On 31 October 2014 18:29, Robert Haas <robertmhaas@gmail.com> wrote:
> Suppose somebody fires off a parallel sort on a text column, or a
> parallel sequential scan involving a qual of the form textcol = 'zog'.
> We launch a bunch of workers to do comparisons; they do lookups
> against pg_collation. After some but not all of them have loaded the
> collation information they need into their local cache, the DBA types
> "cluster pg_collate". It queues up for an AccessExclusiveLock. The
> remaining parallel workers join the wait queue waiting for their
> AccessShareLock. The system is now deadlocked; the only solution is to
> move the parallel workers ahead of the AccessExclusiveLock request,
> but the deadlock detector won't do that unless it knows about the
> relationship among the parallel workers.
It's an obscure case and its not the only solution either.
I'm really surprised that having workers do their own locking doesn't
scare you. Personally, it scares me.
It's not like we're the first do parallel query. Honestly, how many
parallel databases do you think do this?
I can't see this being a practical, workable solution for running a
parallel query across a cluster of many nodes. Shared, distributed
lock table?
-- Simon Riggs http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services