partial TEXT search on an index - Mailing list pgsql-general

I don't think I understand how PG implements fulltext searching or if my
search actually needs to use fulltext search.

basically, I have a btree index on a SERIAL_NUMBER which is of sort like
ABC12345 or AAA123434 or AAB131441

I would like to have search on the specific text of the SERIAL_NUMBER

eg:
select * from table where serial_number LIKE 'AAA%'

does not use the index. What Do i need to do to have it recognise the
"partial index?" (is it called?)

Thanks