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https://issues.apache.org/jira/browse/CASSANDRA-11500?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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ZhaoYang updated CASSANDRA-11500:
---------------------------------
    Fix Version/s: 4.x
                   3.11.x
                   3.0.x

> Obsolete MV entry may not be properly deleted
> ---------------------------------------------
>
>                 Key: CASSANDRA-11500
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-11500
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Materialized Views
>            Reporter: Sylvain Lebresne
>            Assignee: ZhaoYang
>             Fix For: 3.0.x, 3.11.x, 4.x
>
>
> When a Materialized View uses a non-PK base table column in its PK, if an 
> update changes that column value, we add the new view entry and remove the 
> old one. When doing that removal, the current code uses the same timestamp 
> than for the liveness info of the new entry, which is the max timestamp for 
> any columns participating to the view PK. This is not correct for the 
> deletion as the old view entry could have other columns with higher timestamp 
> which won't be deleted as can easily shown by the failing of the following 
> test:
> {noformat}
> CREATE TABLE t (k int PRIMARY KEY, a int, b int);
> CREATE MATERIALIZED VIEW mv AS SELECT * FROM t WHERE k IS NOT NULL AND a IS 
> NOT NULL PRIMARY KEY (k, a);
> INSERT INTO t(k, a, b) VALUES (1, 1, 1) USING TIMESTAMP 0;
> UPDATE t USING TIMESTAMP 4 SET b = 2 WHERE k = 1;
> UPDATE t USING TIMESTAMP 2 SET a = 2 WHERE k = 1;
> SELECT * FROM mv WHERE k = 1; // This currently return 2 entries, the old 
> (invalid) and the new one
> {noformat}
> So the correct timestamp to use for the deletion is the biggest timestamp in 
> the old view entry (which we know since we read the pre-existing base row), 
> and that is what CASSANDRA-11475 does (the test above thus doesn't fail on 
> that branch).
> Unfortunately, even then we can still have problems if further updates 
> requires us to overide the old entry. Consider the following case:
> {noformat}
> CREATE TABLE t (k int PRIMARY KEY, a int, b int);
> CREATE MATERIALIZED VIEW mv AS SELECT * FROM t WHERE k IS NOT NULL AND a IS 
> NOT NULL PRIMARY KEY (k, a);
> INSERT INTO t(k, a, b) VALUES (1, 1, 1) USING TIMESTAMP 0;
> UPDATE t USING TIMESTAMP 10 SET b = 2 WHERE k = 1;
> UPDATE t USING TIMESTAMP 2 SET a = 2 WHERE k = 1; // This will delete the 
> entry for a=1 with timestamp 10
> UPDATE t USING TIMESTAMP 3 SET a = 1 WHERE k = 1; // This needs to re-insert 
> an entry for a=1 but shouldn't be deleted by the prior deletion
> UPDATE t USING TIMESTAMP 4 SET a = 2 WHERE k = 1; // ... and we can play this 
> game more than once
> UPDATE t USING TIMESTAMP 5 SET a = 1 WHERE k = 1;
> ...
> {noformat}
> In a way, this is saying that the "shadowable" deletion mechanism is not 
> general enough: we need to be able to re-insert an entry when a prior one had 
> been deleted before, but we can't rely on timestamps being strictly bigger on 
> the re-insert. In that sense, this can be though as a similar problem than 
> CASSANDRA-10965, though the solution there of a single flag is not enough 
> since we can have to replace more than once.
> I think the proper solution would be to ship enough information to always be 
> able to decide when a view deletion is shadowed. Which means that both 
> liveness info (for updates) and shadowable deletion would need to ship the 
> timestamp of any base table column that is part the view PK (so {{a}} in the 
> example below).  It's doable (and not that hard really), but it does require 
> a change to the sstable and intra-node protocol, which makes this a bit 
> painful right now.
> But I'll also note that as CASSANDRA-1096 shows, the timestamp is not even 
> enough since on equal timestamp the value can be the deciding factor. So in 
> theory we'd have to ship the value of those columns (in the case of a 
> deletion at least since we have it in the view PK for updates). That said, on 
> that last problem, my preference would be that we start prioritizing 
> CASSANDRA-6123 seriously so we don't have to care about conflicting timestamp 
> anymore, which would make this problem go away.



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