Question

I have a stream table (memory table), and a script which is inserting in this table (from 1 row/min to 100 rows/sec). I need to fetch 1000 rows every 5 seconds, i mean select top 1000 rows and then delete that selected rows.

My select query is so simple:

SELECT * FROM vdp_stream WHERE user=xxx

My problem is actually i cannot query a DELETE sql, because maybe some new rows be appended between the time that i SELECT and DELETE. Am i right? Is there any solution to fetch rows from a table?

UPDATE my table structure:

vdp_stream
---------------------
user CHAR(30)
x    INT
y    INT

Answer 1

If you are going to continue doing this using a table I would suggest using UPDATE, SELECT and DELETE statements within a transaction. Something like this perhaps –

START TRANSACTION;

UPDATE vdp_stream
SET selected = 1
ORDER BY id ASC
LIMIT 1000;

SELECT *
FROM vdp_stream
WHERE selected = 1;

DELETE
FROM vdp_stream
WHERE selected = 1;

COMMIT;

UPDATE – as the default isolation level for InnoDB is REPEATABLE READ you can omit the update and just do the SELECT and DELETE based on the same criteria –

START TRANSACTION;

SELECT *
FROM vdp_stream
ORDER BY id ASC
LIMIT 1000;

DELETE
FROM vdp_stream
ORDER BY id ASC
LIMIT 1000;

COMMIT;