Make UPDATE query quicker?

Hi all,

Wondered if someone had any tips about how to do this UPDATE query
quicker. I've got two tables:

CREATE TABLE import_table
( date_time TIMESTAMP
person_id, INTEGER
data REAL)

CREATE TABLE master_table
(date_time TIMESTAMP
person_id INTEGER
data REAL)

Each table has 172800 rows in it.

I want to move the 'data' from the import_table to the master_table by
matching on both the date_time and the person_id.  I do this with this
query:

UPDATE master_table a
SET data =
(SELECT b.date
FROM import_table b
WHERE a.date_time = b.date_time AND a.person_id = b.person_id AND
b.data IS NOT NULL)

However I need to do this about 20 times, and the first time is still
running at the moment (about 15 minutes).

Am I doing something wrong? Should I put some indexes on the columns
somehow to improve performance?

Thanks

James