Question

I have a table, Foo. I run a query on Foo to get the ids from a subset of Foo. I then want to run a more complicated set of queries, but only on those IDs. Is there an efficient way to do this? The best I can think of is creating a query such as:

SELECT ... --complicated stuff
WHERE ... --more stuff
  AND id IN (1, 2, 3, 9, 413, 4324, ..., 939393)

That is, I construct a huge “IN” clause. Is this efficient? Is there a more efficient way of doing this, or is the only way to JOIN with the inital query that gets the IDs? If it helps, I’m using SQLObject to connect to a PostgreSQL database, and I have access to the cursor that executed the query to get all the IDs.

UPDATE: I should mention that the more complicated queries all either rely on these IDs, or create more IDs to look up in the other queries. If I were to make one large query, I’d end up joining six tables at once or so, which might be too slow.

Answer 1

One technique I’ve used in the past is to put the IDs into a temp table, and then use that to drive a sequence of queries. Something like:

BEGIN;
CREATE TEMP TABLE search_result ON COMMIT DROP AS
  SELECT entity_id
  FROM entity /* long complicated search joins and conditions ... */;
-- Fetch primary entities
SELECT entity_id, entity.x /*, ... */
FROM entity JOIN search_result USING (entity_id);
-- Fetch some related entities
SELECT entity_id, related_entity_id, related_entity.x /*, ... */
FROM related_entity JOIN search_result USING (entity_id);
-- And more, as required
END;

This is particularly useful where the search result entities have multiple one-to-many relationships which you want to fetch without either a) doing N*M+1 selects or b) doing a cartesian join of related entities.