InvalidTask is just a common invalid singleton instance initialized by core's empty RootTask.

AccessObject represents what is accessed by an operator.

LogicalOptRule means a logical optimizing rule, which contains de-correlate, ppd, column pruning, etc.

LogicalPlan is a tree of logical operators.

MemTablePredicateExtractor is used to extract some predicates from `WHERE` clause and push the predicates down to the data retrieving on reading memory table stage.

PhysicalPlan is a tree of the physical operators.

Plan is the description of an execution flow.

ShowPredicateExtractor is used to extract some predicates from `PatternLikeOrIlikeExpr` clause and push the predicates down to the data retrieving on reading memory table stage when use ShowStmt.

Task is a new version of `PhysicalPlanInfo`.

BuildPBContext is the context for building `*tipb.Executor`.

PlanContext is the context for building plan.

PlanCounterTp is used in hint nth_plan() to indicate which plan to use.