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https://issues.apache.org/jira/browse/IGNITE-19935?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17742354#comment-17742354
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Pavel Tupitsyn commented on IGNITE-19935:
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The first change to take advantage of the custom order hash combination:
IGNITE-19914
> Allow combining individual colocation hashes
> --------------------------------------------
>
> Key: IGNITE-19935
> URL: https://issues.apache.org/jira/browse/IGNITE-19935
> Project: Ignite
> Issue Type: Improvement
> Affects Versions: 3.0.0-beta1
> Reporter: Pavel Tupitsyn
> Assignee: Pavel Tupitsyn
> Priority: Major
> Labels: ignite-3
> Fix For: 3.0.0-beta2
>
> Time Spent: 50m
> Remaining Estimate: 0h
>
> A hash of two columns currently is
> {code}
> var coloHash = hash(col1, hash(col2, 0));
> {code}
> which means we need to access the *value of col1* after we compute the *hash
> of col2*. In other words, there is no way to calculate individual column
> hashes and combine them later.
> In practice this means we can't combine serialization and hashing when
> colocation column order is different from serialization order. As a result,
> we have to do the same work twice:
> 1. Extract colocation column values (from tuple or POJO) and compute hash
> 2. Extract all column values (from tuple or POJO) and serialize
> This is especially problematic for String, BigInteger and BitMask where we
> have to allocate an array and copy some bytes there.
> *Solution*
> Provide a way to combine hashes:
> {code}
> var coloHash = hashCombine(hash(col2), hash(col1))
> {code}
> This will allow computing individual column hashes and combining them later
> in any order.
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