[SPARK-20431][SS][FOLLOWUP] Specify a schema by using a DDL-formatted string in DataStreamReader

## What changes were proposed in this pull request?

This pr supported a DDL-formatted string in `DataStreamReader.schema`.
This fix could make users easily define a schema without importing the type classes.

For example,

```scala
scala> spark.readStream.schema("col0 INT, col1 DOUBLE").load("/tmp/abc").printSchema()
root
 |-- col0: integer (nullable = true)
 |-- col1: double (nullable = true)
```

## How was this patch tested?

Added tests in `DataStreamReaderWriterSuite`.

Author: hyukjinkwon <[email protected]>

Closes #18373 from HyukjinKwon/SPARK-20431.

Author: HyukjinKwon

python/pyspark/sql/readwriter.py

@@ -98,6 +98,8 @@ def schema(self, schema):
 
         :param schema: a :class:`pyspark.sql.types.StructType` object or a DDL-formatted string
                        (For example ``col0 INT, col1 DOUBLE``).
+
+        >>> s = spark.read.schema("col0 INT, col1 DOUBLE")
         """
         from pyspark.sql import SparkSession
         spark = SparkSession.builder.getOrCreate()

python/pyspark/sql/streaming.py

@@ -319,16 +319,21 @@ def schema(self, schema):
 
         .. note:: Evolving.
 
-        :param schema: a :class:`pyspark.sql.types.StructType` object
+        :param schema: a :class:`pyspark.sql.types.StructType` object or a DDL-formatted string
+                       (For example ``col0 INT, col1 DOUBLE``).
 
         >>> s = spark.readStream.schema(sdf_schema)
+        >>> s = spark.readStream.schema("col0 INT, col1 DOUBLE")
         """
         from pyspark.sql import SparkSession
-        if not isinstance(schema, StructType):
-            raise TypeError("schema should be StructType")
         spark = SparkSession.builder.getOrCreate()
-        jschema = spark._jsparkSession.parseDataType(schema.json())
-        self._jreader = self._jreader.schema(jschema)
+        if isinstance(schema, StructType):
+            jschema = spark._jsparkSession.parseDataType(schema.json())
+            self._jreader = self._jreader.schema(jschema)
+        elif isinstance(schema, basestring):
+            self._jreader = self._jreader.schema(schema)
+        else:
+            raise TypeError("schema should be StructType or string")
         return self
 
     @since(2.0)
@@ -372,7 +377,8 @@ def load(self, path=None, format=None, schema=None, **options):
 
         :param path: optional string for file-system backed data sources.
         :param format: optional string for format of the data source. Default to 'parquet'.
-        :param schema: optional :class:`pyspark.sql.types.StructType` for the input schema.
+        :param schema: optional :class:`pyspark.sql.types.StructType` for the input schema
+                       or a DDL-formatted string (For example ``col0 INT, col1 DOUBLE``).
         :param options: all other string options
 
         >>> json_sdf = spark.readStream.format("json") \\
@@ -415,7 +421,8 @@ def json(self, path, schema=None, primitivesAsString=None, prefersDecimal=None,
 
         :param path: string represents path to the JSON dataset,
                      or RDD of Strings storing JSON objects.
-        :param schema: an optional :class:`pyspark.sql.types.StructType` for the input schema.
+        :param schema: an optional :class:`pyspark.sql.types.StructType` for the input schema
+                       or a DDL-formatted string (For example ``col0 INT, col1 DOUBLE``).
         :param primitivesAsString: infers all primitive values as a string type. If None is set,
                                    it uses the default value, ``false``.
         :param prefersDecimal: infers all floating-point values as a decimal type. If the values
@@ -542,7 +549,8 @@ def csv(self, path, schema=None, sep=None, encoding=None, quote=None, escape=Non
         .. note:: Evolving.
 
         :param path: string, or list of strings, for input path(s).
-        :param schema: an optional :class:`pyspark.sql.types.StructType` for the input schema.
+        :param schema: an optional :class:`pyspark.sql.types.StructType` for the input schema
+                       or a DDL-formatted string (For example ``col0 INT, col1 DOUBLE``).
         :param sep: sets the single character as a separator for each field and value.
                     If None is set, it uses the default value, ``,``.
         :param encoding: decodes the CSV files by the given encoding type. If None is set,

sql/core/src/main/scala/org/apache/spark/sql/streaming/DataStreamReader.scala

@@ -59,6 +59,18 @@ final class DataStreamReader private[sql](sparkSession: SparkSession) extends Lo
     this
   }
 
+  /**
+   * Specifies the schema by using the input DDL-formatted string. Some data sources (e.g. JSON) can
+   * infer the input schema automatically from data. By specifying the schema here, the underlying
+   * data source can skip the schema inference step, and thus speed up data loading.
+   *
+   * @since 2.3.0
+   */
+  def schema(schemaString: String): DataStreamReader = {
+    this.userSpecifiedSchema = Option(StructType.fromDDL(schemaString))
+    this
+  }
+
   /**
    * Adds an input option for the underlying data source.
    *

sql/core/src/test/scala/org/apache/spark/sql/streaming/test/DataStreamReaderWriterSuite.scala

@@ -663,4 +663,16 @@ class DataStreamReaderWriterSuite extends StreamTest with BeforeAndAfter {
     }
     assert(fs.exists(checkpointDir))
   }
+
+  test("SPARK-20431: Specify a schema by using a DDL-formatted string") {
+    spark.readStream
+      .format("org.apache.spark.sql.streaming.test")
+      .schema("aa INT")
+      .load()
+
+    assert(LastOptions.schema.isDefined)
+    assert(LastOptions.schema.get === StructType(StructField("aa", IntegerType) :: Nil))
+
+    LastOptions.clear()
+  }
 }