rdblue commented on a change in pull request #4301:
URL: https://github.com/apache/iceberg/pull/4301#discussion_r823223556
##########
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:
Say I have a table with schema `(id bigint NOT NULL, data string DEFAULT
"--")`. Then I can write to that table and omit the `data` column:
```
INSERT INTO table VALUES (1), (2), (3);
```
The writer must actually insert `(1, "--"), (2, "--"), (3, "--)`.
This is the row with `id=3` in the behavior test:
```sql
create table default_test (id int);
insert into default_test values (1), (2);
alter table default_test add column data int default 0;
alter table default_test alter column data set default 1000;
insert into default_test (id) values (3);
insert into default_test values (4, null);
alter table default_test alter column data set default 2000;
```
```sql
select * from default_test;
```
```
id | data
-- | --
1 | 0
2 | 0
3 | 1000
4 | (null)
```
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