On Fri, Oct 12, 2012 at 4:14 PM, Martin Kersten
<Martin.Kersten@cwi.nl> wrote:
Hello Percy
Thanks for taking the MonetDB tour.
On 10/12/12 8:34 PM, Percy Wegmann wrote:
I'm very new to MonetDB and am just starting to play around with it.
One of the things I'm trying to get my head around is the concurrency
model. I've observed some things that make sense and others that I
don't understand.
1. Concurrent COPY commands into the same table will cause one or the
other to roll back. This makes sense as MonetDB seems to lock at the
table level.
The concurrency control scheme is called 'optimistic concurrency control'
See for details: http://en.wikipedia.org/wiki/Optimistic_concurrency
2. SELECTing data from a table that concurrently has a COPY command
running against it doesn't seem to produce any concurrency errors
They are isolated activities.
3. However, if I INSERT INTO table_B (SELECT ... FROM table_A) while a
COPY command is running against table_A, I do get concurrency conflicts
Yes, that is the basis of OCC.
4. Also, if I have a COPY command running into table_A and concurrently
issue DDL creating a new table_B, I get a concurrency conflict as well
Conflict on the catalog, most likely.
Observations 3+4 seem to indicate that DDL operations and INSERT ...
SELECT FROM ... use some sort of database-wide lock.
This begs the following questions:
a) Is there in fact database-wide locking going on here?
no!
b) Is there a workaround that would allow me to issue DDL and make also
internal copies of tables while concurrently loading data into other tables?
Serialise your update operations using e.g. a funnel
c) Is there something fundamental about the architecture of the database
that requires database-wide locking for these ops or is this just a
detail of the current implementation that could potentially be ironed out?
MonetDB is primarilly aimed at business analytics, which can deal with
a different concurrency control scheme.
Besides, in many instances doing concurrent bulk work not
necessarily leads to performance improvement. For example,
the COPY command already is highly parallel in the number of
cores on your machine.
regards, Martin
Thanks!
Percy
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