cashmand commented on code in PR #461:
URL: https://github.com/apache/parquet-format/pull/461#discussion_r1913567140
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VariantEncoding.md:
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@@ -39,13 +39,42 @@ Another motivation for the representation is that (aside
from metadata) each nes
For example, in a Variant containing an Array of Variant values, the
representation of an inner Variant value, when paired with the metadata of the
full variant, is itself a valid Variant.
This document describes the Variant Binary Encoding scheme.
-[VariantShredding.md](VariantShredding.md) describes the details of the
Variant shredding scheme.
+Variant fields can also be _shredded_.
+Shredding refers to extracting some elements of the variant into separate
columns for more efficient extraction/filter pushdown.
+The [Variant Shredding specification](VariantShredding.md) describes the
details of shredding Variant values as typed Parquet columns.
+
+## Variant in Parquet
-# Variant in Parquet
A Variant value in Parquet is represented by a group with 2 fields, named
`value` and `metadata`.
-Both fields `value` and `metadata` are of type `binary`, and cannot be `null`.
-# Metadata encoding
+* The Variant group must be annotated with the `VARIANT` logical type.
+* Both fields `value` and `metadata` must be of type `binary` (called
`BYTE_ARRAY` in the Parquet thrift definition).
+* The `metadata` field is `required` and must be a valid Variant metadata, as
defined below.
+* The `value` field must be annotated as `required` for unshredded Variant
values, or `optional` if parts of the value are [shredded](VariantShredding.md)
as typed Parquet columns.
+* When present, the `value` field must be a valid Variant value, as defined
below.
+
+This is the expected unshredded representation in Parquet:
+
+```
+optional group variant_name (VARIANT) {
+ required binary metadata;
Review Comment:
Hi @Zouxxyy, in the Spark shredding PRs I've been working on, I put metadata
first. I didn't see much benefit to changing the order in the existing
non-shredded code, but I don't feel too strongly about it either way. The spec
is pretty clear that readers should identify the appropriate columns based on
field names, not field order, and I think things could become quite fragile if
they did rely on field order.
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