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Date: Fri, 1 Jun 2012 20:28:23 +0000 (UTC)
From: "Carl Steinbach (JIRA)" <jira@apache.org>
To: hive-dev@hadoop.apache.org
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Subject: [jira] [Commented] (HIVE-2989) Adding Table Links to Hive
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    [ https://issues.apache.org/jira/browse/HIVE-2989?page=3Dcom.atlassian.=
jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=3D13287=
643#comment-13287643 ]=20

Carl Steinbach commented on HIVE-2989:
--------------------------------------

-1. This patch was committed two minutes after it was marked patch availabl=
e which is unfair to the other committers. Also, there is still an ongoing =
discussion regarding the design proposal.

Please back this patch out.

               =20
> Adding Table Links to Hive
> --------------------------
>
>                 Key: HIVE-2989
>                 URL: https://issues.apache.org/jira/browse/HIVE-2989
>             Project: Hive
>          Issue Type: Improvement
>          Components: Metastore, Query Processor, Security
>    Affects Versions: 0.10.0
>            Reporter: Bhushan Mandhani
>            Assignee: Bhushan Mandhani
>         Attachments: HIVE-2989.1.patch.txt, HIVE-2989.2.patch.txt, HIVE-2=
989.3.patch.txt, HIVE-2989.4.patch.txt, HIVE-2989.5.patch.txt
>
>   Original Estimate: 672h
>  Remaining Estimate: 672h
>
> This will add Table Links to Hive. This will be an alternate mechanism fo=
r a user to access tables and data in a database that is different from the=
 one he is associated with. This feature can be used to provide access cont=
rol (if access to databasename.tablename in queries and "use database X" is=
 turned off in conjunction).
> If db X wants to access one or more partitions from table T in db Y, the =
user will issue:
> CREATE [STATIC] LINK TO T@Y LINKPROPERTIES ('RETENTION'=3D'N')
> New partitions added to T will automatically be added to the link as well=
 and become available to X. However, if the link is specified to be static,=
 that will not be the case. The X user will then have to explicitly import =
each partition of T that he needs. The command above will not actually make=
 any existing partitions of T available to X. Instead, we provide the follo=
wing command to add an existing partition to a link:
> ALTER LINK T@Y ADD PARTITION (ds=3D'2012-04-27')
> The user will need to execute the above for each existing partition that =
needs to be imported. For future partitions, Hive will take care of this. A=
n imported partition can be dropped from a link using a similar command. We=
 just specify "DROP" instead of "ADD". For querying the linked table, the X=
 user will refer to it as T@Y. Link Tables will only have read access and n=
ot be writable. The entire Table Link alongwith all its imported partitions=
 can be dropped as follows:
> DROP LINK TO T@Y
> The above commands are purely MetaStore operations. The implementation wi=
ll rely on replicating the entire partition metadata when a partition is ad=
ded to a link.  For every link that is created, we will add a new row to ta=
ble TBLS. The TBL_TYPE column will have a new kind of value "LINK_TABLE" (o=
r "STATIC_LINK_TABLE" if the link has been specified as static). A new colu=
mn LINK_TBL_ID will be added which will contain the id of the imported tabl=
e. It will be NULL for all other table types including the regular managed =
tables. When a partition is added to a link, the new row in the table PARTI=
TIONS will point to the LINK_TABLE in the same database  and not the master=
 table in the other database. We will replicate all the metadata for this p=
artition from the master database. The advantage of this approach is that f=
ewer changes will be needed in query processing and DDL for LINK_TABLEs. Al=
so, commands like "SHOW TABLES" and "SHOW PARTITIONS" will work as expected=
 for LINK_TABLEs too. Of course, even though the metadata is not shared, th=
e underlying data on disk is still shared. Hive still needs to know that wh=
en dropping a partition which belongs to a LINK_TABLE, it should not drop t=
he underlying data from HDFS. Views and external tables cannot be imported =
from one database to another.
> =20

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