Specifies the action that occurs if the destination table
already exists.
Values:
WRITE_DISPOSITION_UNSPECIFIED (0):
Write disposition is not specified. Defaults to
WRITE_TRUNCATE.
WRITE_TRUNCATE (1):
If the table already exists, BigQuery will
overwrite the table data and use the schema from
the load.
WRITE_APPEND (2):
If the table already exists, BigQuery will
append data to the table.
