Schema.fbs

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//
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/// Logical types, vector layouts, and schemas

namespace org.apache.arrow.flatbuf;

enum MetadataVersion:short {
  /// 0.1.0
  V1,

  /// 0.2.0
  V2,

  /// 0.3.0 -> 0.7.1
  V3,

  /// >= 0.8.0
  V4
}

/// These are stored in the flatbuffer in the Type union below

table Null {
}

/// A Struct_ in the flatbuffer metadata is the same as an Arrow Struct
/// (according to the physical memory layout). We used Struct_ here as
/// Struct is a reserved word in Flatbuffers
table Struct_ {
}

table List {
}

table FixedSizeList {
  /// Number of list items per value
  listSize: int;
}

/// A Map is a logical nested type that is represented as
///
/// List<entry: Struct<key: K, value: V>>
///
/// In this layout, the keys and values are each respectively contiguous. We do
/// not constrain the key and value types, so the application is responsible
/// for ensuring that the keys are hashable and unique. Whether the keys are sorted
/// may be set in the metadata for this field
///
/// In a Field with Map type, the Field has a child Struct field, which then
/// has two children: key type and the second the value type. The names of the
/// child fields may be respectively "entry", "key", and "value", but this is
/// not enforced
///
/// Map
///   - child[0] entry: Struct
///     - child[0] key: K
///     - child[1] value: V
///
/// Neither the "entry" field nor the "key" field may be nullable.
///
/// The metadata is structured so that Arrow systems without special handling
/// for Map can make Map an alias for List. The "layout" attribute for the Map
/// field must have the same contents as a List.
table Map {
  /// Set to true if the keys within each value are sorted
  keysSorted: bool;
}

enum UnionMode:short { Sparse, Dense }

/// A union is a complex type with children in Field
/// By default ids in the type vector refer to the offsets in the children
/// optionally typeIds provides an indirection between the child offset and the type id
/// for each child typeIds[offset] is the id used in the type vector
table Union {
  mode: UnionMode;
  typeIds: [ int ]; // optional, describes typeid of each child.
}

table Int {
  bitWidth: int; // restricted to 8, 16, 32, and 64 in v1
  is_signed: bool;
}

enum Precision:short {HALF, SINGLE, DOUBLE}

table FloatingPoint {
  precision: Precision;
}

/// Unicode with UTF-8 encoding
table Utf8 {
}

table Binary {
}

table FixedSizeBinary {
  /// Number of bytes per value
  byteWidth: int;
}

table Bool {
}

table Decimal {
  /// Total number of decimal digits
  precision: int;
  /// Number of digits after the decimal point "."
  scale: int;
}

enum DateUnit: short {
  DAY,
  MILLISECOND
}

/// Date is either a 32-bit or 64-bit type representing elapsed time since UNIX
/// epoch (1970-01-01), stored in either of two units:
///
/// * Milliseconds (64 bits) indicating UNIX time elapsed since the epoch (no
///   leap seconds), where the values are evenly divisible by 86400000
/// * Days (32 bits) since the UNIX epoch
table Date {
  unit: DateUnit = MILLISECOND;
}

enum TimeUnit: short { SECOND, MILLISECOND, MICROSECOND, NANOSECOND }

/// Time type. The physical storage type depends on the unit
/// - SECOND and MILLISECOND: 32 bits
/// - MICROSECOND and NANOSECOND: 64 bits
table Time {
  unit: TimeUnit = MILLISECOND;
  bitWidth: int = 32;
}

/// Time elapsed from the Unix epoch, 00:00:00.000 on 1 January 1970, excluding
/// leap seconds, as a 64-bit integer. Note that UNIX time does not include
/// leap seconds.
///
/// The Timestamp metadata supports both "time zone naive" and "time zone
/// aware" timestamps. Read about the timezone attribute for more detail
table Timestamp {
  unit: TimeUnit;

  /// The time zone is a string indicating the name of a time zone, one of:
  ///
  /// * As used in the Olson time zone database (the "tz database" or
  ///   "tzdata"), such as "America/New_York"
  /// * An absolute time zone offset of the form +XX:XX or -XX:XX, such as +07:30
  ///
  /// Whether a timezone string is present indicates different semantics about
  /// the data:
  ///
  /// * If the time zone is null or equal to an empty string, the data is "time
  ///   zone naive" and shall be displayed *as is* to the user, not localized
  ///   to the locale of the user. This data can be though of as UTC but
  ///   without having "UTC" as the time zone, it is not considered to be
  ///   localized to any time zone
  ///
  /// * If the time zone is set to a valid value, values can be displayed as
  ///   "localized" to that time zone, even though the underlying 64-bit
  ///   integers are identical to the same data stored in UTC. Converting
  ///   between time zones is a metadata-only operation and does not change the
  ///   underlying values
  timezone: string;
}

enum IntervalUnit: short { YEAR_MONTH, DAY_TIME}
table Interval {
  unit: IntervalUnit;
}

/// ----------------------------------------------------------------------
/// Top-level Type value, enabling extensible type-specific metadata. We can
/// add new logical types to Type without breaking backwards compatibility

union Type {
  Null,
  Int,
  FloatingPoint,
  Binary,
  Utf8,
  Bool,
  Decimal,
  Date,
  Time,
  Timestamp,
  Interval,
  List,
  Struct_,
  Union,
  FixedSizeBinary,
  FixedSizeList,
  Map
}

/// ----------------------------------------------------------------------
/// user defined key value pairs to add custom metadata to arrow
/// key namespacing is the responsibility of the user

table KeyValue {
  key: string;
  value: string;
}

/// ----------------------------------------------------------------------
/// Dictionary encoding metadata

table DictionaryEncoding {
  /// The known dictionary id in the application where this data is used. In
  /// the file or streaming formats, the dictionary ids are found in the
  /// DictionaryBatch messages
  id: long;

  /// The dictionary indices are constrained to be positive integers. If this
  /// field is null, the indices must be signed int32
  indexType: Int;

  /// By default, dictionaries are not ordered, or the order does not have
  /// semantic meaning. In some statistical, applications, dictionary-encoding
  /// is used to represent ordered categorical data, and we provide a way to
  /// preserve that metadata here
  isOrdered: bool;
}

/// ----------------------------------------------------------------------
/// A field represents a named column in a record / row batch or child of a
/// nested type.
///
/// - children is only for nested Arrow arrays
/// - For primitive types, children will have length 0
/// - nullable should default to true in general

table Field {
  // Name is not required, in i.e. a List
  name: string;
  nullable: bool;
  type: Type;

  // Present only if the field is dictionary encoded
  dictionary: DictionaryEncoding;

  // children apply only to Nested data types like Struct, List and Union
  children: [Field];

  // User-defined metadata
  custom_metadata: [ KeyValue ];
}

/// ----------------------------------------------------------------------
/// Endianness of the platform producing the data

enum Endianness:short { Little, Big }

/// ----------------------------------------------------------------------
/// A Buffer represents a single contiguous memory segment
struct Buffer {
  /// The relative offset into the shared memory page where the bytes for this
  /// buffer starts
  offset: long;

  /// The absolute length (in bytes) of the memory buffer. The memory is found
  /// from offset (inclusive) to offset + length (non-inclusive).
  length: long;
}

/// ----------------------------------------------------------------------
/// A Schema describes the columns in a row batch

table Schema {

  /// endianness of the buffer
  /// it is Little Endian by default
  /// if endianness doesn't match the underlying system then the vectors need to be converted
  endianness: Endianness=Little;

  fields: [Field];
  // User-defined metadata
  custom_metadata: [ KeyValue ];
}

root_type Schema;