seq scan instead of index scan - Mailing list pgsql-performance

Hello.

I have a problem I don't understand. I hope it's a simple problem and I'm
just stupid.

When I make a subquery Postgres don't care about my indexes and makes
a seq scan instead of a index scan. Why?

Is it possible that the subquery change the datatype and by this make
a index scan impossible? Can I somehow see the datatypes used by the
query?

Below is the test I'm running.

/ Karl Larsson


CREATE TABLE table_one (
    id bigint PRIMARY KEY NOT NULL
);
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index
"table_one_pkey" for table "table_one"

CREATE TABLE table_two (
    id bigint PRIMARY KEY NOT NULL
);
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index
"table_two_pkey" for table "table_two"




INSERT INTO table_one VALUES (4);
INSERT INTO table_one VALUES (3);
INSERT INTO table_one VALUES (5);
INSERT INTO table_one VALUES (2);
INSERT INTO table_one VALUES (6);
INSERT INTO table_one VALUES (1);

INSERT INTO table_two VALUES (14);
INSERT INTO table_two VALUES (12);
INSERT INTO table_two VALUES (10);
INSERT INTO table_two VALUES (8);
INSERT INTO table_two VALUES (6);
INSERT INTO table_two VALUES (4);
INSERT INTO table_two VALUES (2);



EXPLAIN ANALYZE
SELECT t2.id
FROM table_two AS t2, (
    SELECT id
    FROM table_one AS t1
    WHERE t1.id < 6
  ) AS foo
WHERE t2.id = foo.id;