Index range scan vs index skip scan vs index fast full scan

Question

I have table

test_A(
    id1 number,
    id2 number,
    id3 number,
    name varchar2(10),
    create_dt date
)

I have two indexes one composite index indx1 on (id1,id2) and indx2(id3). Now when I query this table test_A as

select * from test_A where id2=123 and 
create_dt=(select max(create_dt) from test_A where test_A.id2=id2);

I ran explain plan for this above SQL and it is using "index skip scan". If I create another index on create_dt then it using index fast full scan and over all cost and %cpu is showing higher than plan with Index skip scan. It is also using Index range scan after creating index on create_dt.

I could not come to conclusion which should be could? Do I need to create another Index on create_dt or is Index skip scan good? I believe Index skip is a feature of Oracle to run multiple index range scan?

Answer 1

I recommend you to familiarize yourself with this link: http://docs.oracle.com/cd/E16655_01/server.121/e15858/tgsql_optop.htm#CHDFJIJA
It is Oracle 12c related, however it is very usefull to gain understanding how oracle uses different index access pathes in all DBMS versions.


Your subquery is ambigous:

select max(create_dt) from test_A where test_A.id2=id2

both test_A.id2 and id2 references to the same test_A.id2, and the query is equivalent to this:

select * from test_A where id2=123 and 
create_dt=(select max(create_dt) from test_A where id2=id2);

or simply:

select * from test_A where id2=123 and 
create_dt=(select max(create_dt) from test_A where id2 is not null);

I suppose that you want something like this:

select * from test_A where id2=123 and 
create_dt=(select max(create_dt) 
           from test_A ALIAS 
           where test_A.id2=ALIAS.id2);

For the above query a composite index on id2+create_dt most likely give the best results, try it:

CREATE INDEX index_name ON test_A( id2, create_dt);