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https://issues.apache.org/jira/browse/CASSANDRA-4536?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13700642#comment-13700642
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Sylvain Lebresne commented on CASSANDRA-4536:
---------------------------------------------
The main missing part I think is that we should handle composite partition
keys. Meaning that we should allow DISTINCT only if it's on *all* the (CQL3)
partition key columns, but there may be more than one.
Also, in SelectStatement.process(), it seems we only allow CQL3 tables. Why not
just move the isDistinct block at the beginning to include all cases?
Other minor remarks/nits:
* In the parser, I'd rather just have K_DISTINCT optional in front of normal
selectClause and do validation later in SelectStatement, rather than having a
special selectDistinctClause (partly to keep the parser simpler, but also
because we can return better error messages that way). We'd need to support
distinct on multiple columns for composite partition keys anyway.
* In makeFilter(): there's a ColumnSlice.ALL_COLUMNS_ARRAY to shorten that
further. Also, we don't care about reversed, so since reversed slice are
slightly slower, let's never reverse.
> Ability for CQL3 to list partition keys
> ---------------------------------------
>
> Key: CASSANDRA-4536
> URL: https://issues.apache.org/jira/browse/CASSANDRA-4536
> Project: Cassandra
> Issue Type: New Feature
> Components: API
> Affects Versions: 1.1.0
> Reporter: Jonathan Ellis
> Assignee: dan jatnieks
> Priority: Minor
> Labels: cql3
> Fix For: 1.2.7
>
> Attachments: cassandra-4536_1.1.0.patch, cassandra-4536_1.2.2.patch,
> cassandra-4536_1.2.5.patch
>
>
> It can be useful to know the set of in-use partition keys (storage engine row
> keys). One example given to me was where application data was modeled as a
> few 10s of 1000s of wide rows, where the app required presenting these rows
> to the user sorted based on information in the partition key. The partition
> count is small enough to do the sort client-side in memory, which is what the
> app did with the Thrift API--a range slice with an empty columns list.
> This was a problem when migrating to CQL3. {{SELECT mykey FROM mytable}}
> includes all the logical rows, which makes the resultset too large to make
> this a reasonable approach, even with paging.
> One way to add support would be to allow DISTINCT in the special case of
> {{SELECT DISTINCT mykey FROM mytable}}.
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