What's the meaning of the SQL?

Question

I found a sql like the following statement, I can't figure out why it uses "WHERE t2.id = t1.id + 0", has the "+0" have a special purpose? The result is the same when use "WHERE t2.id = t1.id".
SELECT count(*)
FROM t1
WHERE t1.status = 1
AND EXISTS (SELECT 1
FROM t2
WHERE t2.id = t1.id + 0
);

Answer 1

Hi,
This was a trick done in the old rule based optimiser (RBO) days to force the query not to use an index on a particular column. An index wont be used if an indexed column has a function on it and adding 0 to a number or appending null to a varchar was the easiest way to force this. These days with the cost based optimiser (CBO), the same still applies with a function on a column, but the optimiser is now smart enough to make its own mind up whether an index should be used or not.
My recommendation these days, provided you are using the CBO, is to remove these functions and let the optimiser make up its own mind. If in the rare case you still believe you know better than the optimiser, then you should use hints or stored outlines.
Andre