rdblue commented on a change in pull request #4301:
URL: https://github.com/apache/iceberg/pull/4301#discussion_r823202271
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File path: format/spec.md
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@@ -193,10 +193,38 @@ Notes:
For details on how to serialize a schema to JSON, see Appendix C.
+#### Default value
+Default values can be assigned to top-level columns or nested fields. Default
values are used during schema evolution when adding a new column. The default
value is used to read rows belonging to the files that lack the column or
nested field prior to the schema evolution.
+
+Currently, when a default value for a column or nested field is set, it is
considered an incompatible change to change it to another value. However,
changing default values is allowed when calling the `allowIncompatibleChanges`
API explicitly. Changing default values is discouraged since the occurrence of
a rewrite may determine whether the new or old default value is returned during
the read.
+
+Default values are encoded in JSON format. The representation depends on the
type of the corresponding field. The mapping of types and their corresponding
default value JSON representation is described in the following table.
+
+| type | json type | example
| note
|
+|--------------------|---------------|----------------------------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| **`boolean`** | **`boolean`** | true
|
|
+| **`int`** | **`number`** | 1
|
|
Review comment:
In most of the appendix, we are more specific about the type even though
JSON doesn't distinguish. That is so that people know what to convert to and
from. Prefer using "JSON int" or "JSON long" instead of simply "number".
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