regression=# CREATE FUNCTION ab(in text, in text, out ab text) AS $$BEGIN ab := $1 || $2; END;$$ LANGUAGE plpgsql IMMUTABLE;
CREATE FUNCTION
regression=# CREATE table t1 (a text, b text);
CREATE TABLE
regression=# CREATE INDEX t1_ab_idx on t1 (ab(a, b));
CREATE INDEX
regression=# CREATE table t2 (ab text primary key);
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "t2_pkey" for table "t2"
CREATE TABLE
regression=# INSERT INTO t1 VALUES ('a', 'b');
INSERT 0 1
regression=# INSERT INTO t2 VALUES ('ab');
INSERT 0 1
regression=# VACUUM ANALYZE ;
VACUUM
regression=# SET enable_seqscan = false;
SET
regression=# EXPLAIN ANALYZE SELECT * FROM (SELECT ab(a, b) AS ab FROM t1 UNION ALL SELECT * FROM t2) t WHERE ab = 'ab';
                                                         QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
 Result  (cost=10000000000.00..10000000009.53 rows=2 width=18) (actual time=0.052..0.066 rows=2 loops=1)
   ->  Append  (cost=10000000000.00..10000000009.53 rows=2 width=18) (actual time=0.052..0.065 rows=2 loops=1)
         ->  Seq Scan on t1  (cost=10000000000.00..10000000001.26 rows=1 width=32) (actual time=0.051..0.052 rows=1 loops=1)
               Filter: (ab(a, b) = 'ab'::text)
         ->  Index Scan using t2_pkey on t2  (cost=0.00..8.27 rows=1 width=3) (actual time=0.010..0.011 rows=1 loops=1)
               Index Cond: (ab = 'ab'::text)
 Total runtime: 0.106 ms
(7 rows)

regression=# EXPLAIN ANALYZE SELECT * FROM (SELECT ab(a, b) AS ab FROM t1 UNION SELECT * FROM t2) t WHERE ab = 'ab';
                                                           QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------
 Unique  (cost=17.07..17.08 rows=2 width=4) (actual time=0.071..0.073 rows=1 loops=1)
   ->  Sort  (cost=17.07..17.07 rows=2 width=4) (actual time=0.070..0.070 rows=2 loops=1)
         Sort Key: (ab(t1.a, t1.b))
         Sort Method: quicksort  Memory: 25kB
         ->  Append  (cost=0.25..17.06 rows=2 width=4) (actual time=0.050..0.058 rows=2 loops=1)
               ->  Index Scan using t1_ab_idx on t1  (cost=0.25..8.77 rows=1 width=4) (actual time=0.049..0.050 rows=1 loops=1)
                     Index Cond: (ab(a, b) = 'ab'::text)
               ->  Index Scan using t2_pkey on t2  (cost=0.00..8.27 rows=1 width=3) (actual time=0.004..0.005 rows=1 loops=1)
                     Index Cond: (ab = 'ab'::text)
 Total runtime: 0.116 ms
(10 rows)

regression=# EXPLAIN ANALYZE SELECT * FROM (SELECT ab(a, b) AS ab FROM t1) t WHERE ab = 'ab';
                                                  QUERY PLAN
--------------------------------------------------------------------------------------------------------------
 Index Scan using t1_ab_idx on t1  (cost=0.25..8.77 rows=1 width=4) (actual time=0.030..0.032 rows=1 loops=1)
   Index Cond: (ab(a, b) = 'ab'::text)
 Total runtime: 0.048 ms
(3 rows)