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From "Stefania (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-11223) Queries with LIMIT filtering on clustering columns can return less rows than expected
Date Fri, 14 Jul 2017 02:10:02 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-11223?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16086730#comment-16086730
] 

Stefania commented on CASSANDRA-11223:
--------------------------------------

bq. So, in practice changing GroupByPrefixReversed.count() has no effect.

Fine to leave it as is then, but let's add a comment explaining this, so that people don't
find it confusing later on.

bq. The limit is only used for limiting the number of rows stored in the cache for each partition
and we will only count the static row if the partition does not have any row.

Agreed.

bq. ColumnFamily.liveCQL3RowCount() is only used by ColumnFamilyStore::isFilterFullyCoveredBy
to check if the whole partition is cached. That we count the static row or not, the answer
will be correct. We could argue about ColumnFamily.liveCQL3RowCount() independently of its
current use,

You are right in that {{wholePartitionCached}} will be correct regardless and we don't need
to consider any other use cases that don't exist for 2.2. So it's fine as is, I was getting
mixed up with 3.0.

I've checked the 3.11 patch in depth, and the trunk patch with the GH diff:

* I think we can remove {{selectsFullPartitions}} from {{MultiPartitionPager}}.
* The javadoc of {{ReadQuery.selectsFullPartition()}} looks still wrong, I think the return
is the opposite of what the doc says. This applies to 3.0 as well.

There is a conflict with CASSANDRA-13482 for 3.0+ so we need to rebase and rerun CI. Afterwards,
if CI is still OK, we should be able to commit.

> Queries with LIMIT filtering on clustering columns can return less rows than expected
> -------------------------------------------------------------------------------------
>
>                 Key: CASSANDRA-11223
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-11223
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Local Write-Read Paths
>            Reporter: Benjamin Lerer
>            Assignee: Benjamin Lerer
>
> A query like {{SELECT * FROM %s WHERE b = 1 LIMIT 2 ALLOW FILTERING}} can return less
row than expected if the table has some static columns and some of the partition have no rows
matching b = 1.
> The problem can be reproduced with the following unit test:
> {code}
>     public void testFilteringOnClusteringColumnsWithLimitAndStaticColumns() throws
Throwable
>     {
>         createTable("CREATE TABLE %s (a int, b int, s int static, c int,
primary key (a, b))");
>         for (int i = 0; i < 3; i++)
>         {
>             execute("INSERT INTO %s (a, s) VALUES (?, ?)", i,
i);
>                 for (int j = 0; j < 3; j++)
>                     if (!(i == 0 && j
== 1))
>                         execute("INSERT
INTO %s (a, b, c) VALUES (?, ?, ?)", i, j, i + j);
>         }
>         assertRows(execute("SELECT * FROM %s"),
>                    row(1, 0, 1, 1),
>                    row(1, 1, 1, 2),
>                    row(1, 2, 1, 3),
>                    row(0, 0, 0, 0),
>                    row(0, 2, 0, 2),
>                    row(2, 0, 2, 2),
>                    row(2, 1, 2, 3),
>                    row(2, 2, 2, 4));
>         assertRows(execute("SELECT * FROM %s WHERE b = 1 ALLOW FILTERING"),
>                    row(1, 1, 1, 2),
>                    row(2, 1, 2, 3));
>         assertRows(execute("SELECT * FROM %s WHERE b = 1 LIMIT 2 ALLOW
FILTERING"),
>                    row(1, 1, 1, 2),
>                    row(2, 1, 2, 3)); // <--------
FAIL It returns only one row because the static row of partition 0 is counted and filtered
out in SELECT statement
>     }
> {code}



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