On Fri, Oct 2, 2015 at 3:57 PM, Peter Geoghegan <p...@heroku.com> wrote:
> The spinlock acquisition above is actually necessary despite the
> n_writers trick, because that's only used by qtext_store().

Actually, isn't that another bug? The fact that we don't do the same
from within gc_qtexts() in normal cases, even with an exclusive lock
held? We do this:

/* Reset the shared extent pointer */
pgss->extent = extent;

I saw one really weird case on a customer database, with an enormous
although totally repetitive query text and one entry total (I
mentioned this in passing up-thread). Although I'd be willing to
believe it was just a very odd use of the database, since apparently
they were doing some kind of stress-test, perhaps it could be better
explained by a bug like this.

To recap, for other people: pg_stat_statements_internal() may do this
without any shared lock held:

/* No point in loading file now if there are active writers */
if (n_writers == 0)
    qbuffer = qtext_load_file(&qbuffer_size);

Peter Geoghegan

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