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new c7cb964 GH-468: Clarify MAP logical type (#469)
c7cb964 is described below
commit c7cb964d5e215ab3d4b68219bf3a86f39a1d7a0d
Author: Gabor Szadovszky <[email protected]>
AuthorDate: Tue Nov 12 16:12:59 2024 +0100
GH-468: Clarify MAP logical type (#469)
---
LogicalTypes.md | 13 +++++++++----
1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/LogicalTypes.md b/LogicalTypes.md
index 3aa5ceb..7b4b203 100644
--- a/LogicalTypes.md
+++ b/LogicalTypes.md
@@ -745,13 +745,17 @@ to values. `MAP` must annotate a 3-level structure:
* The outer-most level must be a group annotated with `MAP` that contains a
single field named `key_value`. The repetition of this level must be either
- `optional` or `required` and determines whether the list is nullable.
+ `optional` or `required` and determines whether the map is nullable.
* The middle level, named `key_value`, must be a repeated group with a `key`
- field for map keys and, optionally, a `value` field for map values.
+ field for map keys and, optionally, a `value` field for map values. It must
+ not contain any other values.
* The `key` field encodes the map's key type. This field must have
- repetition `required` and must always be present.
+ repetition `required` and must always be present. It must always be the first
+ field of the repeated `key_value` group.
* The `value` field encodes the map's value type and repetition. This field can
- be `required`, `optional`, or omitted.
+ be `required`, `optional`, or omitted. It must always be the second field of
+ the repeated `key_value` group if present. In case of not present, it can be
+ represented as a map with all null values or as a set of keys.
The following example demonstrates the type for a non-null map from strings to
nullable integers:
@@ -777,6 +781,7 @@ keys.
It is required that the repeated group of key-value pairs is named `key_value`
and that its fields are named `key` and `value`. However, these names may not
be used in existing data and should not be enforced as errors when reading.
+(`key` and `value` can be identified by their position in case of misnaming.)
Some existing data incorrectly used `MAP_KEY_VALUE` in place of `MAP`. For
backward-compatibility, a group annotated with `MAP_KEY_VALUE` that is not
