[odb-users] query results not being cached?
boris at codesynthesis.com
Thu Jun 7 03:11:32 EDT 2012
Burton, Craig <crburton at tnsi.com> writes:
> Under many circumstances, our application expects to find exactly zero
> or one instance for many specific queries, but finding two or more would
> lead to an error scenario. Application logic is the following in many
> such cases:
> if zero instances, create a new one
> if one instance, update the one found in the db
> if two or more, do nothing and return an error
If we can assume that the error scenario is quite rare and not performance-
sensitive, this can be handled optimally for the other two cases by doing
a "speculative" load (i.e., with the expectation that there is no error)
and then checking for the error condition:
result r = ...;
if (r.empty ())
// Zero instances.
result::iterator i (r.begin ());
shared_ptr<MyPersistentClass> obj (i.load ());
if (++i != r.end ())
// More than one instance (error).
// One instance.
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