[jira] [Commented] (CASSANDRA-6477) Partitioned indexes

["Jonathan Ellis (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-6477?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13955258#comment-13955258 ] 

Jonathan Ellis commented on CASSANDRA-6477:
-------------------------------------------

bq. The problem is that this means we can't do lazy updates of the index; we need to keep the index perfectly (or, "eventually perfectly") in sync with the base table.

To clarify: Suppose you have you index on the age of users, and we have an entry for {{24: user1}} in the index table.  Now two threads update user1's age; one to 25, and one to 26.  Each thread will

# Read existing age
# Delete index entry for existing age
# Update user record and insert index entry for new age

The problem is if each thread reads the existing age of 24, then we'll end up with both {{25: user1}} and {{26: user1} index entries.  (Atomic batches do not help with this.)  With normal indexes, we clean up stale entries at compaction + read time; we could still do this here but the performance penalty is a lot higher.

Sylvain had another idea.



> Partitioned 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
>             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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