Will Oracle ignore my index? Suppose I have a table (TABLE_1) with some fields, including ID (number), NAME (varchar2) and DATE_TIME (date). There was a need to create a index on those 3 fields to optimize queries like:

select * from TABLE_1 where ID = [SOME_VALUE] and NAME = [SOME_VALUE] and DATE_TIME is not null;


Because DATE_TIME can have a multitude of different datetime values, I decided to create the index as such:

CREATE INDEX IDX_TABLE_1 ON TABLE_1 (ID,NAME,TRUNC(DATE_TIME));


My question is: will Oracle use this index on the select query or will it ignore it and perform a full table scan on TABLE_1?

EDIT1: I forgot to mention that the ID field is a foreign key.
Depends on the data distribution. I can easily construct examples for both cases (index vs table scan).

Just think about the case when the columns are unique or nearly unique (index), or when all the rows are the same (table scan).

Technically your index can be used for the above query.

Given the general nature of a column named ID, I would say the index will be chosen, but it is not guaraanteed.

The predicate DATE_TIME is not null will be processed at the table level, not the index level, even if you make sure that the NULL rows are indexed as well:

create index index_1 on table_1(id, name, trunc(date_time), 1);

PLAN_TABLE_OUTPUT
-------------------------------------------------------
Plan hash value: 852675278

-------------------------------------------------------
| Id  | Operation                           | Name    |
-------------------------------------------------------
|   0 | SELECT STATEMENT                    |         |
|*  1 |  TABLE ACCESS BY INDEX ROWID BATCHED| TABLE_1 |
|*  2 |   INDEX RANGE SCAN                  | INDEX_1 |
-------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter("DATE_TIME" IS NOT NULL)
   2 - access("ID"=1 AND "NAME"='Oracle')