hash vs. sql lookup map speeds

Hi,

I've read several times that postfix-"native" index files are
a lot faster than SQL lookups. While I no problems believing
that statement for maps up to a certain number of entries (below
several thousand, say), I am a little curious if there is a
certain number of entries where the lookup speed is faster
in SQL than in the index files.

As a concrete scenario: Say you're using postfix as a=20
mail gateway to a set of domains that have tens of thousands
of valid email addresses. Would it still make sense to=20
keep these in postfix hash-tables or would MySQL/Postgres make
more sense then?

(As a disclaimer: I have no idea how postfix accesses/caches/re-reads
the index files).

Regs,
Sven