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From "Zoltan Haindrich (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HIVE-18149) Stats: rownum estimation from datasize underestimates in most cases
Date Tue, 02 Jan 2018 16:52:00 GMT

     [ https://issues.apache.org/jira/browse/HIVE-18149?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Zoltan Haindrich updated HIVE-18149:
------------------------------------
    Labels:   (was: TODOC3.0)

Thank you [~leftylev], I've updated the docs.

> Stats: rownum estimation from datasize underestimates in most cases
> -------------------------------------------------------------------
>
>                 Key: HIVE-18149
>                 URL: https://issues.apache.org/jira/browse/HIVE-18149
>             Project: Hive
>          Issue Type: Sub-task
>          Components: Statistics
>            Reporter: Zoltan Haindrich
>            Assignee: Zoltan Haindrich
>             Fix For: 3.0.0
>
>         Attachments: HIVE-18149.01.patch, HIVE-18149.01wip01.patch, HIVE-18149.02.patch,
HIVE-18149.03.patch, HIVE-18149.03wip01.patch, HIVE-18149.03wip02.patch
>
>
> rownum estimation is based on the following fact as of now:
> * datasize being used from the following sources:
> ** basicstats aggregates the loaded "on-heap" row sizes ; other readers are able to give
"raw size" estimation - I've checked orc; but I'm sure others will do the same....api docs
are a bit vague about the methods purpose...
> ** if the basicstats level info is not available; the filesystem level "file-size-sums"
are used as the "raw data size" ; which is multiplied by the [deserialization ratio|https://github.com/apache/hive/blob/d9924ab3e285536f7e2cc15ecbea36a78c59c66d/ql/src/java/org/apache/hadoop/hive/ql/stats/StatsUtils.java#L261]
; which is currently 1.
> the problem with all of this is that deser factor is 1; and that rowsize counts in the
online object headers..
> example; 20 rows are loaded into a partition [columnstats_partlvl_dp.q|https://github.com/apache/hive/blob/d9924ab3e285536f7e2cc15ecbea36a78c59c66d/ql/src/test/queries/clientpositive/columnstats_partlvl_dp.q#L7]
> after HIVE-18108 [this explain|https://github.com/apache/hive/blob/d9924ab3e285536f7e2cc15ecbea36a78c59c66d/ql/src/test/queries/clientpositive/columnstats_partlvl_dp.q#L25]
will estimate the rowsize of the table to be 404 bytes; however the 20 rows of text is only
169 bytes...so it ends up with 0 rows...



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