diff --git a/docs/sql-migration-guide.md b/docs/sql-migration-guide.md
index b52a57acdd7b..8c5721340a30 100644
--- a/docs/sql-migration-guide.md
+++ b/docs/sql-migration-guide.md
@@ -213,7 +213,9 @@ license: |
     - `yesterday [zoneId]` - midnight yesterday
     - `tomorrow [zoneId]` - midnight tomorrow
     - `now` - current query start time
-  For example `SELECT timestamp 'tomorrow';`. 
+  For example `SELECT timestamp 'tomorrow';`.
+
+  - Since Spark 3.0, the `size` function returns `NULL` for the `NULL` input. In Spark version 2.4 and earlier, this function gives `-1` for the same input. To restore the behavior before Spark 3.0, you can set `spark.sql.legacy.sizeOfNull` to `true`.
 
 ## Upgrading from Spark SQL 2.4 to 2.4.1