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https://issues.apache.org/jira/browse/HAWQ-56?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15018366#comment-15018366
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ASF GitHub Bot commented on HAWQ-56:
------------------------------------
Github user shivzone commented on the pull request:
https://github.com/apache/incubator-hawq/pull/121#issuecomment-158464458
+1
> Non-deterministic header results with "HEADER" option from external table
> -------------------------------------------------------------------------
>
> Key: HAWQ-56
> URL: https://issues.apache.org/jira/browse/HAWQ-56
> Project: Apache HAWQ
> Issue Type: Bug
> Components: PXF
> Reporter: Goden Yao
> Assignee: Noa Horn
> Priority: Critical
>
> *Repro Steps*
> External table definition
> {code:sql}
> drop external table if exists testtbl;
> create external table testtbl(i text, j text)
> location
> ('pxf://nakaphdns/tmp/testdata/*?Fragmenter=com.pivotal.pxf.plugins.hdfs.HdfsDataFragmenter&Accessor=com.pivotal.pxf.plugins.hdfs.TextFileAccessor&Resolver=com.pivotal.pxf.plugins.hdfs.TextResolver')
> format 'TEXT' (delimiter ',' header);
> select * from testtbl;
> {code}
> example with 4 segment servers and 4 test files with headers in hdfs
> {code:sql}
> gpadmin=# select * from testtbl ;
> i | j
> ---+---
> 3 | c
> 2 | b
> 1 | a
> 4 | d
> (4 rows)
> {code}
> With 5 test files
> {code:sql}
> gpadmin=# select * from testtbl ;
> i | j
> ----+-------
> 5 | e
> 2 | b
> ID | Value
> 3 | c
> 1 | a
> ID | VALUE
> 4 | d
> (7 rows)
> {code}
> *Analysis*
> When using HEADER option, header line is removed only once per segment.
> As a result there will be different results depending on the number of
> segments/fragments are scanned. If the number of files is greater than the
> number of segments, the header row is included in the data for some files.
> If the number of files is less than or equal to the number of segments, the
> data retrieved is good. Thus, non-deterministic.
> The reason for this behavior is that header line handling is done by the
> external protocol code (fileam.c) which checks if the header_line flag is
> true, and if so skips the first line and sets the flag to false. This code
> calls the custom protocol code (in our case pxf) to get the next tuples, and
> so doesn't know if the tuples are from the same resource or not.
> From what I can see, in gpfdist protocol the problem is solved by letting the
> custom protocol code handle this and marking the flag as false for the
> external protocol infrastructure in fileam.c.
> *Proposed Solution*
> Add a check in pxf validator function (a function that is being called as
> part of external table creation).
> This check will error out if HEADER option is used in a PXF table.
> Currently the validation function API only allows access to the table's URLs
> (in the LOCATION part of the table), and not the format options. In order to
> add the check an API change in the external protocol is required.
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