ISSUE SUMMARY
The query optimizer caches plans for each query shape and reuses these plans for a time. In situations where the performance of the cached plan is poor for a particular instance of the query shape, the optimizer may select a plan with poor performance and fail to evict the cache entry. This behavior may impact deployments where two queries with the same shape have different performance characteristics (generally due to a difference in selectivity).

SERVER-15225 makes the query planner evaluate the cost of the cached query plan, and if the cost of this plan is too high, the query planner switches to a more efficient plan. This more efficient plan is then cached for future use.

This ticket enables the optimization from SERVER-15225 by default.

To disable this behavior users can set internalQueryCacheReplanningEnabled parameter to false using the setParameter command on a running system, or at start time using the setParameter commandline option or setParameter in the configuration file.

For example, to disable using setParameter:

db.runCommand({setParameter: 1, internalQueryCacheReplanningEnabled: false})

This improvement can be re-enabled as follows:

db.runCommand({setParameter: 1, internalQueryCacheReplanningEnabled: true})

See SERVER-15225 for additional details.