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https://bugs.exim.org/show_bug.cgi?id=3084

--- Comment #2 from Marten Lehmann <lehmann@cnm.de> ---
I'm aware of the option to specify the full database connection details on each
lookup. I didn't think of using macros in this context, yet I agree: On the
aesthetic side macros would probably be an easy way. How does exim handle this
internally though?

Specifying server connections globally, I'd assume exim establishes and reuses
these connections, keeping a connection pool. Specifying them per lookup, I'd
assume they get a local scope and the connection would be closed after the
lookup. The latter would be rather bad for performance.

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https://bugs.exim.org/show_bug.cgi?id=3084

--- Comment #3 from Jeremy Harris <jgh146exb@wizmail.org> ---
Macros are evaluated at config load time, which means at every process exec:
every command-line-source message, the daemon, every delivery-routing action
and
every nonlocal transport. The config is loaded in full every time.

Exim runs a pool of DB connections, though because it also traverses several
processes
during the handling of a message, and in large part any given process only
handles one
message, the benefit of the pool is limited. Many DB accesses during the input
ACL
processing for a connection will benefit most. Additionally,
character-identical
lookups will hit in a local cache. New DB connections are made lazily, not
associated with use of the main-config option (mysql_zervers or similar) versus
a
spec local to the lookup in question. The connection is then held open in the
pool
(until overflow or a full tidyup is needed).

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