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From "Prasad Chakka (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HIVE-493) automatically infer existing partitions of table from HDFS files.
Date Mon, 28 Sep 2009 16:57:16 GMT

    [ https://issues.apache.org/jira/browse/HIVE-493?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12760293#action_12760293
] 

Prasad Chakka commented on HIVE-493:
------------------------------------

1) You can do 'alter table <tbl> add partition <partition spec>' at the end of
the map-reduce job that creates the partition. You don't really 'automatic inference' unless
you  do not have any control over the partition creation process

2) Adding new files to existing partitions and tables should not be a problem now. But you
may want to do the above add partition command only when last of the partition files have
been added otherwise the dependent data processes might see incomplete data. With automatic
inferring of partitions, the dependent processes can see incomplete data.

3) Netflix guys wrote a custom map/reducer job to compact/merging process. But you may want
to co-ordinate the partition readers and compactors so that later does not clobber the directory.

> automatically infer existing partitions of table from HDFS files.
> -----------------------------------------------------------------
>
>                 Key: HIVE-493
>                 URL: https://issues.apache.org/jira/browse/HIVE-493
>             Project: Hadoop Hive
>          Issue Type: New Feature
>          Components: Metastore, Query Processor
>    Affects Versions: 0.3.0, 0.3.1, 0.4.0
>            Reporter: Prasad Chakka
>
> Initially partition list for a table is inferred from HDFS directory structure instead
of looking into metastore (partitions are created using 'alter table ... add partition').
but this automatic inferring was removed to favor the later approach during checking-in metastore
checker feature and also to facilitate external partitions.
> Joydeep and Frederick mentioned that it would simple for users to create the HDFS directory
and let Hive infer rather than explicitly add a partition. But doing that raises following...
> 1) External partitions -- so we have to mix both approaches and partition list is merged
list of inferred partitions and registered partitions. and duplicates have to be resolved.
> 2) Partition level schemas can't supported. Which schema to chose for the inferred partitions?
the table schema when the inferred partition is created or the latest tale schema? how do
we know the table schema when the inferred partitions is created?
> 3) If partitions have to be registered the partitions can be disabled without actually
deleting the data. this feature is not supported and may not be that useful but nevertheless
this can't be supported with inferred partitions
> 4) Indexes are being added. So if partitions are not registered then indexes for such
partitions can not be maintained automatically.
> I would like to know what is the general thinking about this among users of Hive. If
inferred partitions are preferred then can we live with restricted functionality that this
imposes?

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