DELETE SELECT Statements
To perform multiple deletes in a single request to Apache Hive, use the DELETE ... SELECT syntax to insert a temporary table of data into Apache Hive. This works by first populating a temporary table with the data you are going to submit to Apache Hive. Once you have all of the data you want to delete, the temporary table is then passed into the table you are inserting to.
Populate the Temporary Table
The temporary table you are populating is dynamic and is created at run time the first time you insert to it. Temporary tables are denoted by a # appearing in their name. When using a temporary table to delete, the temporary table must be named in the format [TableName]#TEMP, where TableName is the
name of the table you are inserting to. For example:
INSERT INTO [CData].[Default].Customers#TEMP (_id) VALUES ('AX1000001'); INSERT INTO [CData].[Default].Customers#TEMP (_id) VALUES ('AX1000002'); INSERT INTO [CData].[Default].Customers#TEMP (_id) VALUES ('AX1000003');
This creates a temporary table called [CData].[Default].Customers#TEMP with one column and three rows of data. Since type cannot be determined on the temporary table itself, all values are stored in memory as strings. They are later converted to the proper type when they are submitted to the [CData].[Default].Customers table.
Delete from the Actual Table
Once your temporary table is populated, it is now time to insert to the actual table in Apache Hive. You can do this by performing a DELETE from the actual table and selecting the input data from the temporary table. For example:
DELETE FROM [CData].[Default].Customers WHERE EXISTS SELECT _id FROM [CData].[Default].Customers#TEMP
In this example, the full contents of the [CData].[Default].Customers#TEMP table are passed into the [CData].[Default].Customers table. This results in fewer requests being submitted to Apache Hive since multiple deletes may be submitted with each request, which is much better for performance if you have many records to delete.
The results of the query are stored in the LastResultInfo#TEMP temporary table. This table is cleared and repopulated the next time data is modified by passing in a temporary table. Please be aware that the LastResultInfo#TEMP table has no predefined schema. You need to check its metadata at run time before reading data.
Temporary Table Life Span
Temporary tables only last as long as the connection remains opened. When the connection to Apache Hive is closed, all temporary tables are cleared, including the LastResultInfo#TEMP table.