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> 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
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