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.