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From Nitan Kainth <ni...@bamlabs.com>
Subject Re: Range deletes, wide partitions, and reverse iterators
Date Tue, 16 May 2017 14:29:41 GMT
Hannu,

How can you read a partition in reverse? 

Sent from my iPhone

> On May 16, 2017, at 9:20 AM, Hannu Kröger <hkroger@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 <ostefano@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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