Bhushan Mandhani created HIVE-2989:
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Summary: 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
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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