alkis commented on code in PR #585:
URL: https://github.com/apache/parquet-format/pull/585#discussion_r3561789828
##########
LogicalTypes.md:
##########
@@ -635,7 +635,133 @@ The type has two type parameters:
The sort order used for `GEOGRAPHY` is undefined. When writing data, no min/max
statistics should be saved for this type and if such non-compliant statistics
-are found during reading, they must be ignored.
+are found during reading, they must be ignored.
+
+### FILE
+
+`FILE` annotates a group that represents a reference to a range of bytes,
which may
+be stored inline in the value, elsewhere within this file, or in an external
file. It
+is intended for use cases such as storing file inventories, manifests, and
unstructured
+data references (e.g., images or audio files stored in object storage).
+
+The annotated group may contain the following fields, identified by name.
Field IDs
+may also be used for projection. All fields are optional:
+
+| Field | Type | Required |
+|----------------|------------|----------|
+| `path` | STRING | No |
+| `offset` | INT64 | No |
+| `size` | INT64 | No |
+| `content_type` | STRING | No |
+| `checksum` | STRING | No |
+| `inline` | BYTE_ARRAY | No |
+
+A value resolves to bytes determined by `inline` / `path` / `offset` / `size`;
+`content_type` and `checksum` are metadata describing whatever is resolved.
+
+#### Fields
+
+##### path
+
+An opaque path string to an external file (e.g., `s3://bucket/file.jpg`). No
special
+encoding (e.g., URI encoding) is applied on top of the user provided data. If
`path` is absent,
+the value refers to this file (a self-reference).
+
+##### offset
+
+A byte offset indicating the start of the byte range within the referenced
data.
+If not provided, readers must treat the value as 0.
+If provided and non-zero, readers must seek to this offset to retrieve the
referenced data.
+
+##### size
+
+The byte length of the referenced data. Must be zero or a positive integer if
provided.
+A value of 0 indicates empty referenced data. If not provided, the range runs
to the end
+of the referenced data.
+
+##### content_type
+
+The media type (MIME type) of the resolved bytes (e.g., `image/png`).
+
+##### checksum
+
+A self-describing integrity token for the resolved bytes, of the form
+`<algorithm>:base64(<digest bytes>)`. It generalizes the storage-system eTag.
The
+recognized algorithms are:
+
+| Algorithm | Notes |
+|-----------|----------------------------------------------------------------|
+| `ETAG` | the object-store eTag — equality-only, not recomputable |
+| `MD5` | the usual S3/HTTP eTag and Content-MD5 |
+| `CRC32` | Parquet's page-checksum algorithm (gzip/zlib) |
+| `CRC32C` | common in object stores, hardware-accelerated |
+| `SHA-256` | e.g. S3 additional checksums |
+
+`checksum` applies to the resolved bytes, except for `ETAG`, which is the
+object-store eTag for the whole file referenced by `path`.
+
+##### inline
+
+The referenced bytes stored inline in the value. If `inline` is set, it
supplies the
+bytes and any locator fields (`path`, `offset`, `size`) that are present are
provenance
+only.
+
+#### Resolution
+
+A value resolves to bytes based on which of `inline`, `path`, `offset`, and
`size` are
+set:
+
+| `inline` | `path` | `offset` | `size` | Resolves to
|
+|----------|--------|----------|--------|----------------------------------------------|
+| set | – | – | – | the inline bytes
|
+| – | set | – | – | whole external file at `path`
|
+| – | set | set | – | external `path`, `[offset, EOF)`
|
+| – | set | – | set | external `path`, `[0, size)`
|
+| – | set | set | set | external `path`, `[offset, offset +
size)` |
+| – | – | set | – | this file, `[offset, EOF)`
(self-reference) |
+| – | – | – | set | this file, `[0, size)`
(self-reference) |
+| – | – | set | set | this file, `[offset, offset + size)`
(self-reference) |
+| – | – | – | – | nothing — invalid
|
+
+A self-reference typically points within the same Parquet file using `offset`
and
+`size`; the bytes are written between column chunks and are not otherwise
referenced by
+the footer. A self-reference is the absence of `path`, never an absolute path
back to
+the current file, so a file containing self-references is renamed or relocated
as a
+single unit.
+
+The referenced bytes are compressed with the same `CompressionCodec` as the one
+specified for the `inline` column.
Review Comment:
The reason is because we are making the assumption that the type of data
stored in the file column is homogeneous. I argue that's a good assumption. If
this column contains text, there is little reason to assume it will be a string
when small and an image when large. Ergo if the writer chose to compress the
column it should compress the self-referenced packed blobs too.
I wanted to make all packed references compressed as well but there was some
pushback for that with the argument that external may be referenced by
different parquet files and it may choose its own compression scheme (or none
at all). I can see arguments both ways - it is a tradeoff.
Making all packed references inherit the CompressionCodec makes the spec
more consistent.
Making only self-references inherit the CompressionCodec is friendlier to
shared external references.
We pick one of the two, it is a tradeoff.
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