ARROW-2843: [Format] Removing field layout from Schema.fbs breaks bac…

format/Schema.fbs

@@ -211,6 +211,33 @@ union Type {
   Map
 }
 
+/// ----------------------------------------------------------------------
+/// The possible types of a vector
+/// VectorType enum is deprecated and only here for backward compatibility
+
+enum VectorType: short {
+  /// used in List type, Dense Union and variable length primitive types (String, Binary)
+  OFFSET,
+  /// actual data, either wixed width primitive types in slots or variable width delimited by an OFFSET vector
+  DATA,
+  /// Bit vector indicating if each value is null
+  VALIDITY,
+  /// Type vector used in Union type
+  TYPE
+}
+
+/// ----------------------------------------------------------------------
+/// represents the physical layout of a buffer
+/// buffers have fixed width slots of a given type
+/// VectorLayout table is deprecated and only here for backward compatibility
+
+table VectorLayout {
+  /// the width of a slot in the buffer (typically 1, 8, 16, 32 or 64)
+  bit_width: short (deprecated);
+  /// the purpose of the vector
+  type: VectorType (deprecated);
+}
+
 /// ----------------------------------------------------------------------
 /// user defined key value pairs to add custom metadata to arrow
 /// key namespacing is the responsibility of the user
@@ -260,6 +287,11 @@ table Field {
   // children apply only to Nested data types like Struct, List and Union
   children: [Field];
 
+  // layout field is deprecated - is derived from ArrowType
+  // still here for backward compatibility to
+  // deserialize Field serialized with older version
+  layout: [ VectorLayout ] (deprecated);
+
   // User-defined metadata
   custom_metadata: [ KeyValue ];
 }