[iceberg] 01/01: Docs: Update wording around nullability

[et...@apache.org]

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etudenhoefner pushed a commit to branch spark-ddl-nullability
in repository https://gitbox.apache.org/repos/asf/iceberg.git

commit 913c05666be73ecdaf1edb6f1b62c4a2c1dd7f1f
Author: Eduard Tudenhoefner <et...@gmail.com>
AuthorDate: Tue Jul 18 09:36:31 2023 +0200

    Docs: Update wording around nullability
---
 docs/spark-ddl.md | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/docs/spark-ddl.md b/docs/spark-ddl.md
index 6c80b5b2bc..282aa938c8 100644
--- a/docs/spark-ddl.md
+++ b/docs/spark-ddl.md
@@ -306,12 +306,17 @@ ALTER TABLE prod.db.sample ALTER COLUMN col FIRST
 ALTER TABLE prod.db.sample ALTER COLUMN nested.col AFTER other_col
 ```
 
-Nullability can be changed using `SET NOT NULL` and `DROP NOT NULL`:
+Nullability for a non-nullable column can be changed using `DROP NOT NULL`:
 
 ```sql
 ALTER TABLE prod.db.sample ALTER COLUMN id DROP NOT NULL
 ```
 
+{{< hint info >}}
+It is not possible to change a nullable column to non-nullable with `SET NOT NULL` because Iceberg doesn't know whether there is existing data with null values.
+{{< /hint >}}
+
+
 {{< hint info >}}
 `ALTER COLUMN` is not used to update `struct` types. Use `ADD COLUMN` and `DROP COLUMN` to add or remove struct fields.
 {{< /hint >}}
@@ -526,4 +531,4 @@ Tags can be removed via the `DROP TAG` sql
 
 ```sql
 ALTER TABLE prod.db.sample DROP TAG historical-tag
-```
\ No newline at end of file
+```