[BUGS] I have a question about using index in order statement.

[kevin]

Question: 
I have a question about using index in order statement.
Why index ix_2 work by Seq Scan and index ix_3 work by Index Scan.

Example :

ix_2 condition :
When I try

  explain
  select * from a_test 
  order by code_ desc

Postgresql response 
  Sort  (cost=100001815.08..100001852.56 rows=14990 width=56)
    Sort Key: code_
    ->  Seq Scan on a_test  (cost=100000000.00..100000260.90 rows=14990 
width=56)

ix_3 condition :
When I try

  explain
  select * from a_test 
  order by lower(code_) desc

Postgresql response 
    Index Scan using ix_3 on a_test  (cost=0.00..769.27 rows=14990 width=18)
    

Table schema :

CREATE TABLE a_test
(
  t_key_ bigint NOT NULL,
  code_ character varying(15)
)
WITH (OIDS=TRUE);
ALTER TABLE a_test OWNER TO postgres;

CREATE INDEX ix_2
  ON a_test
  USING btree
  (code_ DESC);

CREATE INDEX ix_3
  ON a_test
  USING btree
  (lower(code_::text) DESC);