Functions as conditions

Hello all,

Let's create a function that checks if column s is over 500:
 

CREATE OR REPLACE FUNCTION test_func(integer)
 RETURNS boolean
 LANGUAGE plpgsql
AS $function$
    BEGIN
        IF $1 > 500 THEN
                return true;
        ELSE
                return false;
        END IF;
    END$function$;

Running:

 EXPLAIN ANALYZE SELECT * FROM t_random WHERE test_func(s)
                                                     QUERY PLAN                                                    
--------------------------------------------------------------------------------------------------------------------
 Seq Scan on t_random  (cost=0.00..187834.00 rows=233333 width=37) (actual time=0.505..572.463 rows=699500 loops=1)
   Filter: test_func(s)
   Rows Removed by Filter: 500
 Planning time: 0.025 ms
 Execution time: 600.917 ms

As opposed to:

 EXPLAIN ANALYZE SELECT * FROM t_random WHERE s > 500;
                                                    QUERY PLAN                                                   
------------------------------------------------------------------------------------------------------------------
 Seq Scan on t_random  (cost=0.00..14584.00 rows=699546 width=37) (actual time=0.080..75.001 rows=699500 loops=1)
   Filter: (s > 500)
   Rows Removed by Filter: 500
 Planning time: 0.029 ms
 Execution time: 98.257 ms

The first query plan has a horrendous query time and estimations using test_func(). I'm assuming because the function has 0 statistics on the table itself. It can't base its' input off any index, meaning it has no sense of how big the table is correct? I'm not exactly sure the reason here.

Let's assume that this function is absolutely necessary and that it changes based on some environment variables, while maintaining only a single table. We can not have a static column with fixed boolean values since this column will always be changing depending on the environment. Would a good solution then be to use views and create a column based on the function?

 CREATE VIEW t_view AS SELECT *, test_func(s) AS passed FROM t_random;

Running:

 EXPLAIN ANALYZE SELECT * FROM t_view WHERE passed = true:

                                                        QUERY PLAN                                                       
--------------------------------------------------------------------------------------------------------------------------
 Subquery Scan on t_view  (cost=0.00..194834.00 rows=350000 width=38) (actual time=2.198..661.008 rows=699500 loops=1)
   Filter: t_view.test_func
   Rows Removed by Filter: 500
   ->  Seq Scan on t_random  (cost=0.00..187834.00 rows=700000 width=37) (actual time=0.026..579.125 rows=700000 loops=1)
 Planning time: 0.040 ms
 Execution time: 690.057 mss

The query plan does a bit better but is this the best plan I can hope to achieve with the specific function? Materialized views are not an option since the environment is changing constantly and I would like an always-up-to-date database.

Thank you,

-Bao