[jira] [Created] (HIVE-18149) Stats: rownum estimation from datasize underestimates in most cases

["Zoltan Haindrich (JIRA)" <ji...@apache.org>]

Zoltan Haindrich created HIVE-18149:
---------------------------------------

             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...




--
This message was sent by Atlassian JIRA
(v6.4.14#64029)