BUG #11671: UNACCENT call with constant arg evaluated for each row

The following bug has been logged on the website:

Bug reference: 11671
Logged by: Thomas
Email address: t(dot)chaumeny(at)gmail(dot)com
PostgreSQL version: 9.3.5
Operating system: OS X 10.9.4
Description:

Hi,

I noticed that filtering on a constant condition using UNACCENT lead to
significantly slower queries than expected :

1 - First query without UNACCENT:

EXPLAIN ANALYZE SELECT first_name, last_name FROM user WHERE first_name =
'a';
QUERY PLAN

------------------------------------------------------------------------------------------------------------
Seq Scan on user (cost=0.00..2509.61 rows=1 width=32) (actual
time=19.814..19.814 rows=0 loops=1)
Filter: ((first_name)::text = 'a'::text)
Rows Removed by Filter: 31409
Total runtime: 19.870 ms
(4 rows)

2 - Second with UNACCENT (>4 times slower):

mydb=# EXPLAIN ANALYZE SELECT first_name, last_name FROM user WHERE
first_name = UNACCENT('a');
QUERY PLAN

------------------------------------------------------------------------------------------------------------
Seq Scan on user (cost=0.00..2588.14 rows=1 width=32) (actual
time=88.913..88.913 rows=0 loops=1)
Filter: ((first_name)::text = unaccent('a'::text))
Rows Removed by Filter: 31409
Total runtime: 88.969 ms
(4 rows)

Time: 89,767 ms

3 - Third with "SELECT UNACCENT(...)" — which seems to force constant
evaluation :

mydb=# EXPLAIN ANALYZE SELECT first_name, last_name FROM user WHERE
first_name = (SELECT UNACCENT('a'));
QUERY PLAN

------------------------------------------------------------------------------------------------------------
Seq Scan on user (cost=0.01..2509.62 rows=1 width=32) (actual
time=16.875..16.875 rows=0 loops=1)
Filter: ((first_name)::text = $0)
Rows Removed by Filter: 31409
InitPlan 1 (returns $0)
-> Result (cost=0.00..0.01 rows=1 width=0) (actual time=0.024..0.025
rows=1 loops=1)
Total runtime: 16.995 ms
(6 rows)

Since UNACCENT function is defined as STABLE, I would expect its result to
be evaluated only once in the second query.