rdblue commented on code in PR #461:
URL: https://github.com/apache/parquet-format/pull/461#discussion_r1859084567
##########
VariantEncoding.md:
##########
@@ -39,13 +39,41 @@ 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.
+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 is required for unshredded Variant values.
+* The `value` field is optional when parts of the Variant value are shredded
according to the [Variant Shredding specification](VariantShredding.md).
+* 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;
+ required binary value;
+}
+```
+
+This is an example representation of a shredded Variant in Parquet:
+```
+optional group shredded_variant_name (VARIANT) {
+ required binary metadata;
+ optional binary value;
+ optional int64 typed_value;
+}
+```
+
+The `VARIANT` annotation places no additional restrictions on the repetition
of Variant groups, but repetition may be restricted by containing types (such
as `MAP` and `LIST`).
Review Comment:
I don't agree that it is considered a primitive type. And we don't need to
in order to state that it places no additional restrictions on the repetition
of Variant groups.
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