Transactions

Dear Postgres developers,

I have some problems but I can't find any answer.

I don't know, what you mean about a transaction block. As far as I know,
CREATE TABLE, DROP TABLE and INSERT instructions need not to be inside a
transaction block, but UPDATE instruction must be inside a transaction
block. If UPDATE isn't inside a transaction block, changes won't be
executed in the database but changes made by a CREATE/DROP TABLE or
INSERT instruction will be executed even if these instructions aren't
inside a transaction block. It is very strange, and I don't know why.
Could you explain it to me? (I use libpq)

My second problem:
I create and drop lots of tables in a cycle many times (of course I
insert many rows into tables).  I execute a VACUUM ANALYZE instruction
in every cycle, but inspite of that fact, the  time of creation of
tables become longer and longer. Why? Could you help me? I don't know
the solution.

Thank you very much in advance!