Tishj commented on code in PR #16777:
URL: https://github.com/apache/iceberg/pull/16777#discussion_r3433175361
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format/spec.md:
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@@ -330,7 +331,15 @@ The `initial-default` is set only when a field is added to
an existing schema. T
The `initial-default` and `write-default` produce SQL default value behavior,
without rewriting data files. SQL default value behavior when a field is added
handles all existing rows as though the rows were written with the new field's
default value. Default value changes may only affect future records and all
known fields are written into data files. Omitting a known field when writing a
data file is never allowed. The write default for a field must be written if a
field is not supplied to a write. If the write default for a required field is
not set, the writer must fail.
-All columns of `unknown`, `variant`, `geometry`, and `geography` types must
default to null. Non-null values for `initial-default` or `write-default` are
invalid.
+Starting in v4, a field's `write-default` may be a [value
expression](expressions-spec.md) rather than a literal. This allows defaults
such as `current_timestamp()` to be evaluated when a row is written. A value
expression `write-default` is subject to the following requirements:
+
+* The expression must be a constant or a function application (apply); [field
references](expressions-spec.md#field-reference) (bound or unbound) are not
allowed in a default value expression
+* The expression must produce a value of the field's type, subject to [type
promotion](#schema-evolution)
+* The `write-default` expression is evaluated to populate the field for any
record written after the field was added when the writer does not supply the
field's value
Review Comment:
I think this is missing clarification for STRUCT defaults, what is the
intended evaluation here?
Should fields of type STRUCT not be allowed to have a non-constant
expression? Or are you proposing it can have an expression that evaluates
either to °NULL` or `{}` ?
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