Re: Queue in SQL

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On 2001 November 27 09:11 am, Gyorgy Molnar wrote:

> I need to store some incoming data and retrieve them one by one (LIFO).
> Different processes will manage the storage and the retrieval.
> How can I retrieve only the first row from a table?

DROP TABLE queue; DROP SEQUENCE queue_id_seq;
CREATE TABLE queue (id SERIAL UNIQUE, data TEXT);

- -- to insert into queue
INSERT INTO queue (data) VALUES ('first');
INSERT INTO queue (data) VALUES ('second');

- -- to remove from queue
BEGIN;
LOCK queue IN EXCLUSIVE MODE;
SELECT * FROM queue ORDER BY id LIMIT 1;
DELETE FROM queue WHERE id = 1;     -- use the id retrieved above
COMMIT;

Key features:
- - the SERIAL data type draws it's values from a SEQUENCE, which allows us to 
easily maintain the order of the queue.
- - by marking it UNIQUE, we have implicitly defined an index on the column, 
which will make the ORDER BY clause in the SELECT and the WHERE clause in the 
DELETE more efficient.
- - wrapping the whole thing in a transaction and using a LOCK should ensure 
correct behaviour in a concurrent situation.

- -- 
Andrew G. Hammond     mailto:drew@xyzzy.dhs.org   http://xyzzy.dhs.org/~drew/
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"To blow recursion you must first blow recur" -- me
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