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new f2cbc7803c18 [SPARK-57643][DOCS] Document MERGE INTO schema evolution
f2cbc7803c18 is described below
commit f2cbc7803c18648130db862cd85581a57c1dad98
Author: Szehon Ho <[email protected]>
AuthorDate: Tue Jun 23 20:28:39 2026 -0700
[SPARK-57643][DOCS] Document MERGE INTO schema evolution
### What changes were proposed in this pull request?
This PR documents `MERGE WITH SCHEMA EVOLUTION` in the MERGE INTO SQL
reference (`docs/sql-ref-syntax-dml-merge-into.md`), mirroring the existing
INSERT INTO schema evolution documentation. Specifically it:
- Adds `[ WITH SCHEMA EVOLUTION ]` to the **Syntax** and a `WITH SCHEMA
EVOLUTION` entry to the **Parameters**.
- Adds notes describing what triggers schema evolution: only columns
referenced by a direct assignment are added/widened; `UPDATE SET *` / `INSERT
*` count as direct assignments of every target column; and directly assigning a
struct column counts as a direct assignment of every nested field.
- Adds a `Schema Evolution` examples subsection (add a top-level column,
add a nested struct field, widen a column's type).
- Adds a `Schema Evolution Triggers` examples subsection (an extra column
used only in an expression is not added; a wider value produced by an
expression is not widened and overflows; directly assigning a struct adds its
new nested fields).
- Adds a `Schema Evolution When the Source Omits Columns` examples
subsection.
The examples and behavior were cross-checked against the
`MergeIntoSchemaEvolution*` test suites under `sql/core`.
### Why are the changes needed?
`MERGE WITH SCHEMA EVOLUTION` is supported but undocumented in the SQL
reference, while the analogous `INSERT WITH SCHEMA EVOLUTION` is documented.
This brings the MERGE INTO reference to parity.
### Does this PR introduce _any_ user-facing change?
No (documentation-only change).
### How was this patch tested?
Documentation-only change. The example behavior was verified against the
existing `MergeIntoSchemaEvolution*` test suites.
### Was this patch authored or co-authored using generative AI tooling?
Generated-by: Cursor (Claude Opus 4.8)
Closes #56710 from szehon-ho/merge-into-schema-evolution-docs.
Authored-by: Szehon Ho <[email protected]>
Signed-off-by: Szehon Ho <[email protected]>
---
docs/sql-ref-syntax-dml-merge-into.md | 270 +++++++++++++++++++++++++++++++++-
1 file changed, 269 insertions(+), 1 deletion(-)
diff --git a/docs/sql-ref-syntax-dml-merge-into.md
b/docs/sql-ref-syntax-dml-merge-into.md
index c6b74dcad4b6..1ae72ca63b7f 100644
--- a/docs/sql-ref-syntax-dml-merge-into.md
+++ b/docs/sql-ref-syntax-dml-merge-into.md
@@ -32,7 +32,7 @@ apply. All of these row-level changes are performed as a
single atomic operation
### Syntax
```sql
-MERGE INTO target_table [ [ AS ] target_alias ]
+MERGE [ WITH SCHEMA EVOLUTION ] INTO target_table [ [ AS ] target_alias ]
USING { source_table | ( source_query ) } [ [ AS ] source_alias ]
ON merge_condition
[ WHEN MATCHED [ AND matched_condition ] THEN matched_action ] [ ... ]
@@ -51,6 +51,12 @@ not_matched_by_source_action
### Parameters
+* **WITH SCHEMA EVOLUTION**
+
+ Enables automatic schema evolution for this `MERGE` operation. When
enabled, the schema
+ of the target table is automatically evolved to add new columns and widen
data types based
+ on the source table or query, subject to the capabilities of the
underlying connector.
+
* **target_table**
Specifies the table to be merged into, which may be optionally qualified
with a database name.
@@ -111,6 +117,12 @@ not_matched_by_source_action
once. This check is skipped when there are no `WHEN MATCHED` clauses, or
when the only `WHEN MATCHED`
action is an unconditional `DELETE`, because in those cases the outcome does
not depend on the number
of matching source rows.
+- When `WITH SCHEMA EVOLUTION` is specified, only the columns that are
referenced in the merge clauses
+ by a direct assignment are added or have their data type widened.
+- `UPDATE SET *` and `INSERT *` count as direct assignments of every target
column, so schema
+ evolution is triggered for all of them.
+- An `UPDATE` or `INSERT` that directly assigns a struct column counts as a
direct assignment of every
+ nested field in that struct, so schema evolution is triggered for all of
those fields.
