Impala Public Jenkins has submitted this change and it was merged. ( 
http://gerrit.cloudera.org:8080/12636 )

Change subject: IMPALA-8257: Parquet writer sometimes hits DCHECK when handling 
empty string
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IMPALA-8257: Parquet writer sometimes hits DCHECK when handling empty string

We had a too rigorous DCHECK in the code of
ColumnStats<StringValue>::Merge(). The DCHECK makes sure that we copy
the StringValues into their own buffer from the RowBatch memory.
Otherwise their value can be overwritten by following row batches.

The internal pointer of empty StringValues are NULL, so there is no
need to copy them to another buffer, therefore the DCHECKs are
unnecessary and moreover, they can result in crashes.

Now we only evaluate the DCHECKs when the corresponding StringValues
are not empty strings.

Testing:
I added an e2e test that inserts a lot of empty strings into a table.

Change-Id: I934b53c17720e41231e4d614fbc70f1937e19289
Reviewed-on: http://gerrit.cloudera.org:8080/12636
Reviewed-by: Impala Public Jenkins <[email protected]>
Tested-by: Impala Public Jenkins <[email protected]>
---
M be/src/exec/parquet/parquet-column-stats.inline.h
M testdata/workloads/tpch/queries/insert_parquet.test
2 files changed, 19 insertions(+), 5 deletions(-)

Approvals:
  Impala Public Jenkins: Looks good to me, approved; Verified

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Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-MessageType: merged
Gerrit-Change-Id: I934b53c17720e41231e4d614fbc70f1937e19289
Gerrit-Change-Number: 12636
Gerrit-PatchSet: 3
Gerrit-Owner: Zoltan Borok-Nagy <[email protected]>
Gerrit-Reviewer: Impala Public Jenkins <[email protected]>
Gerrit-Reviewer: Tim Armstrong <[email protected]>

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