How to Convert an External Table to a Managed Table in Hive: A Comprehensive Guide

Managing tables in Hive, whether they are external or managed, is a fundamental task for any data engineer or data analyst. One such common requirement is converting an external table to a managed table. While Hive does not offer a direct command for this conversion, it is still quite simple and straightforward to perform. This guide will walk you through the process and highlight important considerations to keep in mind.

Steps to Convert an External Table to a Managed Table

Converting an external table to a managed table in Hive can be done in a few steps. Here's how you can do it:

Create a New Managed Table

The first step is to create a new managed table with the same schema as the external table. This involves specifying the table name, columns, and the file format.

CREATE TABLE new_managed_table_name (column1 datatype, column2 datatype, ... STORED AS file_format) -- Specify the file format e.g. ORC, Parquet, etc.

Insert Data from the External Table

After creating the new managed table, the next step is to insert data from the external table into the newly created managed table. This typically involves using a SELECT statement to transfer the data.

INSERT INTO new_managed_table_name SELECT * FROM external_table_name;

Drop the External Table

If the external table is no longer needed, it can be dropped after the data has been successfully copied to the managed table. Dropping the external table is a crucial step, so ensure that you have no further use for it.

DROP TABLE external_table_name;

Rename the Managed Table (Optional)

If you prefer that the managed table have the same name as the original external table, you can rename it using the ALTER TABLE command.

ALTER TABLE new_managed_table_name RENAME TO external_table_name;

Important Considerations

While converting an external table to a managed table, there are a few considerations you should keep in mind:

Data Location

When you create a managed table, Hive manages the data location. If the external table points to a specific location in HDFS, be aware that creating a managed table will lead to Hive managing the data in its own way. This includes the potential risk of data deletion if the table is dropped.

Data Backup

Always ensure that you have backups or that you are aware of the implications of dropping tables, especially if they contain valuable data. Data loss can be catastrophic, so it's essential to have a backup strategy in place.

Additional Method: Using the ALTER TABLE Command

Another method to convert an external Hive table to a managed table is by using the ALTER TABLE command. This command allows you to set the TBLPROPERTIES to change the table's property from EXTERNALTRUE to EXTERNALFALSE.

ALTER TABLE employee SET TBLPROPERTIES ('EXTERNAL''FALSE');

This method is a simpler one-line command that achieves the same goal, making it a preferred choice in some scenarios.

Conclusion

Converting an external table to a managed table in Hive can be effectively executed by following the steps outlined above. Whether you choose to create a new managed table and copy data, or use the ALTER TABLE command, both methods are reliable and efficient. Always remember to handle your data with care and to back up important data to prevent any potential loss.