Function Based Index not improving query performance

I have created view and in this view i have added the below case statement which i need and for which i already create exactly the function based index. The view has 1900000 records. When i tried to execute the view it takes hours to run and the performance of this view is very low. I dont understand how can i improve the performance.

CREATE OR REPLACE VIEW
    TST_AGG
    (
    ROOT) AS
    Select
     CASE
                WHEN regexp_like(ticker, '\s.*\s')
                THEN SUBSTR(ticker, 1, instr(ticker, ' ')-1)
                WHEN regexp_like(ticker, '\s')
                THEN
                    CASE
                        WHEN regexp_like(SUBSTR(ticker, 1, instr(ticker, ' ')-1), '(P|C)$')
                        AND LENGTH(SUBSTR(ticker, 1, instr(ticker, ' ')-1)) >= 4
                        THEN SUBSTR(SUBSTR(ticker, 1, instr(ticker, ' ')-1), 1, LENGTH(SUBSTR(ticker, 1
                            , instr(ticker, ' ')-1))-3)
                        WHEN regexp_like(SUBSTR(ticker, 1, instr(ticker, ' ')-1), '\w\d\d\w\d$')
                        THEN SUBSTR(SUBSTR(ticker, 1, instr(ticker, ' ')-1), 1, LENGTH(SUBSTR(ticker, 1
                            , instr(ticker, ' ')-1))-5)
                        WHEN regexp_like(SUBSTR(ticker, 1, instr(ticker, ' ')), '\w\d\w\d$')
                        THEN SUBSTR(SUBSTR(ticker, 1, instr(ticker, ' ')-1), 1, LENGTH(SUBSTR(ticker, 1
                            , instr(ticker, ' ')-1))-4)
                        ELSE SUBSTR(ticker, 1, instr(ticker, ' ')-1)
                    END
                WHEN regexp_like(ticker, '(P|C)$')
                AND LENGTH(ticker) >= 4
                THEN SUBSTR(ticker, 1, LENGTH(ticker)-3)
                WHEN regexp_like(ticker, '\w\d\d\w\d$')
                THEN SUBSTR(ticker, 1, LENGTH(ticker)-5)
                WHEN regexp_like(ticker, '\w\d\w\d$')
                THEN SUBSTR(ticker, 1, LENGTH(ticker)-4)
                ELSE ticker
            END ) AS ROOT
FROM TTT_IMP

Below is the functional based index i have created:

CREATE INDEX "IDX_ROOT" ON "TTT_IMP" (CASE  WHEN  REGEXP_LIKE ("TICKER",'\s.*\s') THEN SUBSTR("TICKER",1,INSTR("TICKER",' ')-1) WHEN  REGEXP_LIKE ("TICKER",'\s') THEN CASE  WHEN ( REGEXP_LIKE (SUBSTR("TICKER",1,INSTR("TICKER",' ')-1),'(P|C)$') AND LENGTH(SUBSTR("TICKER",1,INSTR("TICKER",' ')-1))>=4) THEN SUBSTR(SUBSTR("TICKER",1,INSTR("TICKER",' ')-1),1,LENGTH(SUBSTR("TICKER",1,INSTR("TICKER",' ')-1))-3) WHEN  REGEXP_LIKE (SUBSTR("TICKER",1,INSTR("TICKER",' ')-1),'\w\d\d\w\d$') THEN SUBSTR(SUBSTR("TICKER",1,INSTR("TICKER",' ')-1),1,LENGTH(SUBSTR("TICKER",1,INSTR("TICKER",' ')-1))-5) WHEN  REGEXP_LIKE (SUBSTR("TICKER",1,INSTR("TICKER",' ')),'\w\d\w\d$') THEN SUBSTR(SUBSTR("TICKER",1,INSTR("TICKER",' ')-1),1,LENGTH(SUBSTR("TICKER",1,INSTR("TICKER",' ')-1))-4) ELSE SUBSTR("TICKER",1,INSTR("TICKER",' ')-1) END  WHEN ( REGEXP_LIKE ("TICKER",'(P|C)$') AND LENGTH("TICKER")>=4) THEN SUBSTR("TICKER",1,LENGTH("TICKER")-3) WHEN  REGEXP_LIKE ("TICKER",'\w\d\d\w\d$') THEN SUBSTR("TICKER",1,LENGTH("TICKER")-5) WHEN  REGEXP_LIKE ("TICKER",'\w\d\w\d$') THEN SUBSTR("TICKER",1,LENGTH("TICKER")-4) ELSE "TICKER" END );

The index can be used in two scenarios.

