[SPARK-15794] Should truncate toString() of very wide plans

## What changes were proposed in this pull request?

With very wide tables, e.g. thousands of fields, the plan output is unreadable and often causes OOMs due to inefficient string processing. This truncates all struct and operator field lists to a user configurable threshold to limit performance impact.

It would also be nice to optimize string generation to avoid these sort of O(n^2) slowdowns entirely (i.e. use StringBuilder everywhere including expressions), but this is probably too large of a change for 2.0 at this point, and truncation has other benefits for usability.

## How was this patch tested?

Added a microbenchmark that covers this case particularly well. I also ran the microbenchmark while varying the truncation threshold.

```
numFields = 5
wide shallowly nested struct field r/w:  Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------
2000 wide x 50 rows (write in-mem)            2336 / 2558          0.0       23364.4       0.1X

numFields = 25
wide shallowly nested struct field r/w:  Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------
2000 wide x 50 rows (write in-mem)            4237 / 4465          0.0       42367.9       0.1X

numFields = 100
wide shallowly nested struct field r/w:  Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------
2000 wide x 50 rows (write in-mem)          10458 / 11223          0.0      104582.0       0.0X

numFields = Infinity
wide shallowly nested struct field r/w:  Best/Avg Time(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------
[info]   java.lang.OutOfMemoryError: Java heap space
```

Author: Eric Liang <ekl@databricks.com>
Author: Eric Liang <ekhliang@gmail.com>

Closes #13537 from ericl/truncated-string.

2 files changed, 55 insertions, 0 deletions

diff --git a/core/src/main/scala/org/apache/spark/util/Utils.scala b/core/src/main/scala/org/apache/spark/util/Utils.scala
index 1a9dbcae8c..f9d05409e1 100644
--- a/core/src/main/scala/org/apache/spark/util/Utils.scala
+++ b/core/src/main/scala/org/apache/spark/util/Utils.scala
@@ -26,6 +26,7 @@ import java.nio.charset.StandardCharsets
 import java.nio.file.Files
 import java.util.{Locale, Properties, Random, UUID}
 import java.util.concurrent._
+import java.util.concurrent.atomic.AtomicBoolean
 import javax.net.ssl.HttpsURLConnection
 
 import scala.annotation.tailrec
@@ -78,6 +79,52 @@ private[spark] object Utils extends Logging {
   private val MAX_DIR_CREATION_ATTEMPTS: Int = 10
   @volatile private var localRootDirs: Array[String] = null
 
+  /**
+   * The performance overhead of creating and logging strings for wide schemas can be large. To
+   * limit the impact, we bound the number of fields to include by default. This can be overriden
+   * by setting the 'spark.debug.maxToStringFields' conf in SparkEnv.
+   */
+  val DEFAULT_MAX_TO_STRING_FIELDS = 25
+
+  private def maxNumToStringFields = {
+    if (SparkEnv.get != null) {
+      SparkEnv.get.conf.getInt("spark.debug.maxToStringFields", DEFAULT_MAX_TO_STRING_FIELDS)
+    } else {
+      DEFAULT_MAX_TO_STRING_FIELDS
+    }
+  }
+
+  /** Whether we have warned about plan string truncation yet. */
+  private val truncationWarningPrinted = new AtomicBoolean(false)
+
+  /**
+   * Format a sequence with semantics similar to calling .mkString(). Any elements beyond
+   * maxNumToStringFields will be dropped and replaced by a "... N more fields" placeholder.
+   *
+   * @return the trimmed and formatted string.
+   */
+  def truncatedString[T](
+      seq: Seq[T],
+      start: String,
+      sep: String,
+      end: String,
+      maxNumFields: Int = maxNumToStringFields): String = {
+    if (seq.length > maxNumFields) {
+      if (truncationWarningPrinted.compareAndSet(false, true)) {
+        logWarning(
+          "Truncated the string representation of a plan since it was too large. This " +
+          "behavior can be adjusted by setting 'spark.debug.maxToStringFields' in SparkEnv.conf.")
+      }
+      val numFields = math.max(0, maxNumFields - 1)
+      seq.take(numFields).mkString(
+        start, sep, sep + "... " + (seq.length - numFields) + " more fields" + end)
+    } else {
+      seq.mkString(start, sep, end)
+    }
+  }
+
+  /** Shorthand for calling truncatedString() without start or end strings. */
+  def truncatedString[T](seq: Seq[T], sep: String): String = truncatedString(seq, "", sep, "")
 
   /** Serialize an object using Java serialization */
   def serialize[T](o: T): Array[Byte] = {
diff --git a/core/src/test/scala/org/apache/spark/util/UtilsSuite.scala b/core/src/test/scala/org/apache/spark/util/UtilsSuite.scala
index 6698749866..a5363f0bfd 100644
--- a/core/src/test/scala/org/apache/spark/util/UtilsSuite.scala
+++ b/core/src/test/scala/org/apache/spark/util/UtilsSuite.scala
@@ -40,6 +40,14 @@ import org.apache.spark.network.util.ByteUnit
 
 class UtilsSuite extends SparkFunSuite with ResetSystemProperties with Logging {
 
+  test("truncatedString") {
+    assert(Utils.truncatedString(Nil, "[", ", ", "]", 2) == "[]")
+    assert(Utils.truncatedString(Seq(1, 2), "[", ", ", "]", 2) == "[1, 2]")
+    assert(Utils.truncatedString(Seq(1, 2, 3), "[", ", ", "]", 2) == "[1, ... 2 more fields]")
+    assert(Utils.truncatedString(Seq(1, 2, 3), "[", ", ", "]", -5) == "[, ... 3 more fields]")
+    assert(Utils.truncatedString(Seq(1, 2, 3), ", ") == "1, 2, 3")
+  }
+
   test("timeConversion") {
     // Test -1
     assert(Utils.timeStringAsSeconds("-1") === -1)