JOIN vs. WHERE ... IN (subselect)

Using PostgreSQL 7.3.2 on Linux.

One of the types of queries that I like to do on our database is:

SELECT * FROM table1   WHERE indexed_column1 IN   (SELECT column2 FROM table2 WHERE <condition>);

However, in our database table1 is quite large (~1M rows), and the above query takes "forever", EVEN IF table2 CONSISTS
OFONLY ONE ROW!
 

However, my third-party SQL book says that the above is equivalent to:

SELECT table1.* FROM table1, table2   WHERE indexed_column1 = column2 AND <condition>;

And indeed, the above query executes virtually instantaneously if "<condition>" results in a small number of rows from
table2.

I'm fairly new to SQL;  are they really equivalent?  If so, is there some reason that the first form is not
optimized/transformedinto the second form, or is this a planned enhancement?
 

-- Dean

ps: If indexed_column1 has the same name as column2, then the query:

SELECT table1.* from table1   JOIN table2 USING( column )   WHERE <condition>;

Also executes quickly.  I just like the readability of the first query above, but as they say, you gotta do what works
...