Re: views, queries, and locks

On Tue, Apr 3, 2012 at 12:16 PM, Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:
> On Tue, Apr 3, 2012 at 12:01 PM, Jon Nelson <jnelson+pgsql(at)jamponi(dot)net> wrote:
>> I have a situation that I'd like some help resolving.
>> Using PostgreSQL 8.4.<reasonably recent> on Linux, I have three things
>> coming together that cause me pain. I have a VIEW used by a bunch of
>> queries. Usually, these queries are fairly short (subsecond) but
>> sometimes they can be very long (days). I also update this view with
>> CREATE OR REPLACE VIEW every 15-30 minutes. What I observe, sometimes,
>> is this:
>>
>> 1. query A comes in. It's a big one.
>> 2. another process comes along, needs to update the view definition.
>> It issues create or replace view. It blocks on [1].
>> 3. queries B through N come in, are blocked by [2], which is blocked by [1].
>> 4. pandemonium!
>>
>> I can reduce (some) but not eliminate the need to update the view
>> multiple times a day. What might be some good ways to prevent queries
>> B through N blocking?
>>
>> Addendum: I can work around the issue by timing out and failing the
>> CREATE OR REPLACE VIEW (by canceling the query) after a short
>> duration, but is there a better way?
>
> Yeah -- this is just asking for trouble.  Why do you have to replace
> the view every 30 minutes?  Your solution is probably going to involve
> not doing that.

As I said, I can reduce the frequency, but not eliminate it. I'm
curious - if views are little more than sql macros, by the time the
query has begun to execute why is a lock still needed on the view
definition?

--
Jon