Guten Tag Thorsten Schöning,
am Montag, 8. Juni 2020 um 10:14 schrieben Sie:
> When the table needs to be created, when is it visible to other
> threads using the same transaction, before or after executing the
> additional query?
There is a misconception here: Multiple concurrent exec doesn't seem
to be implemented at all, even though the JDBC-docs seem to allow the
sharing of connections. Execution of statements seem to lock the
underlying connection, preventing truly concurrent access by multiple
threads.
https://stackoverflow.com/a/52798543/2055163
https://github.com/pgjdbc/pgjdbc/blob/ecbc48ff6ad293a2555465d4f7fb6a33a403af26/pgjdbc/src/main/java/org/postgresql/core/v3/QueryExecutorImpl.java#L276
https://docs.oracle.com/javadb/10.8.3.0/devguide/cdevconcepts23499.html
https://docs.oracle.com/javadb/10.8.3.0/devguide/cdevconcepts89498.html
So while things don't work as I originally expected, this makes
answering my question easy: Because "CREATE TEMP TABLE IF NOT EXISTS
[...] AS [...]" is ONE statement only forwarded to the driver, its
execution should be atomic with respect to other threads using the
same connection.
It shouldn't make any difference how long the calculation of the
associated query takes, because all other threads need to wait for the
whole statement anyway. And if some other thread comes with the query
subsequently, "IF NOT EXISTS" comes into play and the query should
succeed instantly.
> Am I correct that with using "IF NOT EXISTS" the associated query is
> only executed as well if the table needs to be created?[...]
An answer to that question would still be of interest to me.
> Am I correct that in my described setup I need to make sure on my own
> that only one thread creates each individual temporary table and
> executes the associated query?[...]
No, not unless truly concurrent access is available in the
JDBC-driver. If it would be OTOH, I guess the answer would be yes.
> Am I correct that because of the same transaction used by multiple
> threads I need to synchronize them on web service-level?[...]
Yes and this should be the most performant implementation anyway. In
case of really one and the same connection, everything is handled by
the same JVM already, so can be synchronized within that JVM without
roundtrips to Postgres as well.
[1]: https://stackoverflow.com/a/24089729/2055163
Mit freundlichen Grüßen,
Thorsten Schöning
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Thorsten Schöning E-Mail: Thorsten.Schoening@AM-SoFT.de
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