[Impala-CR](cdh5-trunk) PREVIEW: Basic column-wise slot materialization in Parquet scanner.

["Alex Behm (Code Review)" <ge...@cloudera.org>]

Alex Behm has uploaded a new change for review.

  http://gerrit.cloudera.org:8080/2779

Change subject: PREVIEW: Basic column-wise slot materialization in Parquet scanner.
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PREVIEW: Basic column-wise slot materialization in Parquet scanner.

This change is a first step towards a more efficient Parquet scanner.
The focus is on presenting the new code flow that materializes
the table-level slots in a column-wise fashion, without going deep
into actually improving scan efficieny.

After these changes there are several obvious places that should
be optimized to realize efficiency gains.

Summary of changes
- the table-level tuples are materialized in a column-wise fashion
  with new ColumnReader::ReadValueBatch() functions
- this is done by materializing a 'scratch' batch, and transferring
  scratch tuples that survive filters/conjuncts to the output batch
- the tuples of nested collections are still materialized in
  a row-wise fashion using the ColumnReader::ReadValue() function,
  just as before

Mini benchmark
I ran the following queries on a single impalad before and after my
change using a synthetic 'huge_lineitem' table.
I modified hdfs-scan-node.cc to set the number of rows of any row
batch to 0 to focus the measurement on the scan time.

Query options:
set num_scanner_threads=1;
set disable_codegen=true;
set num_nodes=1;

select * from huge_lineitem;
Before: 22.39s
Afer:   18.50s

select * from huge_lineitem where l_linenumber < 0;
Before: 25.11s
After:  20.56s

select * from huge_lineitem where l_linenumber % 2 = 0;
Before: 26.32s
After:  21.82s

Change-Id: I72a613fa805c542e39df20588fb25c57b5f139aa
---
M be/src/exec/hdfs-parquet-scanner.cc
M be/src/exec/hdfs-parquet-scanner.h
2 files changed, 373 insertions(+), 144 deletions(-)


  git pull ssh://gerrit.cloudera.org:29418/Impala refs/changes/79/2779/1
-- 
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Gerrit-MessageType: newchange
Gerrit-Change-Id: I72a613fa805c542e39df20588fb25c57b5f139aa
Gerrit-PatchSet: 1
Gerrit-Project: Impala
Gerrit-Branch: cdh5-trunk
Gerrit-Owner: Alex Behm <al...@cloudera.com>