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https://issues.apache.org/jira/browse/CASSANDRA-8717?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14482154#comment-14482154
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Evan Volgas commented on CASSANDRA-8717:
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The ability to drop in a custom indexer would be a game changer. I agree with
[~slebresne]'s point about "sort" being the wrong name for this method. I also
really like the idea of hijacking the 2i in the short run and revisiting it
later to consider a more generalized version of transforming the result set.
As far as 2.1 vs 3.0... this is such a big win that I'd hate to see it pushed
too far back into later. Is it maybe still possible to try and get this into
the 2.1 line?
> Top-k queries with custom secondary indexes
> -------------------------------------------
>
> Key: CASSANDRA-8717
> URL: https://issues.apache.org/jira/browse/CASSANDRA-8717
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Andrés de la Peña
> Assignee: Andrés de la Peña
> Priority: Minor
> Labels: 2i, secondary_index, sort, sorting, top-k
> Fix For: 3.0
>
> Attachments: 0001-Add-support-for-top-k-queries-in-2i.patch
>
>
> As presented in [Cassandra Summit Europe
> 2014|https://www.youtube.com/watch?v=Hg5s-hXy_-M], secondary indexes can be
> modified to support general top-k queries with minimum changes in Cassandra
> codebase. This way, custom 2i implementations could provide relevance search,
> sorting by columns, etc.
> Top-k queries retrieve the k best results for a certain query. That implies
> querying the k best rows in each token range and then sort them in order to
> obtain the k globally best rows.
> For doing that, we propose two additional methods in class
> SecondaryIndexSearcher:
> {code:java}
> public boolean requiresFullScan(List<IndexExpression> clause)
> {
> return false;
> }
> public List<Row> sort(List<IndexExpression> clause, List<Row> rows)
> {
> return rows;
> }
> {code}
> The first one indicates if a query performed in the index requires querying
> all the nodes in the ring. It is necessary in top-k queries because we do not
> know which node are the best results. The second method specifies how to sort
> all the partial node results according to the query.
> Then we add two similar methods to the class AbstractRangeCommand:
> {code:java}
> this.searcher =
> Keyspace.open(keyspace).getColumnFamilyStore(columnFamily).indexManager.searcher(rowFilter);
> public boolean requiresFullScan() {
> return searcher == null ? false : searcher.requiresFullScan(rowFilter);
> }
> public List<Row> combine(List<Row> rows)
> {
> return searcher == null ? trim(rows) : trim(searcher.sort(rowFilter,
> rows));
> }
> {code}
> Finnally, we modify StorageProxy#getRangeSlice to use the previous method, as
> shown in the attached patch.
> We think that the proposed approach provides very useful functionality with
> minimum impact in current codebase.
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