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https://issues.apache.org/jira/browse/CASSANDRA-18515?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Caleb Rackliffe updated CASSANDRA-18515:
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Reviewers: Caleb Rackliffe
> Optimize Initial Concurrency Selection for Range Read Algorithm During SAI
> Queries
> ----------------------------------------------------------------------------------
>
> Key: CASSANDRA-18515
> URL: https://issues.apache.org/jira/browse/CASSANDRA-18515
> Project: Cassandra
> Issue Type: Improvement
> Components: Feature/2i Index
> Reporter: Mike Adamson
> Assignee: Mike Adamson
> Priority: Normal
>
> The range read algorithm relies on the Index API’s notion of estimated result
> rows to decide how many replicas to contact in parallel during its first
> round of requests. The more results expected from a replica for a token
> range, the fewer replicas the range read will initially try to contact. Like
> SASI, SAI floors that estimate to a huge negative number to make sure it’s
> selected over other indexes, and this floors the concurrency factor to 1. The
> actual formula looks like this:
> {code:java}
> // resultsPerRange, from SAI, is a giant negative number
> concurrencyFactor = Math.max(1, Math.min(ranges.rangeCount(), (int)
> Math.ceil(command.limits().count() / resultsPerRange)));
> {code}
> Although that concurrency factor is updated as actual results stream in, only
> sending a single range request to a single replica in every case for SAI is
> not ideal. For example, assume I have a 3 node cluster and a keyspace at
> RF=1, with 10 rows spread across the 3 nodes, without vnodes. Issuing a query
> that matches all 10 rows with a LIMIT of 10 will make 2 or 3 serial range
> requests from the coordinator, one to each of the 3 nodes.
> This can be fixed by allowing indexes to bypass the initial concurrency
> calculation allowing SAI queries to contact the entire ring in a single round
> of queries, or at worst the minimum number of rounds as bounded by the
> existing statutory maximum ranges per round.
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