Zoltan Haindrich created HIVE-18149:
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Summary: 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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