rok commented on code in PR #624:
URL: https://github.com/apache/parquet-format/pull/624#discussion_r4087587441
##########
LogicalTypes.md:
##########
@@ -1097,6 +1097,88 @@ optional group my_map (MAP_KEY_VALUE) {
}
```
+### Vectors
+
+`VECTOR` is used to annotate fixed-length ordered sequences of finite and
+non-null elements.
+
+`VectorType` annotation has one required parameter, `num_elements`, which is
+the number of elements in each non-null vector and must be greater than zero.
+
+`VECTOR` must always annotate the canonical 3-level structure:
+
+```
+<vector-repetition> group <name> (VECTOR(<num_elements>)) {
+ repeated group list {
+ required <element-type> element;
+ }
+}
+```
+
+* The outer level must be a group with `logicalType` set to `VECTOR` and
+ `converted_type` set to `LIST`. Its repetition must either be `optional` or
+ `required` and determines whether the vector may be null. It must contain a
+ single field named `list`.
+* The middle level must be a repeated group named `list` with a single field
+ named `element`.
+* The element MUST be a `required` primitive field, individual elements MUST
+ NOT be null. Group elements, including `LIST`, `MAP`, and `VECTOR`, are not
+ allowed.
+
+The element types supported are:
+
+* unannotated `BOOLEAN`, `FLOAT` or `DOUBLE`
+* `INT32` or `INT64`, either unannotated or annotated with `INTEGER` or
+ `DECIMAL`
+* `FIXED_LEN_BYTE_ARRAY` annotated with `FLOAT16` or `DECIMAL`.
+
+The 2-level structures accepted for `LIST` under its backward-compatibility
+rules are not valid for `VECTOR`.
+
+Every numeric element MUST be finite. For `FLOAT`, `DOUBLE`, and `FLOAT16`,
+this excludes NaN, positive infinity, and negative infinity. Whole vectors
+may still be null when the outer group is `optional`.
+
+For example, a nullable vector containing 768 required `FLOAT` elements is:
+
+```
+optional group embedding (VECTOR(768)) {
+ repeated group list {
+ required float element;
+ }
+}
+```
+
+Every non-null vector must contain exactly `num_elements` elements.
+Because `num_elements` is greater than zero, a non-null vector cannot be empty.
+Writers MUST enforce element count, non-null-element, and finite-element
+requirements. Readers MAY rely on these requirements without validating them. A
+reader that detects a different element count, a null element, or a non-finite
+element MUST treat the data as invalid.
+
+For example, with `num_elements = 3` and a `required float element`,
+`[1.0, 2.0, 3.0]` is valid. `[1.0, null, 3.0]`, `[null, null, null]`,
+`[1.0, NaN, 3.0]`, and `[1.0, +Infinity, 3.0]` are invalid. A null vector
+is valid only when the outer group is `optional`.
+
+Definition and repetition levels, value counts, encodings, compression,
+encryption, page boundaries, statistics, column indexes, size statistics, and
+Bloom filters are those of the primitive element column in an ordinary `LIST`.
+Encodings and metadata apply to elements rather than complete vectors.
+`VECTOR` adds no requirement that one vector be contained in a single data
+page.
+
+The sort order of `VECTOR` values is undefined and no statistics are defined
+for whole vectors. Min/max values, `distinct_count`, column-index bounds,
+and Bloom filters retain their ordinary LIST meaning. Bounds and distinct
+counts describe scalar elements. Writers may omit them.
+
Review Comment:
@alamb is this the right amount of detail?
```suggestion
The sort order of `VECTOR` values is undefined and no statistics are defined
for whole vectors.
```
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