fix: migration script can't drop constraint (apache#15807)

scripts/benchmark_migration.py

@@ -30,7 +30,7 @@
 from flask_migrate import downgrade, upgrade
 from graphlib import TopologicalSorter  # pylint: disable=wrong-import-order
 from progress.bar import ChargingBar
-from sqlalchemy import create_engine, inspect, Table
+from sqlalchemy import create_engine, inspect
 from sqlalchemy.ext.automap import automap_base
 
 from superset import db
@@ -172,7 +172,6 @@ def main(
         rows = session.query(model).count()
         print(f"- {model.__name__} ({rows} rows in table {model.__tablename__})")
         model_rows[model] = rows
-    session.close()
 
     print("Benchmarking migration")
     results: Dict[str, float] = {}

superset/migrations/versions/49b5a32daba5_add_report_schedules.py

@@ -28,6 +28,7 @@
 
 import sqlalchemy as sa
 from alembic import op
+from sqlalchemy.engine.reflection import Inspector
 from sqlalchemy.exc import OperationalError
 
 
@@ -119,13 +120,17 @@ def upgrade():
     )
 
 
+def has_unique_constraint(constraint_name: str, table_name: str) -> bool:
+    bind = op.get_bind()
+    inspector = Inspector.from_engine(bind)
+    unique_constraints = inspector.get_unique_constraints(table_name)
+    return constraint_name in {constraint["name"] for constraint in unique_constraints}
+
+
 def downgrade():
     op.drop_index(op.f("ix_report_schedule_active"), table_name="report_schedule")
-    try:
+    if has_unique_constraint("uq_report_schedule_name", "report_schedule"):
         op.drop_constraint("uq_report_schedule_name", "report_schedule", type_="unique")
-    except Exception:
-        # Expected to fail on SQLite
-        pass
 
     op.drop_table("report_execution_log")
     op.drop_table("report_recipient")