression will not return null values and an index can be used:
select regexp_replace(film.title, '(\w+).*$','\1') first_word
from film
where regexp_replace(film.title, '(\w+).*$','\1') is not null;
Oracle can use an index like a skinny version of a table. Many queries only contain a small subset of the columns in a table. If all the columns in that set are part of the same index, Oracle can use that index instead of the table. This will be either an INDEX FAST FULL SCAN or an INDEX FULL SCAN. The data may be read similar to the way a regular table scan works. But since the index is much smaller than the table, that access method can be much faster.
But function-based indexes do not store NULLs. Oracle cannot use an index scan if it thinks there is a NULL that is not stored in the index. In this case, if the base column was defined as NOT NULL, the regular expression would always return a non-null value. But unsurprisingly, Oracle has not built code to determine whether or not a regular expression could return NULL. That sounds like an impossible task, similar to the halting problem.
There are several ways to convince Oracle that the expression is not null. The simplest may be to repeat the predicate and add an IS NOT NULL condition.
Sample Schema
create table film (
film_id number(5) not null,
title varchar2(255) not null);
insert into film select rownumber, column_value
from
(
select rownum rownumber, column_value from table(sys.odcivarchar2list(
q'<The Shawshank Redemption>',
q'<The Godfather>',
q'<The Godfather: Part II>',
q'<The Dark Knight>',
q'<Pulp Fiction>',
q'<The Good, the Bad and the Ugly>',
q'<Schindler's List>',
q'<12 Angry Men>',
q'<The Lord of the Rings: The Return of the King>',
q'<Fight Club>'))
);
create index film_idx1 on film(regexp_replace(title, '(\w+).*$','\1'));
begin
dbms_stats.gather_table_stats(user, 'FILM');
end;
/
Query that does not use index
Even with an index hint, the normal query will not use an index. Remember that hints are directives, and this query would use the index if it was possible.
explain plan for
select /*+ index_ffs(film) */ regexp_replace(title, '(\w+).*$','\1') first_word
from film;
select * from table(dbms_xplan.display);
Plan hash value: 1232367652
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 50 | 3 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| FILM | 10 | 50 | 3 (0)| 00:00:01 |
--------------------------------------------------------------------------
Query that uses index
Now add the extra condition and the query will use the index. I'm not sure why it uses an INDEX FULL SCAN instead of an INDEX FAST FULL SCAN. With such small sample data it doesn't matter. The important point is that an index is used.
explain plan for
select regexp_replace(film.title, '(\w+).*$','\1') first_word
from film
where regexp_replace(film.title, '(\w+).*$','\1') is not null;
select * from table(dbms_xplan.display);
Plan hash value: 1151375616
------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 50 | 1 (0)| 00:00:01 |
|* 1 | INDEX FULL SCAN | FILM_IDX1 | 10 | 50 | 1 (0)| 00:00:01 |
------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter( REGEXP_REPLACE ("TITLE",'(\w+).*$','\1') IS NOT NULL)