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Postgresql COALESCE performance issue

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 ~ 50,000 rows in my_table.

The question is why the request:

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

slower than this:

SELECT * FROM my_table WHERE value = ?

Is there any solution besides optimizing the query string at the application level?

EDIT: The practical question is how to rewrite the query select * from my_table where id=coalesce(?, id) and value=?so that the worst case performance is less than that select * from my_table where value=?of Postgresql 9.0

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2 answers

Try rewriting the form request

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
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, , , , EXPLAIN ANALYZE, , , , :

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

- :

Filter: (value = 3)

, "".

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


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

, , , db a) COALESCE(), , b) .

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