Postgresql COALESCE performance problem

I have this table in Postgresql:

CREATE TABLE my_table
(
    id bigint NOT NULL,
    value bigint,
    CONSTRAINT my_table_pkey PRIMARY KEY (id)
);

There are ~50000 rows in my_table.

The question is, why the query:

SELECT * FROM my_table WHERE id = COALESCE(null, id) and value = ?

is slower than this one:

SELECT * FROM my_table WHERE value = ?

Is there any solution, other than 开发者_C百科optimizing the query string in app-layer?

EDIT: Practically, the question is how to rewrite the query select * from my_table where id=coalesce(?, id) and value=? to have worst case performance not less than that of select * from my_table where value=? in Postgresql 9.0

Try rewriting the query of the form

SELECT *
  FROM my_table
 WHERE value = ?
   AND (? IS NULL OR id = ?)

From my own quick tests

INSERT INTO my_table select generate_series(1,50000),1;
UPDATE my_table SET value = id%17;

CREATE INDEX val_idx ON my_table(value);

VACUUM ANALYZE my_table;

\set idval 17
\set pval   0

explain analyze 
SELECT *
  FROM my_table
 WHERE value = :pval
   AND (:idval IS NULL OR id = :idval);

Index Scan using my_table_pkey on my_table  (cost=0.00..8.29 rows=1 width=16) (actual time=0.034..0.035 rows=1 loops=1)
   Index Cond: (id = 17)
   Filter: (value = 0)
 Total runtime: 0.064 ms

\set idval null

explain analyze 
SELECT *
  FROM my_table
 WHERE value = :pval
   AND (:idval IS NULL OR id = :idval);

Bitmap Heap Scan on my_table  (cost=58.59..635.62 rows=2882 width=16) (actual time=0.373..1.594 rows=2941 loops=1)
   Recheck Cond: (value = 0)
   ->  Bitmap Index Scan on validx  (cost=0.00..57.87 rows=2882 width=0) (actual time=0.324..0.324 rows=2941 loops=1)
         Index Cond: (value = 0)
 Total runtime: 1.811 ms

From creating a similar table, populating it, updating statistics, and finally looking at the output of EXPLAIN ANALYZE, the only difference I see is that the first query filters like this:

Filter: ((id = COALESCE(id)) AND (value = 3))

and the second one filters like this:

Filter: (value = 3)

I see substantially different performance and execution plans when there's an index on the column "value". In the first case

Bitmap Heap Scan on my_table  (cost=19.52..552.60 rows=5 width=16) (actual time=19.311..20.679 rows=1000 loops=1)
  Recheck Cond: (value = 3)
  Filter: (id = COALESCE(id))
  ->  Bitmap Index Scan on t2  (cost=0.00..19.52 rows=968 width=0) (actual time=19.260..19.260 rows=1000 loops=1)
        Index Cond: (value = 3)
Total runtime: 22.138 ms

and in the second

Bitmap Heap Scan on my_table  (cost=19.76..550.42 rows=968 width=16) (actual time=0.302..1.293 rows=1000 loops=1)
  Recheck Cond: (value = 3)
  ->  Bitmap Index Scan on t2  (cost=0.00..19.52 rows=968 width=0) (actual time=0.276..0.276 rows=1000 loops=1)
        Index Cond: (value = 3)
Total runtime: 2.174 ms

So I'd say it's slower because the db engine a) evaluates the COALESCE() expression rather than optimizing it away, and b) evaluating it involves an additional filter condition.