[jira] [Updated] (CASSANDRA-8717) Top-k queries with custom secondary indexes

["Sam Tunnicliffe (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/CASSANDRA-8717?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Sam Tunnicliffe updated CASSANDRA-8717:
---------------------------------------
    Attachment: 8717-v5.txt

Thanks, LGTM.

I've attached a v5 with a few minor codestyle nits fixed and I've also added an additional {{Tracing#isTracing}} call in {{SIS#highestSelectivityPredicate}} as previously we'd go to the effort of building the string args even when tracing is not enabled.

[~iamaleksey], I'm +1 if you're happy to commit this.

> 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.x
>
>         Attachments: 0001-Add-support-for-top-k-queries-in-2i.patch, 0002-Add-support-for-top-k-queries-in-2i.patch, 0003-Add-support-for-top-k-queries-in-2i.patch, 0004-Add-support-for-top-k-queries-in-2i.patch, 8717-v5.txt
>
>
> 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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