Re: [Xen-devel] [HACKERS] [OSSTEST PATCH 0/1] PostgreSQL db: Retry on constraint violation [and 2 more messages] [and 1 more messages]

[Kevin Grittner <kgrittn@xxxxxxxxx>]

On Thu, Dec 15, 2016 at 9:53 AM, Ian Jackson <ian.jackson@xxxxxxxxxxxxx> wrote:

> I don't think "set max_pred_locks_per_transaction generously" is a
> practical thing to write in the documentation, because the application
> programmer, or admin, has no sensible way to calculate what a
> sufficiently generous value is.

ok

> You seem to be implying that code relying on the summarised data might
> make over-optimistic decisions.  That seems dangerous to me, but (with
> my very dim view of database innards) I can't immediately see how to
> demonstrate that it must in any case be excluded.

No, with any of these conditions, the information on which a
decision to generate a serialization failure is summarized into
something less granular, and in all cases we turn any "in doubt"
situations into serialization failures; that is, you can get false
positives (a serialization failure exception where complete
information could have avoided it) but not false negatives (a
serialization anomaly appearing in the database or query results
from a transaction which commits).  Based on that alone, I think it
is fair to say that it does not weaken guarantees about
serialization failures for read-only transactions not being
possible on commit unless the initial exception is suppressed in a
subtransaction nor that anomalous results are not possible in a
read-only transaction.  The granularity promotion of predicate
locks could not affect the guarantees about never seeing a
serialization failure on the first statement that reads data in a
read-only transaction, but I would need to take a very close look
at how the SLRU summarization of committed transactions might
affect that one -- we lose some of the detail about the relative
order of the commits and snapshot acquisitions, and that might be
enough to allow a false positive on that first statement.  That
would require more study than I can give it this month.

I do remember that Dan ran some saturation workloads to stress this
feature for days and weeks at a time pushing things to the point of
using the SLRU summarization, and I remember thinking it odd that
certain tests generated zero errors on the read-only transactions,
which I'm pretty sure were single-statement transactions.  It was
only during this week's discussion that I had the epiphany about
that only being possible if the read-only transaction had multiple
statements; but the fact that such long saturation runs with SLRU
summarization showed no errors on read-only transactions supports
the idea that such summarization doesn't compromise that guarantee.
Unfortunately, it falls short of proof.  :-(

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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