[SPARK-12928][SQL] Oracle FLOAT datatype is not properly handled when reading via JDBC

The contribution is my original work and that I license the work to the project under the project's open source license.

Author: poolis <gmichalopoulos@gmail.com>
Author: Greg Michalopoulos <gmichalopoulos@gmail.com>

Closes #10899 from poolis/spark-12928.

sql/core/src/main/scala/org/apache/spark/sql/jdbc/OracleDialect.scala

@@ -38,6 +38,12 @@ private case object OracleDialect extends JdbcDialect {
       // This is sub-optimal as we have to pick a precision/scale in advance whereas the data
       //  in Oracle is allowed to have different precision/scale for each value.
       Option(DecimalType(DecimalType.MAX_PRECISION, 10))
+    } else if (sqlType == Types.NUMERIC && md.build().getLong("scale") == -127) {
+      // Handle FLOAT fields in a special way because JDBC ResultSetMetaData converts
+      // this to NUMERIC with -127 scale
+      // Not sure if there is a more robust way to identify the field as a float (or other
+      // numeric types that do not specify a scale.
+      Option(DecimalType(DecimalType.MAX_PRECISION, 10))
     } else {
       None
     }

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

@@ -600,6 +600,15 @@ class JDBCSuite extends SparkFunSuite
     assert(derbyDialect.getJDBCType(BooleanType).map(_.databaseTypeDefinition).get == "BOOLEAN")
   }
 
+  test("OracleDialect jdbc type mapping") {
+    val oracleDialect = JdbcDialects.get("jdbc:oracle")
+    val metadata = new MetadataBuilder().putString("name", "test_column").putLong("scale", -127)
+    assert(oracleDialect.getCatalystType(java.sql.Types.NUMERIC, "float", 1, metadata) ==
+      Some(DecimalType(DecimalType.MAX_PRECISION, 10)))
+    assert(oracleDialect.getCatalystType(java.sql.Types.NUMERIC, "numeric", 0, null) ==
+      Some(DecimalType(DecimalType.MAX_PRECISION, 10)))
+  }
+
   test("table exists query by jdbc dialect") {
     val MySQL = JdbcDialects.get("jdbc:mysql://127.0.0.1/db")
     val Postgres = JdbcDialects.get("jdbc:postgresql://127.0.0.1/db")