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https://issues.apache.org/jira/browse/CASSANDRA-13857?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16272230#comment-16272230
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ZhaoYang commented on CASSANDRA-13857:
--------------------------------------
Sorry for the late reply. I don't see any technical issue of supporting those
cases.
> 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
>
> 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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