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https://issues.apache.org/jira/browse/DRILL-5941?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16250120#comment-16250120
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ASF GitHub Bot commented on DRILL-5941:
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Github user paul-rogers commented on the issue:
https://github.com/apache/drill/pull/1030
For FWIW, the native CSV reader does the following:
* To read the header, it seeks to offset 0 in the file, regardless of the
block being read, then reads the header, which may be a remote read.
* The reader then seeks to the start of its block. If this is the first
block, it skips the header, else it searches for the start of the next record.
Since Hive has the same challenges, Hive must have solved this, we have
only to research that existing solution.
One simple solution is:
* If block number is 0, skip the header.
* If block number is 1 or larger, look for the next record separator.
> Skip header / footer logic works incorrectly for Hive tables when file has
> several input splits
> -----------------------------------------------------------------------------------------------
>
> Key: DRILL-5941
> URL: https://issues.apache.org/jira/browse/DRILL-5941
> Project: Apache Drill
> Issue Type: Bug
> Components: Storage - Hive
> Affects Versions: 1.11.0
> Reporter: Arina Ielchiieva
> Assignee: Arina Ielchiieva
> Fix For: Future
>
>
> *To reproduce*
> 1. Create csv file with two columns (key, value) for 3000029 rows, where
> first row is a header.
> The data file has size of should be greater than chunk size of 256 MB. Copy
> file to the distributed file system.
> 2. Create table in Hive:
> {noformat}
> CREATE EXTERNAL TABLE `h_table`(
> `key` bigint,
> `value` string)
> ROW FORMAT DELIMITED
> FIELDS TERMINATED BY ','
> STORED AS INPUTFORMAT
> 'org.apache.hadoop.mapred.TextInputFormat'
> OUTPUTFORMAT
> 'org.apache.hadoop.hive.ql.io.HiveIgnoreKeyTextOutputFormat'
> LOCATION
> 'maprfs:/tmp/h_table'
> TBLPROPERTIES (
> 'skip.header.line.count'='1');
> {noformat}
> 3. Execute query {{select * from hive.h_table}} in Drill (query data using
> Hive plugin). The result will return less rows then expected. Expected result
> is 3000028 (total count minus one row as header).
> *The root cause*
> Since file is greater than default chunk size, it's split into several
> fragments, known as input splits. For example:
> {noformat}
> maprfs:/tmp/h_table/h_table.csv:0+268435456
> maprfs:/tmp/h_table/h_table.csv:268435457+492782112
> {noformat}
> TextHiveReader is responsible for handling skip header and / or footer logic.
> Currently Drill creates reader [for each input
> split|https://github.com/apache/drill/blob/master/contrib/storage-hive/core/src/main/java/org/apache/drill/exec/store/hive/HiveScanBatchCreator.java#L84]
> and skip header and /or footer logic is applied for each input splits,
> though ideally the above mentioned input splits should have been read by one
> reader, so skip / header footer logic was applied correctly.
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