danielcweeks commented on code in PR #16777: URL: https://github.com/apache/iceberg/pull/16777#discussion_r4495617471
########## format/spec.md: ########## @@ -372,6 +373,14 @@ 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. +In v4, a field's `write-default` may be a [value expression](expressions-spec.md). 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) 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'm not clear on the exact scenario you have in mind, but this should still be consistent for MSTs. There may be scenarios where the MTS would have to fail if there's an unresolvable conflict, but how the write default is assigned when the record is created. Even in an MST, the default value exists and can be used with the transaction prior to committing the whole MST. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
