On 12 Jun 2013, at 14:12, lists@??? wrote:
> In this example, instead of 5 queries (in reality each with 1-2 joins) it is only a single query and even easier to read/work with.
http://www.exim.org/exim-html-current/doc/html/spec_html/ch-file_and_database_lookups.html
"8. Lookup caching
Exim caches all lookup results in order to avoid needless repetition of lookups. However, because (apart from the daemon) Exim operates as a collection of independent, short-lived processes, this caching applies only within a single Exim process. There is no inter-process lookup caching facility."
So, in the first router, if you did a query which SELECTed multiple records then that would be saved into address_data. If the *exact same* lookup happened in sequential routers, the cached results would be used - thus cutting the number of queries.
If however the query was spawned from a different process (as can happen with redirection or multiple deliveries with breaks on the queue), additional queries will take place.
Graeme
