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

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gszadovszky commented on a change in pull request #164:
URL: https://github.com/apache/parquet-format/pull/164#discussion_r567688108



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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 certain release makes a compliance level for
+implementations. If a writer implementation claims that it is at a certain
+compliance level then it must use only features from the *core feature list* of
+that parquet-format release. If a reader implementation claims the same if must
+implement all of the listed features. 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 the *core feature 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. We also require
+to implement interoperability tests for that feature to prove one
+implementation can read the data written by the other one and vice versa.
+
+## Core feature 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

Review comment:
       The parts describing how the parquet-mr implementations work were not meant to be part of the final document. As I don't know too much about other implementations I've felt it is good to note the current behavior so others may understand the situation better. After a consensus of the related topic made I would remove this note.




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