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https://issues.apache.org/jira/browse/SOLR-13399?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16895494#comment-16895494
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Yonik Seeley commented on SOLR-13399:
-------------------------------------
OK, figured out the issue...
It turns out that if you have foo!, foo!bar! will normally not nest under it.
The number of bits used for the first part of the hash is dynamic depending on
the number of levels in the composite hash ID. That's unfortunate for a number
of reasons. It also breaks the initial bi-level hash that guaranteed that you
could just add a prefix to any document id without any escaping (i.e. if your
ID happens to contain "!", it can cause the document hash to fall outside of
the parent hash prefix.)
It looks like is working as designed (according to SOLR-5320), but it was
certainly surprising since it prevents hash routing from working out-of-the-box
in conjunction with tri-level ids without explicitly specifying bits with the
"/" notation.
> compositeId support for shard splitting
> ---------------------------------------
>
> Key: SOLR-13399
> URL: https://issues.apache.org/jira/browse/SOLR-13399
> Project: Solr
> Issue Type: New Feature
> Reporter: Yonik Seeley
> Assignee: Yonik Seeley
> Priority: Major
> Fix For: 8.3
>
> Attachments: SOLR-13399.patch, SOLR-13399.patch
>
>
> Shard splitting does not currently have a way to automatically take into
> account the actual distribution (number of documents) in each hash bucket
> created by using compositeId hashing.
> We should probably add a parameter *splitByPrefix* to the *SPLITSHARD*
> command that would look at the number of docs sharing each compositeId prefix
> and use that to create roughly equal sized buckets by document count rather
> than just assuming an equal distribution across the entire hash range.
> Like normal shard splitting, we should bias against splitting within hash
> buckets unless necessary (since that leads to larger query fanout.) . Perhaps
> this warrants a parameter that would control how much of a size mismatch is
> tolerable before resorting to splitting within a bucket.
> *allowedSizeDifference*?
> To more quickly calculate the number of docs in each bucket, we could index
> the prefix in a different field. Iterating over the terms for this field
> would quickly give us the number of docs in each (i.e lucene keeps track of
> the doc count for each term already.) Perhaps the implementation could be a
> flag on the *id* field... something like *indexPrefixes* and poly-fields that
> would cause the indexing to be automatically done and alleviate having to
> pass in an additional field during indexing and during the call to
> *SPLITSHARD*. This whole part is an optimization though and could be split
> off into its own issue if desired.
>
>
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