Update Benchmark comments

Author: IvanVergiliev

sql/core/src/test/scala/org/apache/spark/sql/execution/benchmark/FilterPushdownBenchmark.scala

@@ -412,6 +412,13 @@ object FilterPushdownBenchmark extends BenchmarkBase with SQLHelper {
     }
 
     runBenchmark(s"Predicate conversion benchmark with unbalanced Expression") {
+      // This benchmark tests a very isolated part of the predicate pushdown process - specifically,
+      // the individual action of converting a Spark `Expression` to an ORC `SearchArgument`.
+      // This results in more granular numbers that can help highlight small performance
+      // differences in this part of the code that would be hidden by slower components that
+      // get run when a full Spark job is executed.
+      // The benchmark below runs a more complete, end-to-end test which covers the whole pipeline
+      // and can uncover high-level performance problems, but is bad at discriminating details.
       val numRows = 1
       val width = 2000
 
@@ -421,7 +428,8 @@ object FilterPushdownBenchmark extends BenchmarkBase with SQLHelper {
         val whereColumn = (1 to numFilter)
             .map(i => col("c1") === lit(i))
             .foldLeft(lit(false))(_ || _)
-        val benchmark = new Benchmark(s"Select 1 row with $numFilter filters",
+        val benchmark = new Benchmark(
+          s"Convert a filter with $numFilter columns to ORC filter",
           numRows, minNumIters = 5, output = output)
         val name = s"Native ORC Vectorized (Pushdown)"
         benchmark.addCase(name) { _ =>