Hi,
i have a table with a huge amount of rows (actually 4 millions and a half), defined like this:
CREATE TABLE rtp_frame (
i_len integer NOT NULL,
i_file_offset bigint NOT NULL,
i_file_id integer NOT NULL, -- foreign key
i_timestamp bigint NOT NULL,
i_loop integer NOT NULL,
i_medium_id integer NOT NULL, -- foreign key
PRIMARY KEY(i_medium_id, i_loop, i_timestamp)
);
The primary key creates the btree index.
If I ask the database something like this:
SELECT ((max(i_timestamp) - min(i_timestamp))::double precision / <rate>)
FROM rtp_frame
WHERE i_medium_id = <medium> AND i_loop = <loop>;
it replies istantaneously.
But if i ask
DECLARE blablabla INSENSITIVE NO SCROLL CURSOR WITHOUT HOLD FOR
SELECT i_file_id, i_len, i_file_offset, i_timestamp
FROM rtp_frame WHERE i_medium_id = <medium>
AND i_loop = <loop>
AND i_timestamp BETWEEN 0 and 5400000
ORDER BY i_timestamp
on a medium with, say, 4 millions rows co-related, it takes 15 seconds to reply, even with a different clause on i_timestamp (say i_timestamp >= 0), even with the ORDER BY clause specified on the three indexed columns (ORDER BY i_medium_id, i_loop, i_timestamp).
Issued on a medium with "just" some hundred thousand rows, it runs instantaneously.
If I add a single btree index on i_timestamp, it runs instantaneously event on a medium with millions rows (so having a btree(i_medium_id, i_loop, i_timestamp) and btree(i_timestamp)).
With (btree(i_medium_id, i_loop) and btree(i_timestamp)), the first for sure takes 15 seconds to run, the second i think too but not sure atm.
can anybody explain me why this happens ? and if i should try different indexes ?
thanks a lot
Emiliano