Re: Queue in SQL

Hi Gyorgy,

Try this:

BEGIN;
SELECT * FROM table ORDER BY oid LIMIT 1 FOR UPDATE;
DELETE FROM table WHERE oid=(SELECT MIN(oid) FROM table);
COMMIT;

Few notes:

1. You might want to add an index over the oid column:
CREATE INDEX "my_idx" ON table(oid);

2. If you are executing this series from a programming language, you can
probably just change the first SELECT to "SELECT oid, * FROM ..." and just
grab out the oid and pass it is a parameter to the DELETE, rather than
having to do the aggregate subselect.

Chris

> -----Original Message-----
> From: pgsql-sql-owner(at)postgresql(dot)org
> [mailto:pgsql-sql-owner(at)postgresql(dot)org]On Behalf Of Gyorgy Molnar
> Sent: Tuesday, 27 November 2001 10:11 PM
> To: pgsql-sql(at)postgresql(dot)org
> Subject: [SQL] Queue in SQL
>
>
> Hi!
>
> Question:
> 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?
> ==================
>
> Explanation
> I intend to use libpq and C. as far as I know to retrieve the
> result from a
> query I need to do the following:
>
> BEGIN A TRANSACTION
> DECLARE CURSOR cursor FOR SELECT
> FETCH one row from cursor
> Process
> CLOSE cursor
> DELETE row
> COMMIT
>
> Let say I have 100,000 records in my table, I have to pick up
> only the first
> row. I do not have any special criteria to execute a query, and I
> only need
> the oldest added row (let say the first row if the table is indexed).
> How can I narrow the search criteria, not to receive all of the 100,000
> record in the result?
> Unfortunately, I do not know the internal working of the SELECT.
> I think it
> should create a temporary object to store the result. This object size may
> depend on the number of the rows in the result and the size of the stored
> data per row. I can fetch the rows one by one using this temporary object.
>
> Kind Regards,
> Gyorgy Molnar
>
>
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