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https://issues.apache.org/jira/browse/KYLIN-1186?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15075657#comment-15075657
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liyang commented on KYLIN-1186:
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- FunctionDesc.getParameter().getColRefs() will return any columns if they are
the function's parameter.
- Note the new measure must count another column (not SELLER_ID), or the query
engine may get confused.
- maxLength() is needed to allocate temp buffer to hold a row. 1.x-staging
should use this info sooner or later if it hasn't yet.
- peekLength() is used to determine the length of a byte chunk which is
previously written by calling serialize(). The current position of input buffer
is guaranteed to be at the beginning of the serialization.
> Support precise Count Distinct using bitmap
> -------------------------------------------
>
> Key: KYLIN-1186
> URL: https://issues.apache.org/jira/browse/KYLIN-1186
> Project: Kylin
> Issue Type: Improvement
> Components: Job Engine
> Affects Versions: v1.1
> Reporter: Yerui Sun
> Assignee: Yerui Sun
> Fix For: v2.0, v1.3
>
> Attachments: KYLIN-1186-1.x-staging.patch
>
> Original Estimate: 168h
> Remaining Estimate: 168h
>
> For now, kylin only support non-precise count distinct by hyperloglog.
> In our production scenario, there're strongly requirements for precise count
> distinct, mainly for the column of type int or bigint, such as user-id,
> product-id, etc.
> Implementing of precise count distinct for all types is difficult and not
> efficiency. However, only supporting int or bigint make this much easier. The
> values can be projected into a bitmap, which is easy to be compressed and
> stored, and easy to count.
> I've created a POC based on RoaringBitmap, proving that worked. There's some
> more work to be done:
> * RoaringBitmap only support int, there need a solution to support bigint;
> * Add a new measure and codec, like HyperLogLogPlusCounter, make it easy to
> use;
> * Add new measure on web ui, and check that whether the column type is int
> or bigint;
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