generating unique ids in hive

Not sure if this is all that helpful, but here goes...

Consider the native MapReduce analog: assuming your input data set is text based, the input Mapper's key (and hence unique ID) would be, for each line, the name of the file plus its byte offset.

When you are loading the data into Hive, if you can create an extra 'column' that has this info, you get your rowID for free. It's semantically meaningless, but so too is the approach you mention above.

Use the reflect UDF to generate UUIDs.

reflect("java.util.UUID", "randomUUID")

Update (2019)

For a long time, UUIDs were your best bet for getting unique values in Hive. As of Hive 4.0, Hive offers a surrogate key UDF which you can use to generate unique values which will be far more performant than UUID strings. Documentation is a bit sparse still but here is one example:

create table customer (
  id bigint default surrogate_key(),
  name string, 
  city string, 
  primary key (id) disable novalidate
);

To have Hive generate IDs for you, use a column list in the insert statement and don't mention the surrogate key column:

-- staging_table would have two string columns.
insert into customer (name, city) select * from staging_table;