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https://issues.apache.org/jira/browse/HIVE-18149?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16266711#comment-16266711
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Zoltan Haindrich commented on HIVE-18149:
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I think setting {{hive.stats.deserialization.factor}} to about {{10.0}} might
possibly yield more realistic estimates... ; for the above example it would
estimate 4 rows which is much better than zero rows....
> 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
> Reporter: Zoltan Haindrich
>
> 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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