test=# create index idx_tbl_expression_a on tbl_expression using btree (a);
CREATE INDEX
test=# create index idx_tbl_expression_b on tbl_expression using btree (b);
CREATE INDEX
test=# 
test=# explain analyze select * from tbl_expression where a = 'TEST';
                                                              QUERY 
 
PLAN                                                             
 
-------------------------------------------------------------------------------------------------------------------------------------
-
 Index Scan using idx_tbl_expression_a on tbl_expression  (cost=0.29..8.30 rows=1 width=15) (actual time=
 
0.130..0.130 rows=0 loops=1)
   Index Cond: ((a)::text = 'TEST'::text)
 Planning time: 0.667 ms
 Execution time: 0.168 ms
(4 rows)

test=# explain analyze select * from tbl_expression where b = 'you';
                                                              QUERY PLAN                                                             
 
--
 
-----------------------------------------------------------------------------------------------------------------------------------
-
 Index Scan using idx_tbl_expression_b on tbl_expression  (cost=0.29..8.30 rows=1 width=15) (actual time=0.171..0.171 rows=0 loops=1)
   Index Cond: ((b)::text = 'you'::text)
 Planning time: 0.126 ms
 Execution time: 0.206 ms
(4 rows)