etseidl commented on code in PR #466:
URL: https://github.com/apache/parquet-format/pull/466#discussion_r1829600414
##########
LogicalTypes.md:
##########
@@ -648,49 +650,92 @@ optional group my_list (LIST) {
}
```
-Some existing data does not include the inner element layer. For
-backward-compatibility, the type of elements in `LIST`-annotated structures
-should always be determined by the following rules:
+##### 2-level structure
+
+Some existing data does not include the inner element layer, meaning that
`LIST`
+annotates a 2-level structure. In contrast to 3-level structure, the repetition
+of 2-level structure can be `optional`, `required`, or `repeated`.
+
+```
+<list-repetition> group <name> (LIST) {
+ repeated <element-type> <element-name>;
+}
+```
+
+For backward-compatibility, the type of elements in `LIST`-annotated 2-level
+structures should always be determined by the following rules:
1. If the repeated field is not a group, then its type is the element type and
elements are required.
-2. If the repeated field is a group with multiple fields, then its type is the
- element type and elements are required.
-3. If the repeated field is a group with one field and is named either `array`
- or uses the `LIST`-annotated group's name with `_tuple` appended then the
- repeated type is the element type and elements are required.
-4. Otherwise, the repeated field's type is the element type with the repeated
- field's repetition.
+2. If the repeated field is a group with multiple fields, then its type (Struct
+ type with multiple fields) is the element type and elements are required.
+3. If the repeated field is a group with one `required` or `optional` field,
+ and is named either `array` or uses the `LIST`-annotated group's name with
+ `_tuple` appended, then the repeated type (Struct type with single field) is
+ the element type and elements are required.
+4. If the repeated field is a `LIST`-annotated group with one `repeated` field,
+ then the element type is a list type with 2-level structure and elements are
+ required.
+5. Otherwise, the repeated field's type is the element type with the repeated
+ field's repetition. Please note that the repeated field here (a group with
+ one field) cannot be `LIST`-annotated or `MAP`-annotated 3-level structure,
+ as such a group's repetition must be `required` or `optional`.
Examples that can be interpreted using these rules:
```
-// List<Integer> (nullable list, non-null elements)
+// Rule 1: List<Integer> (nullable list, non-null elements)
optional group my_list (LIST) {
repeated int32 element;
}
-// List<Tuple<String, Integer>> (nullable list, non-null elements)
+// Rule 2: List<Struct<String, Integer>> (nullable list, non-null elements)
optional group my_list (LIST) {
repeated group element {
required binary str (STRING);
required int32 num;
};
}
-// List<OneTuple<String>> (nullable list, non-null elements)
+// Rule 3: List<Struct<String>> (nullable list, non-null elements)
optional group my_list (LIST) {
repeated group array {
required binary str (STRING);
};
}
-// List<OneTuple<String>> (nullable list, non-null elements)
+// Rule 3: List<Struct<String>> (nullable list, non-null elements)
optional group my_list (LIST) {
repeated group my_list_tuple {
required binary str (STRING);
};
}
+
+// List<List<Integer>>
+// Rule 4: nullable outer list with non-null elements
+// Rule 1: non-null inner list with non-null elements
+optional group my_list (LIST) {
+ repeated group array (LIST) {
+ repeated int32 array;
+ }
+}
+```
+
+##### 1-level structure without `LIST` annotation
+
+Some existing data does not even have the `LIST` annotation and simply uses
+`repeated` repetition to annotate the element type. In this case, the element
+type MUST be a primitive type and both the list and elements are required.
Review Comment:
https://github.com/apache/parquet-format/pull/17 is an interesting read, and
explains why the unannotated version exists (allows direct conversion from
protobuf). I _would_ prefer there be a single, canonical way to represent
lists, so perhaps we should bring up deprecating the direct form on the ML. I
don't know how relevant it is now (and in fact only learned of its existence
last week).
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