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From "Tyler Hobbs (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (CASSANDRA-8180) Optimize disk seek using min/max colunm name meta data when the LIMIT clause is used
Date Wed, 04 Mar 2015 23:33:39 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-8180?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Tyler Hobbs updated CASSANDRA-8180:
-----------------------------------
    Fix Version/s: 3.0

> Optimize disk seek using min/max colunm 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: Core
>         Environment: Cassandra 2.0.10
>            Reporter: DOAN DuyHai
>            Assignee: Stefania
>            Priority: Minor
>             Fix For: 3.0
>
>
> 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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