I have no idea what you mean by "custom index"

please explain.



yes, an index will affect performance - it will possibly make some things much much much faster, and some other things slower.


You put an index on a table (say on the ENAME column in the EMP table) so that a query like:


select * from emp where ename = 'TKYTE'

would be able to return the result virtually instantaneously.

That same index however will necessarily slow down:

insert into emp(ename,hiredate) values ('someone', sysdate );


That insert will have to maintain the index - extra work. A small price to pay for being able to get to the individual row much faster.