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https://issues.apache.org/jira/browse/HIVE-2989?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13280575#comment-13280575
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Namit Jain commented on HIVE-2989:
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The goal is to do much better accounting. Take a look at
https://cwiki.apache.org/confluence/display/Hive/Links
> 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
> Reporter: Bhushan Mandhani
> Assignee: Bhushan Mandhani
> Fix For: 0.10.0
>
> Original Estimate: 672h
> Remaining Estimate: 672h
>
> This will add Table Links to Hive. This will be an alternate mechanism for 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 control (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'='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 following
> command to add an existing partition to a link:
> ALTER LINK T@Y ADD PARTITION (ds='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. An
> 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 not
> 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 will
> rely on replicating the entire partition metadata when a partition is added
> to a link. For every link that is created, we will add a new row to table
> TBLS. The TBL_TYPE column will have a new kind of value "LINK_TABLE" (or
> "STATIC_LINK_TABLE" if the link has been specified as static). A new column
> LINK_TBL_ID will be added which will contain the id of the imported table. 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 PARTITIONS 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 partition from
> the master database. The advantage of this approach is that fewer changes
> will be needed in query processing and DDL for LINK_TABLEs. Also, commands
> like "SHOW TABLES" and "SHOW PARTITIONS" will work as expected for
> LINK_TABLEs too. Of course, even though the metadata is not shared, the
> underlying data on disk is still shared. Hive still needs to know that when
> dropping a partition which belongs to a LINK_TABLE, it should not drop the
> underlying data from HDFS. Views and external tables cannot be imported from
> one database to another.
>
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