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From "Stefania (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8180) Optimize disk seek using min/max column name meta data when the LIMIT clause is used
Date Wed, 27 Jan 2016 08:56:40 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8180?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15118892#comment-15118892
] 

Stefania commented on CASSANDRA-8180:
-------------------------------------

The latest commit is ready for review [here|https://github.com/stef1927/cassandra/commit/fb1a4d93dc101143eb3d4062d577e643e8824c73].
CI is still pending however.

bq. Isn't the static row is outside the iteration?

That's a very good point, thanks for pointing this out. So we don't need to worry about this.

bq. So we can't really trust hasTombstones()? This may be a significant bug.

This method was introduced in this patch but currently the same logic is used in {{SinglePartitionReadCommand.queryMemtableAndDiskInternal()}}
and {{queryMemtableAndSSTablesInTimestampOrder}} to determine which non overlapping sstables
contain no tombstones at all, see especially the comment in the second method:

{code}
        // This mean that nothing queried by the filter can be in the sstable. One exception
is the top-level partition deletion
        // however: if it is set, it impacts everything and must be included. Getting that
top-level partition deletion costs us
        // some seek in general however (unless the partition is indexed and is in the key
cache), so we first check if the sstable
        // has any tombstone at all as a shortcut.
        if (sstable.getSSTableMetadata().maxLocalDeletionTime == Integer.MAX_VALUE)
            continue; // Means no tombstone at all, we can skip that sstable
{code}

The trouble is that in other places we assume the opposite, for example {{CompactionController.getFullyExpiredSSTables()}}
requires {{maxDeletionTime}} to be {{Integer.MAX_VALUE}} in the presence of live cells or
else it might expire the sstable. I think the code in {{SinglePartitionReadCommand}} is wrong
and we should have looked at the {{minDeletionTime}} instead, so I've updated the new method
accordingly.

I've had some failures with upgrade dtests, due to the clustering values of old sstables ("jb"
format) containing more values than the comparator types. I've added a check on {{Version.hasNewStatsFile()}}
to prevent this and some assertions to spot it sooner, and this seems to have fixed the failing
tests.

To wrap up the discussion on upgrading sstable stats, this would allow applying this optimization
also to sstables with tombstones and that are smaller than {{DatabaseDescriptor.column_index_size_in_kb}},
64kb by default. For larger sstables with tombstones, the lower bound comes from the partition
index and this will be read anyway in the presence of tombstones when {{partitionDeletion()}}
is called. This requires also initializing the iterator, which will cause a disk seek to the
partition start if the partition index is not available (sstables smaller than 64kb) or there
are static columns in the query, see the {{AbstractSSTableIterator}} constructor. 

Therefore I don't see the point in upgrading the stats, but should we wish to do it, here
is the work required:
- upgrade the sstable version from "ma" to "mb" (unsure what this entails exactly, other than
changing {{BigVersion}} and {{BigFormat}})
- introduce a serializer for clustering prefixes that is able to distinguish between clusterings
and bounds by using functionality similar to what's in {{UnfilteredSerializer}}
- pass the clustering types, or ideally the {{SerializationHeader}}, to the {{StatsMetadata}}
serialize and deserialize methods - or alternatively add the min and max clustering prefixes
to {{SerializationHeader.Component}}, which is also a metadata component. However in this
case we'd have to do some more work to pass to it the min and max clustering prefixes, which
can instead be easily collected for {{StatsMetadata}} by using the existing method {{MetadataCollector.updateClusteringValues()}}.
I suppose they could also go into {{EncodingStats}}, which is serialized in the header, but
I am not sure if this would increase the size of partitions excessively because it seems we
also use {{EncodingStats}} for partitions.