1. To reduce the records selected, ie

   SELECT ... FROM TST_AGG where ROOT = ...

2. To avoid querying a table, and to avoid expensive calcuations

   SELECT ROOT FROM TST_AGG

I am assuming the latter here. The optimizer can only use an index (in place of a table) if it knows that the index entries are one for one with the table. Because it does not know if that expression may return a NULL (which are NOT stored in the index) it cannot make that direct swap unless you inform the optimizer.

Hence

SELECT ROOT FROM TST_AGG

will not have the chance to use the index, but

SELECT ROOT FROM TST_AGG WHERE ROOT IS NOT NULL

should be able to.

One other thing to be careful of, is that we might alter the expression syntax that we used to store the index. So check out USER_IND_EXPRESSIONS, and perhaps put that expression back into the definition of the view.

I would suggest to review your data model, the regex is really ugly. Store relevant information directly in column instead of somewhere hidden in a ticket string.

Anyway, I would propose to create a virtual column instead of view. Then you can create an index on this virtual column and it should also be used. Would be similar to this:

ALTER TABLE TTT_IMP ADD (ROOT VARCHAR2(20) GENERATED ALWAYS AS (
CAST(
    CASE
    WHEN regexp_like(ticker, '\s.*\s')
    THEN SUBSTR(ticker, 1, instr(ticker, ' ')-1)
    WHEN regexp_like(ticker, '\s')
    THEN
        CASE
            WHEN regexp_like(SUBSTR(ticker, 1, instr(ticker, ' ')-1), '(P|C)$')
            AND LENGTH(SUBSTR(ticker, 1, instr(ticker, ' ')-1)) >= 4
            THEN SUBSTR(SUBSTR(ticker, 1, instr(ticker, ' ')-1), 1, LENGTH(SUBSTR(ticker, 1
                , instr(ticker, ' ')-1))-3)
            WHEN regexp_like(SUBSTR(ticker, 1, instr(ticker, ' ')-1), '\w\d\d\w\d$')
            THEN SUBSTR(SUBSTR(ticker, 1, instr(ticker, ' ')-1), 1, LENGTH(SUBSTR(ticker, 1
                , instr(ticker, ' ')-1))-5)
            WHEN regexp_like(SUBSTR(ticker, 1, instr(ticker, ' ')), '\w\d\w\d$')
            THEN SUBSTR(SUBSTR(ticker, 1, instr(ticker, ' ')-1), 1, LENGTH(SUBSTR(ticker, 1
                , instr(ticker, ' ')-1))-4)
            ELSE SUBSTR(ticker, 1, instr(ticker, ' ')-1)
        END
    WHEN regexp_like(ticker, '(P|C)$')
    AND LENGTH(ticker) >= 4
    THEN SUBSTR(ticker, 1, LENGTH(ticker)-3)
    WHEN regexp_like(ticker, '\w\d\d\w\d$')
    THEN SUBSTR(ticker, 1, LENGTH(ticker)-5)
    WHEN regexp_like(ticker, '\w\d\w\d$')
    THEN SUBSTR(ticker, 1, LENGTH(ticker)-4)
    ELSE ticker
    END
AS VARCHAR2(20))
) VIRTUAL);

To add to @ConnorMcDonnald’s great answer, here’s a blurb from the docs:

12.4.2 Disadvantages of Function-Based Indexes

> If the index expression is a function invocation, then the function
> return type cannot be constrained.
>
> Because you cannot constrain the function return type with NOT NULL,
> you must ensure that the query that uses the index cannot fetch NULL
> values. Otherwise, the database performs a full table scan.