Thank you all for your insights. When spark-connector adds allows filtering to a query, it makes the query to just ‘run’ no matter if it is expensive for larger table OR not so expensive for table with fewer rows. In my particular case, nodes are reaching 2TB/per node load in 50 node cluster. When bunch of such queries run , causes impact on server resources.
Since allow filtering is an expensive operation - I’m trying find knobs which if I turn, mitigate the impact. What I think , correct me if I am wrong , is – it is query design itself which is not optimized per table design - that in turn causing connector to add allow filtering implicitly. I’m not thinking to add secondary indexes on tables because they’ve their own overheads. kindly share if there are other means which we can use to influence connector not to use allow filtering. Thanks again. Asad From: Jeff Jirsa [mailto:jji...@gmail.com] Sent: Thursday, July 25, 2019 10:24 AM To: cassandra <user@cassandra.apache.org> Subject: Re: Performance impact with ALLOW FILTERING clause. "unpredictable" is such a loaded word. It's quite predictable, but it's often mispredicted by users. "ALLOW FILTERING" basically tells the database you're going to do a query that will require scanning a bunch of data to return some subset of it, and you're not able to provide a WHERE clause that's sufficiently fine grained to avoid the scan. It's a loose equivalent of doing a full table scan in SQL databases - sometimes it's a valid use case, but it's expensive, you're ignoring all of the indexes, and you're going to do a lot more work. It's predictable, though - you're probably going to walk over some range of data. Spark is grabbing all of the data to load into RDDs, and it probably does it by slicing up the range, doing a bunch of range scans. It's doing that so it can get ALL of the data and do the filtering / joining / searching in-memory in spark, rather than relying on cassandra to do the scanning/searching on disk. On Thu, Jul 25, 2019 at 6:49 AM ZAIDI, ASAD A <az1...@att.com<mailto:az1...@att.com>> wrote: Hello Folks, I was going thru documentation and saw at many places saying ALLOW FILTERING causes performance unpredictability. Our developers says ALLOW FILTERING clause is implicitly added on bunch of queries by spark-Cassandra connector and they cannot control it; however at the same time we see unpredictability in application performance – just as documentation says. I’m trying to understand why would a connector add a clause in query when this can cause negative impact on database/application performance. Is that data model that is driving connector make its decision and add allow filtering to query automatically or if there are other reason this clause is added to the code. I’m not a developer though I want to know why developer don’t have any control on this to happen. I’ll appreciate your guidance here. Thanks Asad
