GitHub user jianqiao opened a pull request:

    https://github.com/apache/incubator-quickstep/pull/239

    Add ThreadPrivateCompactKeyHashTable for aggregation.

    This PR implements a new specialized aggregation hash table that is 
preferable for two-phase (i.e. aggregate locally and then merge) aggregation 
with small-cardinality group-by keys. To use this hash table, it also requires 
that the group-by keys have fixed-length types with total byte size no greater 
than 8 (so that each composite key can be packed into a 64-bit QWORD).
    
    This PR together with #237 can improve the performance TPC-H Q01 from 
~11.5s to ~5.3s, with scale factor 100 on a cloud lab machine.

You can merge this pull request into a Git repository by running:

    $ git pull https://github.com/jianqiao/incubator-quickstep 
thread-private-compact-ht

Alternatively you can review and apply these changes as the patch at:

    https://github.com/apache/incubator-quickstep/pull/239.patch

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

    This closes #239
    
----
commit 99af4690ae4462e7b6843bc92c4cb3ef31a1d75a
Author: Jianqiao Zhu <jianq...@cs.wisc.edu>
Date:   2017-04-22T04:23:13Z

    Add ThreadPrivateCompactKeyHashTable as a fast path data structure for 
aggregation.

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