### Examples
@@ -217,6 +229,262 @@ SELECT * FROM target;
+--+------+----------+
```
+#### Schema Evolution
+
+```sql
+SELECT * FROM students;
++-------------+--------------------------+----------+
+| name| address|student_id|
++-------------+--------------------------+----------+
+| Amy Smith| 123 Park Ave, San Jose| 111111|
+| Bob Brown| 456 Taylor St, Cupertino| 222222|
+| Helen Davis| 469 Mission St, San Diego| 999999|
++-------------+--------------------------+----------+
+
+-- This example uses a source table that has an extra "email" column not
present in the target.
+SELECT * FROM new_students;
++-------------+--------------------------+----------+-----------------+
+| name| address|student_id| email|
++-------------+--------------------------+----------+-----------------+
+| Amy Smith| 123 Park Ave, San Jose| 111111| [email protected]|
+|Cathy Johnson| 789 Race Ave, Palo Alto| 333333|[email protected]|
++-------------+--------------------------+----------+-----------------+
+
+-- Evolve the students table schema to add the new email column from the
source.
+MERGE WITH SCHEMA EVOLUTION INTO students t
+ USING new_students s
+ ON t.student_id = s.student_id
+ WHEN MATCHED THEN UPDATE SET *
+ WHEN NOT MATCHED THEN INSERT *;
+
+SELECT * FROM students;
++-------------+--------------------------+----------+-----------------+
+| name| address|student_id| email|
++-------------+--------------------------+----------+-----------------+
+| Amy Smith| 123 Park Ave, San Jose| 111111| [email protected]|
+| Bob Brown| 456 Taylor St, Cupertino| 222222| NULL|
+|Cathy Johnson| 789 Race Ave, Palo Alto| 333333|[email protected]|
+| Helen Davis| 469 Mission St, San Diego| 999999| NULL|
++-------------+--------------------------+----------+-----------------+
+```
+
+Schema evolution also applies to nested fields. In the following example the
source's `address`
+struct has an extra `zip` field that is added to the target's `address` struct.
+
+```sql
+SELECT * FROM student_addresses;
++----------+-----------+
+|student_id| address|
++----------+-----------+
+| 111111| {San Jose}|
+| 222222|{Cupertino}|
++----------+-----------+
+
+SELECT * FROM address_updates;
++----------+------------------+
+|student_id| address|
++----------+------------------+
+| 111111| {San Jose, 95110}|
+| 333333|{Palo Alto, 94301}|
++----------+------------------+
+
+-- Evolve the address struct to add the new nested zip field.
+MERGE WITH SCHEMA EVOLUTION INTO student_addresses t
+ USING address_updates s
+ ON t.student_id = s.student_id
+ WHEN MATCHED THEN UPDATE SET *
+ WHEN NOT MATCHED THEN INSERT *;
+
+SELECT * FROM student_addresses;
++----------+------------------+
+|student_id| address|
++----------+------------------+
+| 111111| {San Jose, 95110}|
+| 222222| {Cupertino, NULL}|
+| 333333|{Palo Alto, 94301}|
++----------+------------------+
+```
+
+Schema evolution can also widen the data type of an existing column. In the
following example the
+source's `credits` column is a `BIGINT` that holds a value too large for the
target's `INT` column,
+so the target's `credits` column is widened from `INT` to `BIGINT`.
+
+```sql
+SELECT * FROM student_credits;
++----------+-------+
+|student_id|credits|
++----------+-------+
+| 111111| 30|
+| 222222| 45|
++----------+-------+
+
+-- The source's credits column is a BIGINT and contains a value that does not
fit in an INT.
+SELECT * FROM credit_updates;
++----------+-----------+
+|student_id| credits|
++----------+-----------+
+| 111111|10000000000|
+| 333333| 60|
++----------+-----------+
+
+-- Widen the target's credits column from INT to BIGINT to accommodate the
source values.
+MERGE WITH SCHEMA EVOLUTION INTO student_credits t
+ USING credit_updates s
+ ON t.student_id = s.student_id
+ WHEN MATCHED THEN UPDATE SET *
+ WHEN NOT MATCHED THEN INSERT *;
+
+SELECT * FROM student_credits;
++----------+-----------+
+|student_id| credits|
++----------+-----------+
+| 111111|10000000000|
+| 222222| 45|
+| 333333| 60|
++----------+-----------+
+```
+
+#### Schema Evolution Triggers
+
+Schema evolution adds or widens a column only when a source column is
referenced by a direct
+assignment to a target column. Referencing a source column only inside an
expression does not evolve
+the target. Conversely, directly assigning a whole struct counts as
referencing it, so all of its
+new nested fields are added.
+
+In the following example the source has an extra `bonus` column that is only
used in an expression,
+so the target table is not evolved and no `bonus` column is added.
+
+```sql
+SELECT * FROM student_credits;
++----------+-------+
+|student_id|credits|
++----------+-------+
+| 111111| 30|
+| 222222| 45|
++----------+-------+
+
+-- The source has an extra bonus column not present in the target.
