rdblue commented on a change in pull request #4301:
URL: https://github.com/apache/iceberg/pull/4301#discussion_r824309140
##########
File path: format/spec.md
##########
@@ -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.
Review comment:
It isn't close to option 1 because the write default can still freely
change. The only thing we need to put behind `alloweIncompatibleChanges` is
modifying the initial default.
This is fine:
```java
table.updateSchema()
.addRequiredColumn("test", IntegerType.get(), 34) // sets the initial
and write defaults to 34
.setColumnDefault("test", 35) // sets the write default to 35
.commit();
```
This is what would expose the initial default and would be a bit confusing:
```java
table.updateSchema()
.addRequiredColumn("test", IntegerType.get(), 34) // sets the initial
and write defaults to 34
.commit();
table.updateSchema()
.allowIncompatibleChanges()
.setInitialColumnDefault("test", 35) // sets the initial default to 35
.commit();
```
I think we could get away without adding `setInitialColumnDefault` because
you can always go drop the column and re-add it if it really is a safe change.
This is equivalent to the above:
```java
table.updateSchema()
.addRequiredColumn("test", IntegerType.get(), 34) // sets the initial
and write defaults to 34
.commit();
table.updateSchema()
.dropColumn("test")
.commit();
table.updateSchema()
.addRequiredColumn("test", IntegerType.get(), 35) // sets the initial
and write defaults to 35
.commit();
```
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