I understand how to use the WITH
clause for recursive queries (!!), but I\'m having problems understanding its general use / power.
For example the followin
WITH
lets you define "temporary tables" for use in a SELECT
query. For example, I recently wrote a query like this, to calculate changes between two sets:
-- Let o be the set of old things, and n be the set of new things.
WITH o AS (SELECT * FROM things(OLD)),
n AS (SELECT * FROM things(NEW))
-- Select both the set of things whose value changed,
-- and the set of things in the old set but not in the new set.
SELECT o.key, n.value
FROM o
LEFT JOIN n ON o.key = n.key
WHERE o.value IS DISTINCT FROM n.value
UNION ALL
-- Select the set of things in the new set but not in the old set.
SELECT n.key, n.value
FROM o
RIGHT JOIN n ON o.key = n.key
WHERE o.key IS NULL;
By defining the "tables" o
and n
at the top, I was able to avoid repeating the expressions things(OLD)
and things(NEW)
.
Sure, we could probably eliminate the UNION ALL
using a FULL JOIN
, but I wasn't able to do that in my particular case.
If I understand your query correctly, it does this:
Find the oldest row in global.prospect whose status is 'new' or 'reset'.
Mark it by adding an asterisk to its status
Return the row (including our tweak to status
).
I don't think WITH
will simplify anything in your case. It may be slightly more elegant to use a FROM
clause, though:
update global.prospect psp
set status = status || '*'
from ( select psp_id
from global.prospect
where status = 'new' or status = 'reset'
order by request_ts
limit 1
) p2
where psp.psp_id = p2.psp_id
returning psp.*;
Untested. Let me know if it works.
It's pretty much exactly what you have already, except:
This can be easily extended to update multiple rows. In your version, which uses a subquery expression, the query would fail if the subquery were changed to yield multiple rows.
I did not alias global.prospect
in the subquery, so it's a bit easier to read. Since this uses a FROM
clause, you'll get an error if you accidentally reference the table being updated.
In your version, the subquery expression is encountered for every single item. Although PostgreSQL should optimize this and only evaluate the expression once, this optimization will go away if you accidentally reference a column in psp
or add a volatile expression.