Is there a similar function in postgresql for mysql's SQL_CALC_FOUND_ROWS? - postgresql

Question

everybody using mysql knows:
SELECT SQL_CALC_FOUND_ROWS ..... FROM table WHERE ... LIMIT 5, 10;
and right after run this :
SELECT FOUND_ROWS();
how do i do this in postrgesql? so far, i found only ways where i have to send the query twice...

Answer 1

No, there is not (at least not as of July 2007). I'm afraid you'll have to resort to:
BEGIN ISOLATION LEVEL SERIALIZABLE;
SELECT id, username, title, date FROM posts ORDER BY date DESC LIMIT 20;
SELECT count(id, username, title, date) AS total FROM posts;
END;
The isolation level needs to be SERIALIZABLE to ensure that the query does not see concurrent updates between the SELECT statements.
Another option you have, though, is to use a trigger to count rows as they're INSERTed or DELETEd. Suppose you have the following table:
CREATE TABLE posts (
id SERIAL PRIMARY KEY,
poster TEXT,
title TEXT,
time TIMESTAMPTZ DEFAULT now()
);
INSERT INTO posts (poster, title) VALUES ('Alice', 'Post 1');
INSERT INTO posts (poster, title) VALUES ('Bob', 'Post 2');
INSERT INTO posts (poster, title) VALUES ('Charlie', 'Post 3');
Then, perform the following to create a table called post_count that contains a running count of the number of rows in posts:
-- Don't let any new posts be added while we're setting up the counter.
BEGIN;
LOCK TABLE posts;
-- Create and initialize our post_count table.
SELECT count(*) INTO TABLE post_count FROM posts;
-- Create the trigger function.
CREATE FUNCTION post_added_or_removed() RETURNS TRIGGER AS $$
BEGIN
IF TG_OP = 'DELETE' THEN
UPDATE post_count SET count = count - 1;
ELSIF TG_OP = 'INSERT' THEN
UPDATE post_count SET count = count + 1;
END IF;
RETURN NULL;
END;
$$ LANGUAGE plpgsql;
-- Call the trigger function any time a row is inserted.
CREATE TRIGGER post_added_or_removed_tgr
AFTER INSERT OR DELETE
ON posts
FOR EACH ROW
EXECUTE PROCEDURE post_added_or_removed();
COMMIT;
Note that this maintains a running count of all of the rows in posts. To keep a running count of certain rows, you'll have to tweak it:
SELECT count(*) INTO TABLE post_count FROM posts WHERE poster <> 'Bob';
CREATE FUNCTION post_added_or_removed() RETURNS TRIGGER AS $$
BEGIN
-- The IF statements are nested because OR does not short circuit.
IF TG_OP = 'DELETE' THEN
IF OLD.poster <> 'Bob' THEN
UPDATE post_count SET count = count - 1;
END IF;
ELSIF TG_OP = 'INSERT' THEN
IF NEW.poster <> 'Bob' THEN
UPDATE post_count SET count = count + 1;
END IF;
END IF;
RETURN NULL;
END;
$$ LANGUAGE plpgsql;

Answer 2

There is a simple way, but keep in mind, that following COUNT(*) aggr function will be applied to all rows returned after where and before limit/offset (may be costy)
SELECT
id,
"count" (*) OVER () AS cnt
FROM
objects
WHERE
id > 2
OFFSET 50
LIMIT 5

Answer 3

No, PostgreSQL doesn't try to count all relevant results when you only need 10 results. You need a seperate COUNT to count all results.