SQL Server Execution Plans - Red Gate Software

Chapter 5: Controlling Execution Plans with Hints
While this will eliminate errors, it will almost certainly result in longer query times since
the optimizer won't be able to choose the optimal plan over the "robust" plan.
You should only ever use this hint if you have a set of wide rows that cause this rare
error condition.
KEEP PLAN
As the data in a table changes, gets inserted, or deleted, the statistics describing the
data also change. Once the volume of statistics changed passes a certain threshold, any
queries referencing those statistics are marked for recompile in the plan cache, on the
assumption that the optimizer might, based on the new statistics, be able to generate a
more efficient plan.
Setting the KEEP PLAN hint doesn't prevent recompiles, but it does cause the optimizer to
use less stringent rules when determining the need for a recompile. This means that, with
more volatile data, you can keep recompiles to a minimum. The hint causes the optimizer
to treat temporary tables within the plan in the same way as permanent tables, reducing
the number of recompiles caused by the temporary table. This reduces the time and cost
of recompiling a plan, which can be quite large, depending on the query.
However, problems may arise because the old plans may not be as efficient as newer plans
could be. This happens because of the changing data; a plan that was previously adequate
for a given data set is no longer appropriate because of the changes to that data set.
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