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From Jörn Franke <jornfra...@gmail.com>
Subject Re: The build-in indexes in ORC file does not work.
Date Wed, 16 Mar 2016 11:26:22 GMT
How much data are you querying? What is the query? How selective it is supposed to be? What
is the block size?

> On 16 Mar 2016, at 11:23, Joseph <wxy810xl@sina.com> wrote:
> 
> Hi all,
> 
> I have known that ORC provides three level of indexes within each file, file level, stripe
level, and row level. 
> The file and stripe level statistics are in the file footer so that they are easy to
access to determine if the rest of the file needs to be read at all. 
> Row level indexes include both column statistics for each row group and position for
seeking to the start of the row group. 
> 
> The following is my understanding:
> 1. The file and stripe level indexes are forcibly generated, we can not control them.
> 2. The row level indexes can be configured by "orc.create.index"(whether to create row
indexes) and "orc.row.index.stride"(number of rows between index entries).
> 3. Each Index has statistics of min, max for each column, so sort data by the filter
column will bring better performance.
> 4. To use any one of the three level of indexes,we should enable predicate push-down
by setting spark.sql.orc.filterPushdown=true (in sparkSQL) or hive.optimize.ppd=true (in hive).
> 
> But I found the  build-in indexes in ORC files did not work both in spark 1.5.2 and hive
1.2.1:
> First, when the query statement with where clause did't match any record (the filter
column had a value beyond the range of data),  the performance when enabled  predicate push-down
was almost the same with when disabled predicate push-down.  I think, when the filter column
has a value beyond the range of data, all of the orc files will not be scanned if use file
level indexes,  so the performance should improve obviously.
> 
> The second, when enabled "orc.create.index" and sorted data by filter column and where
clause can only match a few records, the performance when enabled  predicate push-down was
almost the same with when disabled predicate push-down. 
> 
> The third, when enabled  predicate push-down and "orc.create.index", the performance
when  filter column had a value beyond the range of data was almost the same with when filter
column had a value covering almost the whole data. 
> 
> So,  has anyone used ORC's build-in indexes before (especially in spark SQL)?  What's
my issue?
> 
> Thanks!
> 
> Joseph

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