index with basic compression for filter predicates

example.sql

create table t (a int,b int, c int, d int);
insert into t select 1, 1, level,level from dual connect by level<=100;

insert into t select 1, 5, level,level from dual connect by level<=100000;
insert into t select 1, 6, level,level from dual connect by level<=100000;
insert into t select 1, 7, level,level from dual connect by level<=100000;

insert into t select 10, 10, level,level from dual connect by level<=1;
insert into t select 11, 11, level,level from dual connect by level<=1;

create index t_ix on t(a,b,c) compress 2;

call dbms_stats.gather_table_stats('','T');

select leaf_blocks from user_indexes where index_name='T_IX';
select blocks from user_segments where segment_name='T_IX';
select/*+ index_rs_asc(t) gather_plan_statistics */ count(*) from t where a<3 and b<3;
select * from dbms_xplan.display_cursor('','','allstats last');

output.sql

SQL> select leaf_blocks from user_indexes where index_name='T_IX';

LEAF_BLOCKS
-----------
        666

1 row selected.

SQL> select blocks from user_segments where segment_name='T_IX';

    BLOCKS
----------
       768

1 row selected.

SQL> select/*+ index_rs_asc(t) gather_plan_statistics */ count(*) from t where a<3 and b<3;

  COUNT(*)
----------
       100

1 row selected.

SQL> select * from dbms_xplan.display_cursor('','','allstats last');

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------------------
SQL_ID  7xwh91zzmznv0, child number 0
-------------------------------------
select/*+ index_rs_asc(t) gather_plan_statistics */ count(*) from t
where a<3 and b<3

Plan hash value: 3836941823

---------------------------------------------------------------------------------------------
| Id  | Operation         | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |      1 |        |      1 |00:00:00.02 |     670 |      1 |
|   1 |  SORT AGGREGATE   |      |      1 |      1 |      1 |00:00:00.02 |     670 |      1 |
|*  2 |   INDEX RANGE SCAN| T_IX |      1 |  20007 |    100 |00:00:00.02 |     670 |      1 |
---------------------------------------------------------------------------------------------

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

   2 - access("A"<3 AND "B"<3)
       filter("B"<3)


21 rows selected.