+SELECT * FROM credit_transfers;
++----------+-------+-----+
+|student_id|credits|bonus|
++----------+-------+-----+
+| 111111| 30| 12|
+| 333333| 60| 5|
++----------+-------+-----+
+
+-- bonus is only referenced inside an expression, so the target schema is left
unchanged.
+MERGE WITH SCHEMA EVOLUTION INTO student_credits t
+ USING credit_transfers s
+ ON t.student_id = s.student_id
+ WHEN MATCHED THEN UPDATE SET credits = s.credits + s.bonus
+ WHEN NOT MATCHED THEN INSERT (student_id, credits) VALUES (s.student_id,
s.credits + s.bonus);
+
+-- The target schema is unchanged: no bonus column is added.
+SELECT * FROM student_credits;
++----------+-------+
+|student_id|credits|
++----------+-------+
+| 111111| 42|
+| 222222| 45|
+| 333333| 65|
++----------+-------+
+```
+
+Type widening follows the same rule. If a wider value is produced by an
expression instead of a
+direct assignment, the target column keeps its type, so a value that does not
fit can no longer be
+stored and the statement fails. The example below reuses the `credit_updates`
table, whose `credits`
+column is a `BIGINT` value too large for an `INT`.
+
+```sql
+-- credits is produced by an expression, so the target column is not widened
from INT to BIGINT.
+MERGE WITH SCHEMA EVOLUTION INTO student_credits t
+ USING credit_updates s
+ ON t.student_id = s.student_id
+ WHEN MATCHED THEN UPDATE SET credits = s.credits + 100;
+Error: CAST_OVERFLOW_IN_TABLE_INSERT
+```
+
+Directly assigning a struct column from the source, on the other hand, counts
as referencing it, so
+its new nested fields are added even though they are not named individually.
In the following example
+assigning the whole struct with `UPDATE SET address = s.address` adds the
source's new `zip` field to
+the target's `address` struct.
+
+```sql
+SELECT * FROM student_addresses;
++----------+-----------+
+|student_id| address|
++----------+-----------+
+| 111111| {San Jose}|
+| 222222|{Cupertino}|
++----------+-----------+
+
+-- The source's address struct has an extra zip field not present in the
target.
+SELECT * FROM address_changes;
++----------+------------------+
+|student_id| address|
++----------+------------------+
+| 222222|{Cupertino, 95014}|
++----------+------------------+
+
+-- Assigning the whole struct directly adds the new zip nested field.
+MERGE WITH SCHEMA EVOLUTION INTO student_addresses t
+ USING address_changes s
+ ON t.student_id = s.student_id
+ WHEN MATCHED THEN UPDATE SET address = s.address;
+
+SELECT * FROM student_addresses;
++----------+------------------+
+|student_id| address|
++----------+------------------+
+| 111111| {San Jose, NULL}|
+| 222222|{Cupertino, 95014}|
++----------+------------------+
+```
+
+#### Schema Evolution When the Source Omits Columns
+
+With schema evolution enabled, `UPDATE SET *` and `INSERT *` also tolerate a
source that is missing
+some of the target's columns. A missing column keeps its existing value on an
update and is set to
+its default value (or `NULL`) on an insert. Without schema evolution the same
statement fails,
+because `*` requires the source to provide every target column.
+
+```sql
+SELECT * FROM students;
++-------------+--------------------------+----------+
+| name| address|student_id|
++-------------+--------------------------+----------+
+| Amy Smith| 123 Park Ave, San Jose| 111111|
+| Bob Brown| 456 Taylor St, Cupertino| 222222|
+| Helen Davis| 469 Mission St, San Diego| 999999|
++-------------+--------------------------+----------+
+
+-- The source is missing the address column that exists in the target.
+SELECT * FROM name_updates;
++-------------+----------+
+| name|student_id|
++-------------+----------+
+| Amy Smith| 111111|
+|Cathy Johnson| 333333|
++-------------+----------+
+
+MERGE WITH SCHEMA EVOLUTION INTO students t
+ USING name_updates s
+ ON t.student_id = s.student_id
+ WHEN MATCHED THEN UPDATE SET *
+ WHEN NOT MATCHED THEN INSERT *;
+
+-- Student 111111 keeps its address on update; student 333333 is inserted with
a NULL address.
+SELECT * FROM students;
++-------------+--------------------------+----------+
+| name| address|student_id|
++-------------+--------------------------+----------+
+| Amy Smith| 123 Park Ave, San Jose| 111111|
+| Bob Brown| 456 Taylor St, Cupertino| 222222|
+|Cathy Johnson| NULL| 333333|
+| Helen Davis| 469 Mission St, San Diego| 999999|
++-------------+--------------------------+----------+
+```
+
### Related Statements
* [INSERT TABLE statement](sql-ref-syntax-dml-insert-table.html)
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