OK - I see. And to add insult to injury, I tried creating a temporary table to store the intermediate results. Then I was going to just do an insert... select... to insert the rows. That would de-couple the nextval() from the query.
Strangely, the first query I tried it on worked great. But, when I tried to add a second set of data with a similar query to the same temporary table, it slowed right down again. And, of course, when I remove the insert, it's fine.
I am not entirely sure I am understanding your process properly, but just a note- If you are getting acceptable results creating the temp table, and the issue is just that you get very bad plans when using it in some query that follows, then it is worth noting that autovacuum does nothing on temp tables and for me it is nearly always worth the small cost to perform an analyze (at least on key fields) after creating a temp table, or rather after inserting/updating/deleting records in a significant way.
