[IMPALA-4335] fetch calls may now produce empty row batches

• Type: Bug
• Status: Resolved
• Priority: Critical
• Resolution: Fixed
• Affects Version/s: Impala 2.8.0
• Fix Version/s: Impala 2.8.0
• Component/s: Distributed Exec
• Labels: query_generator
• Target Version: Impala 2.8.0
• Assignee: Henry Robinson
• Reporter: Matthew Jacobs
• Created: 21/Oct/16 17:15
• Updated: 25/Oct/16 18:15
• Resolved: 22/Oct/16 21:33

Description

After recent changes (likely IMPALA-2905), Impala may return empty row batches on fetch() calls. This shouldn't be a breaking issue for well written clients, but misbehaving clients may break. (e.g. Impyla isn't handling this correctly, I have a patch for it.)

Also this results in extra, unnecessary RPCs. We should make sure the server is skipping empty row batches.

I'm marking this as 'distributed exec' rather than 'client' because I think it's related to the recent coordinator fragment changes.

This is easy to reproduce with a simple query. Executed through impyla results in 0 rows because the first row batch is empty.

#!/usr/bin/env impala-python

import impala.dbapi
with impala.dbapi.connect(host='127.0.0.1', port=21050) as conn:
  with conn.cursor() as cursor:
    cursor.execute('select * from tpch_kudu.region')
    kudu_results = cursor.fetchall()
    for row in kudu_results:
      print row