Oracle PLAN_TABLE: column SEARCH_COLUMNS

create table tq84_abc (
   val     number,
   col_1   varchar2(10),
   col_2   varchar2(10),
   col_3   varchar2(10),
   col_4   varchar2(10)
);

create index tq84_abc_ix on tq84_abc(col_1, col_2, col_3, col_4);

explain plan
   set statement_id = 'abc'
for
   select *
   from
      tq84_abc
   where
   --
   -- Use only three columns of the available four
   -- in the index:
   --
      col_1 = 'A' and
      col_2 = 'B' and
      col_3 = 'C'
;

select
   lpad(' ', depth*2) || operation || ' ' || options op,
   object_name,
   search_columns
from
   plan_table
where
   statement_id = 'abc'
order by
   id;

-- OP                                     OBJECT_NAME   SEARCH_COLUMNS
-- -------------------------------------  ------------  --------------
-- SELECT STATEMENT
--   TABLE ACCESS BY INDEX ROWID BATCHED  TQ84_ABC
--     INDEX RANGE SCAN                   TQ84_ABC_IX   3