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Re: Caching of Queries

From: "Iain" <iain(at)mst(dot)co(dot)jp>
To: "Jim C(dot) Nasby" <decibel(at)decibel(dot)org>
Cc: <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Caching of Queries
Date: 2004-09-28 02:06:17
Message-ID: 00b201c4a4ff$bd73b8b0$7201a8c0@mst1x5r347kymb (view raw or flat)
Thread:
Lists: pgsql-performance
Jim,

I can only tell you (roughly) how it works wth Oracle, and it's a very well
documented and laboured point over there - it's the cornerstone of Oracle's
scalability architecture, so if you don't believe me, or my explanation is
just plain lacking, then it wouldn't be a bad idea to check it out. The
"other Tom" aka Tomas Kyte runs the Ask Tom site which is a great source of
info on this. It's also very well explained in his book "Expert one on one
Oracle" I think it was called. I havn't seen any reason yet as to why the
same issues shouldn't, don't or  wouldn't apply to pg.

Your comment is both right and wrong. Yes, metadata lookups are essentially
the same as as access methods for normal queries. Any time you read data in
the DB you have to place a shared lock, often called a latch - it's a
lightweight type of lock. The trouble is that while a data page can have
multiple latches set at any time, only 1 process can be placing a  a latch
on a page at a time. This doesn't sound so serious so far, latches are
"lightweight" afterall, however... even in a database of a billion rows and
100+ tables, the database metadata is a very _small_ area. You must put
latches on the metadata tables to do optimization, so for example, if you
are optimizing a 10 table join, you must queue up 10 times to place your
latchs. You then do your optimization and queue up 10 more times to remove
your latches. In fact it is worse than this, because you won't queue up 10
times it's more likely to be a hundred times since it is far more complex
than 1 latch per table being optimized (you will be looking up statistics
and other things).

As I already said, even in a huge DB of a billion rows, these latches are
happening on a realatively small and concentrated data set - the metadata.
Even if there is no contention for the application data, the contention for
the metadata may be furious. Consider this scenario, you have a 1000 users
constantly submitting queries that must not only be soft parsed (SQL
statement syntax) but hard parsed (optimized) because you have no query
cache. Even if they are looking at completely different data, they'll all be
queuing up for latches on the same little patch of metadata. Doubling your
CPU speed or throwing in a fibre channel disk array will not help here, the
system smply won't scale.

Tom Lane noted that since the query cache would be in shared memory the
contention issue does not go away. This is true, but I don't think that it's
hard to see that the amount of contention is consderably less in any system
that is taking advantage of the caching facility - ie applications using
bind variables to reduce hard parsing. However, badly written applications
(from the point of view of query cache utilization) could very well
experience a degradation in performance. This could be handled with an
option to disable caching - or even better to disable caching of any sql not
using binds. I don't think even the mighty Oracle has that option.

As you may have guessed, my vote is for implementing a query cache that
includes plans.

I have no specific preference as to data caching. It doesn't seem to be so
important to me.

Regards
Iain



> On Thu, Sep 23, 2004 at 08:29:25AM -0700, Mr Pink wrote:
> > Not knowing anything about the internals of pg, I don't know how this
relates, but in theory,
> > query plan caching is not just about saving time re-planning queries,
it's about scalability.
> > Optimizing queries requires shared locks on the database metadata,
which, as I understand it
> > causes contention and serialization, which kills scalability.
>
> One of the guru's can correct me if I'm wrong here, but AFAIK metadata
> lookups use essentially the same access methods as normal queries. This
> means MVCC is used and no locking is required. Even if locks were
> required, they would be shared read locks which wouldn't block each
> other.
> -- 
> Jim C. Nasby, Database Consultant               decibel(at)decibel(dot)org
> Give your computer some brain candy! www.distributed.net Team #1828
>
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