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https://issues.apache.org/jira/browse/CASSANDRA-7282?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14133122#comment-14133122
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Benedict commented on CASSANDRA-7282:
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bq. I wouldn't even call this a hashCode to avoid the confusion, perhaps an
"ordering key" or "ordering prefix"?
Creating an interface would be problematic, since we need to have our map key
be a shared type for both CSLM keys and NBHOM keys. So I'm going to stick with
the current situation.
If you meant from a purely documentation point of view, it is absolutely
essential that the value is a _hash_, otherwise performance will be O\(n\^2\),
so whilst it may be worth clarifying it is essential we call it a hashCode().
To elaborate on this in documentation, I've included the following extra comment
{quote}
This data structure essentially only works for keys that are first sorted by
some hash value (and may then be sorted
within those hashes arbitrarily), where a 32-bit prefix of the hash we sort by
is returned by hashCode()
{quote}
> Faster Memtable map
> -------------------
>
> Key: CASSANDRA-7282
> URL: https://issues.apache.org/jira/browse/CASSANDRA-7282
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Benedict
> Assignee: Benedict
> Labels: performance
> Fix For: 3.0
>
> Attachments: profile.yaml, reads.svg, run1.svg, writes.svg
>
>
> Currently we maintain a ConcurrentSkipLastMap of DecoratedKey -> Partition in
> our memtables. Maintaining this is an O(lg(n)) operation; since the vast
> majority of users use a hash partitioner, it occurs to me we could maintain a
> hybrid ordered list / hash map. The list would impose the normal order on the
> collection, but a hash index would live alongside as part of the same data
> structure, simply mapping into the list and permitting O(1) lookups and
> inserts.
> I've chosen to implement this initial version as a linked-list node per item,
> but we can optimise this in future by storing fatter nodes that permit a
> cache-line's worth of hashes to be checked at once, further reducing the
> constant factor costs for lookups.
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