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Posted to commits@cassandra.apache.org by "Nicolas Favre-Felix (JIRA)" <ji...@apache.org> on 2014/08/27 22:48:58 UTC
[jira] [Created] (CASSANDRA-7844) Fetching a single static column
requires scanning to the first live CQL row
Nicolas Favre-Felix created CASSANDRA-7844:
----------------------------------------------
Summary: Fetching a single static column requires scanning to the first live CQL row
Key: CASSANDRA-7844
URL: https://issues.apache.org/jira/browse/CASSANDRA-7844
Project: Cassandra
Issue Type: Bug
Components: Core
Reporter: Nicolas Favre-Felix
Reading a single static column seems to do more work than needed, scanning the partition until the first live CQL row before returning a value.
As I understand, static columns are stored separately from clustered rows (see CASSANDRA-6956 for an issue that arised from this storage model). Nevertheless, Cassandra doesn't optimize for the case where only static columns and partition key dimensions are retrieved.
Selecting a static column on its own is possible:
{code}
> create table friends (user text, next_id int static, friend_id int, email text, primary key(user,friend_id));
> insert into friends (user, next_id) values ('user1', 1);
> select * from friends where user = 'user1';
user | friend_id | next_id | email
-------+-----------+---------+-------
user1 | null | 1 | null
(1 rows)
{code}
Let's insert and delete some clustered data:
{code}
> insert into friends (user, next_id, friend_id, email) values ('user1', 2, 1, 'abc@foo');
> insert into friends (user, next_id, friend_id, email) values ('user1', 3, 2, 'def@foo');
> insert into friends (user, next_id, friend_id, email) values ('user1', 4, 3, 'ghi@foo');
> select * from friends where user = 'user1';
user | friend_id | next_id | email
-------+-----------+---------+---------
user1 | 1 | 4 | abc@foo
user1 | 2 | 4 | def@foo
user1 | 3 | 4 | ghi@foo
(3 rows)
> delete from friends where user = 'user1' and friend_id = 1;
> delete from friends where user = 'user1' and friend_id = 2;
> delete from friends where user = 'user1' and friend_id = 3;
{code}
And then fetch the static column again:
{code}
> TRACING ON
Now tracing requests.
> select next_id from friends where user = 'user1' limit 1;
next_id
---------
4
(1 rows)
Tracing session: 597cc970-2e27-11e4-932f-c551d8e65d14
activity | timestamp | source | source_elapsed
---------------------------------------------------------------------------+--------------+-----------+----------------
execute_cql3_query | 13:18:46,792 | 127.0.0.1 | 0
Parsing SELECT next_id from friends where user = 'user1' LIMIT 1; | 13:18:46,792 | 127.0.0.1 | 59
Preparing statement | 13:18:46,792 | 127.0.0.1 | 125
Executing single-partition query on friends | 13:18:46,792 | 127.0.0.1 | 357
Acquiring sstable references | 13:18:46,792 | 127.0.0.1 | 369
Merging memtable tombstones | 13:18:46,792 | 127.0.0.1 | 381
Skipped 0/0 non-slice-intersecting sstables, included 0 due to tombstones | 13:18:46,792 | 127.0.0.1 | 445
Merging data from memtables and 0 sstables | 13:18:46,792 | 127.0.0.1 | 460
Read 1 live and 6 tombstoned cells | 13:18:46,792 | 127.0.0.1 | 504
Request complete | 13:18:46,792 | 127.0.0.1 | 711
{code}
We went over tombstones instead of returning the static column immediately.
Is this possibly related to CASSANDRA-7085?
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