Debugging shows that the bottleneck in the stats calculation is the
number of times we're querying the users table using the same array of
IDs in the `where` condition but each time combined with other
conditions.
So we're inserting the results of querying the users table with the
array of IDs in a temporary table and using this temporary table for the
other calculations. When querying this temporary table, there's no need
to filter for IDs anymore.
For budget stats, the `generate` method is now about 10-20 times faster
for a budget with 20,000 participants. For budgets with only a few dozen
participants, there's no significant difference in performance.
I thought about modifying the `participants` method and use the
temporary table there. The problem, however, is that in this case it
isn't clear when to drop the temporary table, and we could end up with
thousands of temporary tables in the database if we don't do it right.
Creating and dropping the temporary table in the same transaction, on
the other hand, guarantees that won't be the case.
Note there's no risk of duplicate tables since they're created and
dropped inside a transaction, so we're always using the same table name
for the same resource. We're adding a test that fails with a
`PG::DuplicateTable: ERROR: relation "participants__1"` error if we
don't use a transaction.
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