The latest version of the Vitess query planner, Gen4, changes most of the planner's internals.
To ensure Gen4 produces plans that, once executed, give us the same results as what V3 would do, we have developed
a small test tool that runs queries using both Gen4 and V3 planners and compare their results. If the results we got are different
from the two planners, the query will fail and the difference will be printed in VTGate's logs as a warning.
This tool is enabled by the use of a new planner: Gen4CompareV3, to use it, we must start VTGate with the -planner_version
flag set to Gen4CompareV3. Once set, new queries will be tested against both Gen4 and V3.
This new planner should only be used for testing purposes.