If one big query doesn't cause problems

Every row you read becomes a (roughly) RF number of tasks in the cluster. If you ask for 100 rows in one query it will generate 300 tasks that are processed by the read thread pool which as a default of 32 threads. If you ask for a lot of rows and the number of nodes in low there is a chance the client starve others as they wait for all the tasks to be completed. So i tend to like asking for fewer rows. 


Aaron Morton
New Zealand

Co-Founder & Principal Consultant
Apache Cassandra Consulting

On 7/11/2013, at 12:19 pm, Dan Gould <dan@chill.com> wrote:

Thanks Nate,

I assume 10k is the return limit.  I don't think I'll ever get close to 10k matches to the IN query.  That said, you're right: to be safe I'll increase the limit to match the number of items on the IN.

I didn't know CQL supported stored procedures, but I'll take a look.  I suppose my question was asking about parsing overhead, however.  If one big query doesn't cause problems--which I assume it wouldn't since there can be multiple threads parsing and I assume C* is smart about memory when accumulating results--I'd much rather do that.


On 11/6/13 3:05 PM, Nate McCall wrote:
Unless you explicitly set a page size (i'm pretty sure the query is converted to a paging query automatically under the hood) you will get capped at the default of 10k which might get a little weird semantically. That said, you should experiment with explicit page sizes and see where it gets you (i've not tried this yet with an IN clause - would be real curious to hear how it worked). 

Another thing to consider is that it's a pretty big statement to parse every time. You might want to go the (much) smaller batch route so these can be stored procedures? (another thing I havent tried with IN clause - don't see why it would not work though).

On Wed, Nov 6, 2013 at 4:08 PM, Dan Gould <dan@chill.com> wrote:
I was wondering if anyone had a sense of performance/best practices
around the 'IN' predicate.

I have a list of up to potentially ~30k keys that I want to look up in a
table (typically queries will have <500, but I worry about the long tail).  Most
of them will not exist in the table, but, say, about 10-20% will.

Would it be best to do:

1) SELECT fields FROM table WHERE id in (uuid1, uuid2, ...... uuid30000);

2) Split into smaller batches--
for group_of_100 in all_30000:
   // ** Issue in parallel or block after each one??
   SELECT fields FROM table WHERE id in (group_of_100 uuids);

3) Something else?

My guess is that (1) is fine and that the only worry is too much data returned (which won't be a problem in this case), but I wanted to check that it's not a C* anti-pattern before.

[Conversely, is a batch insert with up to 30k items ok?]


Nate McCall
Austin, TX

Co-Founder & Sr. Technical Consultant
Apache Cassandra Consulting