Analyze your Spark application using explain

What this post added

This post details how to use the `explain` API in Spark SQL to retrieve and analyze physical and logical execution plans for Spark queries. It demonstrates the use of `explain()` and `explain(extended=true)` with code examples and explains how to interpret the output, including stages, exchanges, and aggregate functions. It also shows how to view execution plans in the Spark UI and Spark history server.