Well, sstables contain some statistics about the cell timestamps and using that information and the tombstone timestamp it might be possible to skip some data but I’m not sure that Cassandra currently does that. Maybe it would be worth a JIRA ticket and see what the devs think about it. If optimizing this case would make sense.
Hannu > On 16 May 2017, at 18:03, Stefano Ortolani <ostef...@gmail.com> wrote: > > Hi Hannu, > > the piece of data in question is older. In my example the tombstone is the > newest piece of data. > Since a range tombstone has information re the clustering key ranges, and the > data is clustering key sorted, I would expect a linear scan not to be > necessary. > > On Tue, May 16, 2017 at 3:46 PM, Hannu Kröger <hkro...@gmail.com > <mailto:hkro...@gmail.com>> wrote: > Well, as mentioned, probably Cassandra doesn’t have logic and data to skip > bigger regions of deleted data based on range tombstone. If some piece of > data in a partition is newer than the tombstone, then it cannot be skipped. > Therefore some partition level statistics of cell ages would need to be kept > in the column index for the skipping and that is probably not there. > > Hannu > >> On 16 May 2017, at 17:33, Stefano Ortolani <ostef...@gmail.com >> <mailto:ostef...@gmail.com>> wrote: >> >> That is another way to see the question: are reverse iterators range >> tombstone aware? Yes. >> That is why I am puzzled by this afore-mentioned behavior. >> I would expect them to handle this case more gracefully. >> >> Cheers, >> Stefano >> >> On Tue, May 16, 2017 at 3:29 PM, Nitan Kainth <ni...@bamlabs.com >> <mailto:ni...@bamlabs.com>> wrote: >> Hannu, >> >> How can you read a partition in reverse? >> >> Sent from my iPhone >> >> > On May 16, 2017, at 9:20 AM, Hannu Kröger <hkro...@gmail.com >> > <mailto:hkro...@gmail.com>> wrote: >> > >> > Well, I’m guessing that Cassandra doesn't really know if the range >> > tombstone is useful for this or not. >> > >> > In many cases it might be that the partition contains data that is within >> > the range of the tombstone but is newer than the tombstone and therefore >> > it might be still be returned. Scanning through deleted data can be >> > avoided by reading the partition in reverse (if all the deleted data is in >> > the beginning of the partition). Eventually you will still end up reading >> > a lot of tombstones but you will get a lot of live data first and the >> > implicit query limit of 10000 probably is reached before you get to the >> > tombstones. Therefore you will get an immediate answer. >> > >> > Does it make sense? >> > >> > Hannu >> > >> >> On 16 May 2017, at 16:33, Stefano Ortolani <ostef...@gmail.com >> >> <mailto:ostef...@gmail.com>> wrote: >> >> >> >> Hi all, >> >> >> >> I am seeing inconsistencies when mixing range tombstones, wide >> >> partitions, and reverse iterators. >> >> I still have to understand if the behaviour is to be expected hence the >> >> message on the mailing list. >> >> >> >> The situation is conceptually simple. I am using a table defined as >> >> follows: >> >> >> >> CREATE TABLE test_cql.test_cf ( >> >> hash blob, >> >> timeid timeuuid, >> >> PRIMARY KEY (hash, timeid) >> >> ) WITH CLUSTERING ORDER BY (timeid ASC) >> >> AND compaction = {'class' : 'LeveledCompactionStrategy'}; >> >> >> >> I then proceed by loading 2/3GB from 3 sstables which I know contain a >> >> really wide partition (> 512 MB) for `hash = x`. I then delete the oldest >> >> _half_ of that partition by executing the query below, and restart the >> >> node: >> >> >> >> DELETE >> >> FROM test_cql.test_cf >> >> WHERE hash = x AND timeid < y; >> >> >> >> If I keep compactions disabled the following query timeouts (takes more >> >> than 10 seconds to >> >> succeed): >> >> >> >> SELECT * >> >> FROM test_cql.test_cf >> >> WHERE hash = 0x963204d451de3e611daf5e340c3594acead0eaaf >> >> ORDER BY timeid ASC; >> >> >> >> While the following returns immediately (obviously because no deleted >> >> data is ever read): >> >> >> >> SELECT * >> >> FROM test_cql.test_cf >> >> WHERE hash = 0x963204d451de3e611daf5e340c3594acead0eaaf >> >> ORDER BY timeid DESC; >> >> >> >> If I force a compaction the problem is gone, but I presume just because >> >> the data is rearranged. >> >> >> >> It seems to me that reading by ASC does not make use of the range >> >> tombstone until C* reads the >> >> last sstables (which actually contains the range tombstone and is flushed >> >> at node restart), and it wastes time reading all rows that are actually >> >> not live anymore. >> >> >> >> Is this expected? Should the range tombstone actually help in these cases? >> >> >> >> Thanks a lot! >> >> Stefano >> > >> > >> > --------------------------------------------------------------------- >> > To unsubscribe, e-mail: user-unsubscr...@cassandra.apache.org >> > <mailto:user-unsubscr...@cassandra.apache.org> >> > For additional commands, e-mail: user-h...@cassandra.apache.org >> > <mailto:user-h...@cassandra.apache.org> >> > >> > >
