[GitHub] [parquet-format] pitrou commented on a change in pull request #164: PARQUET-1950: Define core features

[GitBox <gi...@apache.org>]

pitrou commented on a change in pull request #164:
URL: https://github.com/apache/parquet-format/pull/164#discussion_r559639264



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File path: CoreFeatures.md
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+
+# Parquet Core Features
+
+This document lists the core features for each parquet-format release. This
+list is a subset of the features which parquet-format makes available.
+
+## Purpose
+
+The list of core features for a certian release makes a compliance level that
+the different implementations can tied to. If an implementation claims that it
+provides the functionality of a parquet-format release core features it must
+implement all of the listed features according the specification (both read and
+write path). This way it is easier to ensure compatibility between the
+different parquet implementations.
+We cannot and don't want to stop our clients to use any features that are not
+on this list but it shall be highlighted that using these features might make
+the written parquet files unreadable by other implementations. We can say that
+the features available in a parquet-format release (and one of the
+implementations of it) and not on this list are experimental.
+
+## Versioning
+
+This document is versioned by the parquet-format releases which follows the
+scheme of semantic versioning. It means that no feature will be deleted from
+this document under the same major version. (We might deprecate some, though.)
+Because of the semantic versioning if one implementation supports the core
+features of the parquet-format release `a.b.x` it must be able to read any
+parquet files written by implementations supporting the release `a.d.y` where
+`b >= d`.
+
+If a parquet file is written according to a released version of this document
+it might be a good idea to write this version into the field `compliance_level`
+in the thrift object `FileMetaData`.
+
+## Adding new features
+
+The idea is to only include features which are specified correctly and proven
+to be useful for everyone. Because of that we require to have at least two
+different implementations that are released and widely tested.
+
+## The "list"
+
+This list is based on the [parquet thrift file](src/main/thrift/parquet.thrift)
+where all the data structures we might use in a parquet file are defined.
+
+### File structure
+
+All of the required fields in the structure (and sub-structures) of
+`FileMetaData` must be set according to the specification.
+The following page types are supported:
+* Data page V1 (see `DataPageHeader`)
+* Dictionary page (see `DictionaryPageHeader`)
+
+**TODO**: list optional fields that must be filled properly.
+
+### Types
+
+#### Primitive types
+
+The following [primitive types](README.md#types) are supported
+* `BOOLEAN`
+* `INT32`
+* `INT64`
+* `FLOAT`
+* `DOUBLE`
+* `BYTE\_ARRAY`
+* `FIXED\_LEN\_BYTE\_ARRAY`
+
+NOTE: The primitive type `INT96` is deprecated so it is intentionally not listed
+here.
+
+#### Logical types
+
+The [logical type](LogicalTypes.md)s are practically annotations helping to
+understand the related primitive type (or structure). Originally we have had
+the `ConvertedType` enum in the thrift file representing all the possible
+logical types. After a while we realized it is hard to extend and so introduced
+the `LogicalType` union. For backward compatibility reasons we allow to use the
+old `ConvertedType` values according to the specified rules but we expect that
+the logical types in the file schema are defined with `LogicalType` objects.
+
+The following LogicalTypes are supported:
+* `STRING`
+* `MAP`
+* `LIST`
+* `ENUM`
+* `DECIMAL` (for which primitives?)
+* `DATE`
+* `TIME`: **(Which unit, utc?)**
+* `TIMESTAMP`: **(Which unit, utc?)**
+* `INTEGER`: (all bitwidth 8, 16, 32, 64) **(unsigned?)**
+* `UNKNOWN` **(?)**
+* `JSON` **(?)**
+* `BSON` **(?)**
+* `UUID` **(?)**
+
+NOTE: The old ConvertedType `INTERVAL` has no representation in LogicalTypes.
+This is becasue `INTERVAL` is deprecated so we do not include it in this list.
+
+### Encodings
+
+The following encodings are supported:
+* [PLAIN](Encodings.md#plain-plain--0)  
+  parquet-mr: Basically all value types are written in this encoding in case of
+  V1 pages
+* [PLAIN\_DICTIONARY](Encodings.md#dictionary-encoding-plain_dictionary--2-and-rle_dictionary--8)
+  **(?)**  
+  parquet-mr: As per the spec this encoding is deprecated while we still use it
+  for V1 page dictionaries.
+* [RLE](Encodings.md#run-length-encoding--bit-packing-hybrid-rle--3)  
+  parquet-mr: Used for both V1 and V2 pages to encode RL and DL and for BOOLEAN
+  values in case of V2 pages
+* [DELTA\_BINARY\_PACKED](Encodings.md#delta-encoding-delta_binary_packed--5)
+  **(?)**  
+  parquet-mr: Used for V2 pages to encode INT32 and INT64 values.
+* [DELTA\_LENGTH\_BYTE\_ARRAY](Encodings.md#delta-length-byte-array-delta_length_byte_array--6)
+  **(?)**  
+  parquet-mr: Not used directly
+* [DELTA\_BYTE\_ARRAY](Encodings.md#delta-strings-delta_byte_array--7)
+  **(?)**  
+  parquet-mr: Used for V2 pages to encode BYTE\_ARRAY and
+  FIXED\_LEN\_BYTE\_ARRAY values
+* [RLE\_DICTIONARY](Encodings.md#dictionary-encoding-plain_dictionary--2-and-rle_dictionary--8)
+  **(?)**  
+  parquet-mr: Used for V2 page dictionaries
+* [BYTE\_STREAM\_SPLIT](Encodings.md#byte-stream-split-byte_stream_split--9)
+  **(?)**  
+  parquet-mr: Not used by default; can be used only via explicit configuration
+
+NOTE: [BIT\_PACKED](Encodings.md#bit-packed-deprecated-bit_packed--4) is
+deprecated and not used directly (boolean values are encoded with this under
+PLAIN) so not included in this list.
+
+**TODO**: In parquet-mr dictionary encoding is not enabled for
+FIXED\_LEN\_BYTE\_ARRAY in case of writing V1 pages. I don't know the reason
+behind. Any experience/idea about this from other implementations?
+
+### Compression
+
+The following compression algorithms are supported (including `UNCOMPRESSED`).
+* `SNAPPY`
+* `GZIP`
+* `LZO` **(?)**
+* `BROTLI` **(?)**
+* `LZ4` **(?)**
+* `ZSTD` **(?)**

Review comment:
       For the record, we (meaning @chairmank) discovered an incompatibility in the LZ4 compatibility code that was added to Parquet C++ to handle Hadoop-produced files: see https://issues.apache.org/jira/browse/ARROW-11301




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