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From David Savage <davemssav...@gmail.com>
Subject CQL Select Map using an IN relationship
Date Thu, 13 Mar 2014 11:12:29 GMT
Hi there,

I'm experimenting using cassandra and have run across an error message
which I need a little more information on.

The use case I'm experimenting with is a series of document updates
(documents being an arbitrary map of key value pairs), I would like to find
the latest document updates after a specified time period. I don't want to
store many copies of the documents (one per update) as the updates are
often only to single keys in the map so that would involve a lot of
duplicated data.

The solution I've found that seems to fit best in terms of performance is
to have two tables.

One that has an event log of timeuuid -> docid and a second that stores the
documents themselves stored by docid -> map<string, string>. I then run two
queries, one to select ids that have changed after a certain time:

SELECT id FROM eventlog WHERE timestamp>=minTimeuuid($minimumTime)

and then a second to select the actual documents themselves

SELECT id, data FROM documents WHERE id IN (0, 1, 2, 3, 4, 5, 6, 7...)

However this then explodes on query with the error message:

"Cannot restrict PRIMARY KEY part id by IN relation as a collection is
selected by the query"

Detective work lead me to these lines in
org.apache.cassandra.cql3.statementsSelectStatement:

                    // We only support IN for the last name and for compact
storage so far
                    // TODO: #3885 allows us to extend to non compact as
well, but that remains to be done
                    if (i != stmt.columnRestrictions.length - 1)
                        throw new
InvalidRequestException(String.format("PRIMARY KEY part %s cannot be
restricted by IN relation", cname));
                    else if (stmt.selectACollection())
                        throw new
InvalidRequestException(String.format("Cannot restrict PRIMARY KEY part %s
by IN relation as a collection is selected by the query", cname));

It seems like #3885 will allow support for the first IF block above, but I
don't think it will allow the second, am I correct?

Any pointers on how I can work around this would be greatly appreciated.

Kind regards,

Dave

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