[SPARK-10180][SQL] JDBC datasource are not processing EqualNullSafe filter

This PR is followed by https://github.com/apache/spark/pull/8391.
Previous PR fixes JDBCRDD to support null-safe equality comparison for JDBC datasource. This PR fixes the problem that it can actually return null as a result of the comparison resulting error as using the value of that comparison.

Author: hyukjinkwon <gurwls223@gmail.com>
Author: HyukjinKwon <gurwls223@gmail.com>

Closes #8743 from HyukjinKwon/SPARK-10180.

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/jdbc/JDBCRDD.scala

@@ -193,6 +193,9 @@ private[sql] object JDBCRDD extends Logging {
   private def compileFilter(f: Filter): Option[String] = {
     Option(f match {
       case EqualTo(attr, value) => s"$attr = ${compileValue(value)}"
+      case EqualNullSafe(attr, value) =>
+        s"(NOT ($attr != ${compileValue(value)} OR $attr IS NULL OR " +
+          s"${compileValue(value)} IS NULL) OR ($attr IS NULL AND ${compileValue(value)} IS NULL))"
       case LessThan(attr, value) => s"$attr < ${compileValue(value)}"
       case GreaterThan(attr, value) => s"$attr > ${compileValue(value)}"
       case LessThanOrEqual(attr, value) => s"$attr <= ${compileValue(value)}"
@@ -320,7 +323,6 @@ private[sql] class JDBCRDD(
     if (sb.length == 0) "1" else sb.substring(1)
   }
 
-
   /**
    * `filters`, but as a WHERE clause suitable for injection into a SQL query.
    */

sql/core/src/test/scala/org/apache/spark/sql/jdbc/JDBCSuite.scala

@@ -185,6 +185,7 @@ class JDBCSuite extends SparkFunSuite
     assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE THEID != 2")).collect().size == 2)
     assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE THEID = 1")).collect().size == 1)
     assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE NAME = 'fred'")).collect().size == 1)
+    assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE NAME <=> 'fred'")).collect().size == 1)
     assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE NAME > 'fred'")).collect().size == 2)
     assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE NAME != 'fred'")).collect().size == 2)
     assert(stripSparkFilter(sql("SELECT * FROM foobar WHERE NAME IN ('mary', 'fred')"))
@@ -473,7 +474,9 @@ class JDBCSuite extends SparkFunSuite
       === "(NOT (col1 IN ('mno', 'pqr')))")
     assert(doCompileFilter(IsNull("col1")) === "col1 IS NULL")
     assert(doCompileFilter(IsNotNull("col1")) === "col1 IS NOT NULL")
-    assert(doCompileFilter(And(EqualNullSafe("col0", "abc"), EqualTo("col1", "def"))) === "")
+    assert(doCompileFilter(And(EqualNullSafe("col0", "abc"), EqualTo("col1", "def")))
+      === "((NOT (col0 != 'abc' OR col0 IS NULL OR 'abc' IS NULL) "
+        + "OR (col0 IS NULL AND 'abc' IS NULL))) AND (col1 = 'def')")
   }
 
   test("Dialect unregister") {