Usage of Composite Index

If I create an index on column (a,b,c) then
a, ab, abc combination of columns in where clause will use the index.
bc, b, and c combinations of columns in where clause will not use the index.

The example is given below.

SQL> create table comp_tab(a number, b varchar2(10), c varchar2(10));
Table created.

SQL> begin
for i in 1 .. 10000
loop
insert into comp_tab values(i,'pc-'||round(dbms_random.value(1,20000),0),'pc-'||round(dbms_random.value(1,20000),0));
end loop;
end;
/
PL/SQL procedure successfully completed.
SQL> create index comp_tab_I on comp_tab(a,b,c);
Index created.


SQL> analyze table comp_tab estimate statistics;
Table analyzed.

SQL> set autot trace

a in where cluase will use the index as a is leading portion of index.
SQL> select a,c,b from comp_tab where a=565;
Execution Plan
----------------------------------------------------------
Plan hash value: 753992557
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 19 | 2 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| COMP_TAB_I | 1 | 19 | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------------

ac combination will use the index.
SQL> select a,c,b from comp_tab where a=565 and c='pc-3722';
Execution Plan
----------------------------------------------------------
Plan hash value: 753992557
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 19 | 2 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| COMP_TAB_I | 1 | 19 | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------------

bc combination is used in where clause so will not use index.
SQL> select b,c,a from comp_tab where b='pc-5895' and c='pc-2893';
Execution Plan
----------------------------------------------------------
Plan hash value: 3767570742
------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 19 | 11 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| COMP_TAB | 1 | 19 | 11 (0)| 00:00:01 |
------------------------------------------------------------------------------


Only b combination is used so will not use the index.
SQL> select a,c,b from comp_tab where b='pc-6735';
Execution Plan
----------------------------------------------------------
Plan hash value: 3767570742
------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 19 | 11 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| COMP_TAB | 1 | 19 | 11 (0)| 00:00:01 |
------------------------------------------------------------------------------


abc combination is used in where clause and so will use the index.
SQL> select b,c,a from comp_tab where b='pc-5895' and c='pc-2893' and a=564;
Execution Plan
----------------------------------------------------------
Plan hash value: 753992557
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 19 | 1 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| COMP_TAB_I | 1 | 19 | 1 (0)| 00:00:01 |
-------------------------------------------------------------------------------

ab combination in where clause will use the index.
SQL> select c from comp_tab where b='pc-5895' and a=564;
Execution Plan
----------------------------------------------------------
Plan hash value: 753992557
-------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 19 | 2 (0)| 00:00:01 |
|* 1 | INDEX RANGE SCAN| COMP_TAB_I | 1 | 19 | 2 (0)| 00:00:01 |
-------------------------------------------------------------------------------