David Rorke created IMPALA-12120:
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Summary: Set appropriate output writer parallelism when using new
processing cost planner
Key: IMPALA-12120
URL: https://issues.apache.org/jira/browse/IMPALA-12120
Project: IMPALA
Issue Type: Improvement
Components: Frontend
Reporter: David Rorke
Assignee: Riza Suminto
The new processing cost based planner changes (IMPALA-11604, IMPALA-12091) will
impact output writer parallelism for insert queries, with the potential for
more small files if the processing cost based planning results in too many
writer fragments. This could further exacerbate a problem that was introduced
with mt_dop (see IMPALA-8125).
There are 2 cases to consider:
# Unpartitioned inserts where the output writer is in the same fragment as the
scan. In this case the output parallelism will be determined by the scan
parallelism which may increase (vs mt_dop) with the changes in IMPALA-12091.
# Partitioned inserts where the output writer fragment typically consists of a
sort followed by the writer, and the parallelism under IMPALA-11604 is driven
by the estimated sort cost. Again we have the potential to overparallelize
resulting in too many small files.
The MAX_FS_WRITERS query option (IMPALA-8125) can help mitigate this but we
should have better default behavior even when MAX_FS_WRITERS isn't set. The
default output writer parallelism with no query options set should avoid
creating excessive writer parallelism for both partitioned and unpartitioned
inserts. We could also consider always including an exchange (even in the
unpartitioned case) to decouple the writer from the scan parallelism.
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