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From "Kurt Greaves (JIRA)" <j...@apache.org>
Subject [jira] [Assigned] (CASSANDRA-13857) Allow MV with only partition key
Date Mon, 25 Jun 2018 03:59:00 GMT

     [ https://issues.apache.org/jira/browse/CASSANDRA-13857?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Kurt Greaves reassigned CASSANDRA-13857:
----------------------------------------

    Assignee: Alexander Ivakov

> Allow MV with only partition key
> --------------------------------
>
>                 Key: CASSANDRA-13857
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13857
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Materialized Views
>            Reporter: Kurt Greaves
>            Assignee: Alexander Ivakov
>            Priority: Major
>
> We currently disallow creation of a view that has the exact same primary key as the base
where no clustering keys are present, however a potential use case would be a view where part
of the PK is filtered so as to have a subset of data in the view which is faster for range
queries. We actually currently allow this, but only if you have a clustering key defined.
If you only have a partitioning key it's not possible.
> From the mailing list, the below example works:
> {code:java}
> CREATE TABLE users (
>   site_id int,
>   user_id text,
>   n int,
>   data set<frozen<text>>,
>   PRIMARY KEY ((site_id, user_id), n));
> user data is updated and read by PK and sometimes I have to fetch all user for some specific
site_id. It appeared that full scan by token(site_id,user_id) filtered by WHERE site_id =
<some id> works much slower than unfiltered full scan on
> CREATE MATERIALIZED VIEW users_1 AS
> SELECT site_id, user_id, n, data
> FROM users
> WHERE site_id = 1 AND user_id IS NOT NULL AND n IS NOT NULL
> PRIMARY KEY ((site_id, user_id), n);
> {code}
> However the following does not:
> {code:java}
> CREATE TABLE users (
> site_id int,
> user_id text,
> data set<text>,
> PRIMARY KEY ((site_id, user_id)));
> CREATE MATERIALIZED VIEW users_1 AS
> SELECT site_id, user_id, data
> FROM users
> WHERE site_id = 1 AND user_id IS NOT NULL 
> PRIMARY KEY ((site_id, user_id));
> InvalidRequest: Error from server: code=2200 [Invalid query] message="No columns are
defined for Materialized View other than primary key"
> {code}
> This is because if the clustering key is empty we assume they've only defined the primary
key in the partition key and we haven't accounted for this use case. 
> On that note, we also don't allow the following narrowing of the partition key:
> {code}
> CREATE TABLE kurt.base (
>     id int,
>     uid text,
>     data text,
>     PRIMARY KEY (id, uid)
> ) 
> CREATE MATERIALIZED VIEW kurt.mv2 AS SELECT * from kurt.base where id IS NOT NULL and
uid='1' PRIMARY KEY ((id, uid));
> InvalidRequest: Error from server: code=2200 [Invalid query] message="No columns are
defined for Materialized View other than primary key"
> {code}
> But we do allow the following, which works because there is still a clustering key, despite
not changing the PK.
> {code}
> CREATE MATERIALIZED VIEW kurt.mv2 AS SELECT * from kurt.base where id IS NOT NULL and
uid='1' PRIMARY KEY (id, uid);
> {code}
> And we also allow the following, which is a narrowing of the partition key as above,
but with an extra clustering key.
> {code}
> create table kurt.base3 (id int, uid int, clus1 int, clus2 int, data text, PRIMARY KEY
((id, uid), clus1, clus2));
> CREATE MATERIALIZED VIEW kurt.mv4 AS SELECT * from kurt.base3 where id IS NOT NULL and
uid IS NOT NULL and clus1 IS NOT NULL AND clus2 IS NOT NULL  PRIMARY KEY ((id, uid, clus1),
clus2);
> {code}
> I _think_ supporting these cases is trivial and mostly already handled in the underlying
MV write path, so we might be able to get away with just a simple change of [this condition|https://github.com/apache/cassandra/blob/83822d12d87dcb3aaad2b1e670e57ebef4ab1c36/src/java/org/apache/cassandra/cql3/statements/CreateViewStatement.java#L291].



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