SparkStrategies包含了一系列特定的Strategies，这些Strategies是继承自QueryPlanner中定义的Strategy，它定义接受一个Logical Plan，生成一系列的Physical Plan
通过Strategies把逻辑计划转换成可以具体执行的物理计划
package object sql {/*** Converts a logical plan into zero or more SparkPlans. This API is exposed for experimenting* with the query planner and is not designed to be stable across spark releases. Developers* writing libraries should instead consider using the stable APIs provided in* [[org.apache.spark.sql.sources]]*/@DeveloperApi@InterfaceStability.Unstabletype Strategy = SparkStrategytype DataFrame = Dataset[Row]
}