> Optimize disk seek using min/max column name meta data when the LIMIT clause is used
> ------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-8180
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8180
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Local Write-Read Paths
>         Environment: Cassandra 2.0.10
>            Reporter: DOAN DuyHai
>            Assignee: Stefania
>            Priority: Minor
>             Fix For: 3.x
>
>         Attachments: 8180_001.yaml, 8180_002.yaml
>
>
> I was working on an example of sensor data table (timeseries) and face a use case where
C* does not optimize read on disk.
> {code}
> cqlsh:test> CREATE TABLE test(id int, col int, val text, PRIMARY KEY(id,col)) WITH
CLUSTERING ORDER BY (col DESC);
> cqlsh:test> INSERT INTO test(id, col , val ) VALUES ( 1, 10, '10');
> ...
> >nodetool flush test test
> ...
> cqlsh:test> INSERT INTO test(id, col , val ) VALUES ( 1, 20, '20');
> ...
> >nodetool flush test test
> ...
> cqlsh:test> INSERT INTO test(id, col , val ) VALUES ( 1, 30, '30');
> ...
> >nodetool flush test test
> {code}
> After that, I activate request tracing:
> {code}
> cqlsh:test> SELECT * FROM test WHERE id=1 LIMIT 1;
>  activity                                                                  | timestamp
   | source    | source_elapsed
> ---------------------------------------------------------------------------+--------------+-----------+----------------
>                                                         execute_cql3_query | 23:48:46,498
| 127.0.0.1 |              0
>                             Parsing SELECT * FROM test WHERE id=1 LIMIT 1; | 23:48:46,498
| 127.0.0.1 |             74
>                                                        Preparing statement | 23:48:46,499
| 127.0.0.1 |            253
>                                   Executing single-partition query on test | 23:48:46,499
| 127.0.0.1 |            930
>                                               Acquiring sstable references | 23:48:46,499
| 127.0.0.1 |            943
>                                                Merging memtable tombstones | 23:48:46,499
| 127.0.0.1 |           1032
>                                                Key cache hit for sstable 3 | 23:48:46,500
| 127.0.0.1 |           1160
>                                Seeking to partition beginning in data file | 23:48:46,500
| 127.0.0.1 |           1173
>                                                Key cache hit for sstable 2 | 23:48:46,500
| 127.0.0.1 |           1889
>                                Seeking to partition beginning in data file | 23:48:46,500
| 127.0.0.1 |           1901
>                                                Key cache hit for sstable 1 | 23:48:46,501
| 127.0.0.1 |           2373
>                                Seeking to partition beginning in data file | 23:48:46,501
| 127.0.0.1 |           2384
>  Skipped 0/3 non-slice-intersecting sstables, included 0 due to tombstones | 23:48:46,501
| 127.0.0.1 |           2768
>                                 Merging data from memtables and 3 sstables | 23:48:46,501
| 127.0.0.1 |           2784
>                                         Read 2 live and 0 tombstoned cells | 23:48:46,501
| 127.0.0.1 |           2976
>                                                           Request complete | 23:48:46,501
| 127.0.0.1 |           3551
> {code}
> We can clearly see that C* hits 3 SSTables on disk instead of just one, although it has
the min/max column meta data to decide which SSTable contains the most recent data.
> Funny enough, if we add a clause on the clustering column to the select, this time C*
optimizes the read path:
> {code}
> cqlsh:test> SELECT * FROM test WHERE id=1 AND col > 25 LIMIT 1;
>  activity                                                                  | timestamp
   | source    | source_elapsed
> ---------------------------------------------------------------------------+--------------+-----------+----------------
>                                                         execute_cql3_query | 23:52:31,888
| 127.0.0.1 |              0
>                Parsing SELECT * FROM test WHERE id=1 AND col > 25 LIMIT 1; | 23:52:31,888
| 127.0.0.1 |             60
>                                                        Preparing statement | 23:52:31,888
| 127.0.0.1 |            277
>                                   Executing single-partition query on test | 23:52:31,889
| 127.0.0.1 |            961
>                                               Acquiring sstable references | 23:52:31,889
| 127.0.0.1 |            971
>                                                Merging memtable tombstones | 23:52:31,889
| 127.0.0.1 |           1020
>                                                Key cache hit for sstable 3 | 23:52:31,889
| 127.0.0.1 |           1108
>                                Seeking to partition beginning in data file | 23:52:31,889
| 127.0.0.1 |           1117
>  Skipped 2/3 non-slice-intersecting sstables, included 0 due to tombstones | 23:52:31,889
| 127.0.0.1 |           1611
>                                 Merging data from memtables and 1 sstables | 23:52:31,890
| 127.0.0.1 |           1624
>                                         Read 1 live and 0 tombstoned cells | 23:52:31,890
| 127.0.0.1 |           1700
>                                                           Request complete | 23:52:31,890
| 127.0.0.1 |           2140
> {code}



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