Why does postgres seach in ALL lines (not optimal!)

Hi,

I have two tables with entries that have the same IDs. My select shall
get all entries from both tables that have an ID > 19000, all in all I
have 19577 entries.

select * from m, t where m.id > 19000 and t.messageid = m.id;

Postgres should immediately get the 577 lines from m, and get the
corresponding 577 lines from table t, using the primary keys. But, it
scans ALL the lines in table t which seems highly inefficient:

db=# EXPLAIN select * from m, t
db-# where m.id > 19000 and t.messageid = m.id;

Merge Join  (cost=0.00..628.96 rows=550 width=172)
  ->  Index Scan using pk_m on m  (cost=0.00..17.43  rows=  550
width=101)
  ->  Index Scan using pk_t on t  (cost=0.00..554.34 rows=19576 (!!!)
width=71)

Why is this and how can I make postgres search fewer rows in the table
t?
Any help is very appreciated!!!
Markus