cubicweb #481715 [multisource] optimize usage of temporary tables [deprecated]

getting data from multiple sources leads to the creation of a temporary table. Those tables are costly to create and should be avoided if possible or at least created only once and not inside every transaction.

Here is a thread about cost of temporary tables on postgresql :

Which is summarized by this sentence :

As I learned today, creating a temporary table in PostgreSQL can easily create four files and do dozens of updates to system tables; that's all before you start actually inserting any data into the temporary table.

We could use ON COMMIT DELETE ROWS if the table are created only once per session.

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