What causes DELTA_SCHEMA_CHANGE_MERGE_FAILED?

A concurrent ALTER TABLE ADD COLUMNS or schema evolution write changed the Delta table schema while a MERGE operation was reading the table's metadata. An automatic schema evolution triggered by a streaming write added new columns to the table schema mid-MERGE. A dbt model with schema_changes = ignore ran simultaneously with a MERGE pipeline targeting the same Delta table

How do I fix DELTA_SCHEMA_CHANGE_MERGE_FAILED?

Step 1: Retry the MERGE operation — after the schema change is committed, the retry will use the updated schema and should succeed.. Step 2: Coordinate schema changes and MERGE operations to not run concurrently; apply schema changes during a maintenance window before resuming MERGE pipelines.. Step 3: If schema evolution is frequent, consider using mergeSchema = true carefully and ensure all writers agree on the new schema before any MERGE is attempted.. Step 4: Use Delta table isolation level Serializable to detect and abort conflicting schema changes early rather than failing mid-operation.

High severityexecution

Power BI Refresh Error:
DELTA_SCHEMA_CHANGE_MERGE_FAILED

What does this error mean?

A Delta Lake MERGE operation failed because a concurrent transaction modified the schema of the target table while the MERGE was in progress, making it impossible to safely resolve the conflict.

Common causes

1A concurrent ALTER TABLE ADD COLUMNS or schema evolution write changed the Delta table schema while a MERGE operation was reading the table's metadata
2An automatic schema evolution triggered by a streaming write added new columns to the table schema mid-MERGE
3A dbt model with schema_changes = ignore ran simultaneously with a MERGE pipeline targeting the same Delta table

How to fix it

1Step 1: Retry the MERGE operation — after the schema change is committed, the retry will use the updated schema and should succeed.
2Step 2: Coordinate schema changes and MERGE operations to not run concurrently; apply schema changes during a maintenance window before resuming MERGE pipelines.
3Step 3: If schema evolution is frequent, consider using mergeSchema = true carefully and ensure all writers agree on the new schema before any MERGE is attempted.
4Step 4: Use Delta table isolation level Serializable to detect and abort conflicting schema changes early rather than failing mid-operation.

Frequently asked questions

Does setting mergeSchema = true prevent this error?

No. mergeSchema = true allows a write to add new columns to the schema, but it does not prevent a concurrent MERGE from failing if it encounters an unexpected schema change mid-execution.

Can streaming pipelines trigger this?

Yes. Delta Lake auto-merge schema evolution in structured streaming can modify the table schema between the MERGE operation's plan phase and its commit phase, causing this error.