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https://issues.apache.org/jira/browse/CASSANDRA-6477?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14252369#comment-14252369
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Michaël Figuière commented on CASSANDRA-6477:
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Something that hasn't been mentioned so far in this thread is client side's
Token Aware Balancing. So far when a query relies on a Secondary Index, it
doesn't have any partition key specified in its where clause which means that
the driver will fall back on the underlying balancing policy, which is DC aware
round robin by default in the DS Java Driver. That's the appropriate behavior
as there's no node that can be better than another as a coordinator in this
situation.
With Global Indexes, in order for the Driver to still be able to perform Token
Aware Balancing, it'll need to be able to figure out which index will be used,
which doesn't seems to be always trivial in the above examples, especially if
several columns and/or indexes are involved in the {{WHERE}} clause. So here we
might need to include an extra information about it in the Prepare response
message of the Native Protocol, if the indexes to be involved can be figured
out at Prepare-time.
> Global indexes
> --------------
>
> Key: CASSANDRA-6477
> URL: https://issues.apache.org/jira/browse/CASSANDRA-6477
> Project: Cassandra
> Issue Type: New Feature
> Components: API, Core
> Reporter: Jonathan Ellis
> Assignee: Carl Yeksigian
> Labels: cql
> Fix For: 3.0
>
>
> Local indexes are suitable for low-cardinality data, where spreading the
> index across the cluster is a Good Thing. However, for high-cardinality
> data, local indexes require querying most nodes in the cluster even if only a
> handful of rows is returned.
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