Postgresql COALESCE performance problem - postgresql

Question

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 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

Answer 1

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

Answer 2

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.