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From "Michael Shuler (JIRA)" <j...@apache.org>
Subject [jira] [Resolved] (CASSANDRA-8180) Optimize disk seek using min/max colunm name meta data when the LIMIT clause is used
Date Thu, 23 Oct 2014 23:29:34 GMT

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

Michael Shuler resolved CASSANDRA-8180.
---------------------------------------
    Resolution: Not a Problem

Querying for all the rows with the primary key {{WHERE id=1}} indeed requires scanning all
the rows. By limiting your query only to those clustering column rows that also have {{col
> 25}} is precisely the kind of query optimization that users should be doing with well
thought out schema and queries that answer efficiently. This is by design. If I'm misunderstanding
something here, please re-open and explain further :)

> 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
>            Priority: Minor
>
> 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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