I have an addresses table with currently 850,000 records. In my
application, to check for duplicates before entering it into the
database, I'm doing a select on all the fields with the data I'm about
to enter. If it is found, then I do not insert the record, but rather
just return the id for my app. Currently the only field that is
indexed is the id field.

This query is taking about 2 seconds to complete. So now I'm trying to
find the best way and fastest way to check for duplicates. I thought
about:
1.) Indexing every field (line1, line2, city, state, zip, country)
2.) Creating a hash of the values and adding the hashed value in
another column and just index that column
3.) make every field a unique index and use REPLACE INTO instead of
INSERT

What are your thougths? Is there's a better way to do this?

Thanks for your help.