Create index to find non empty titles for assigments

A big time of the query time of get_assignments is spent finding
assignments with not null titles.

Create an index on that condition trying to speed it up.

lms/migrations/versions/b39108c0cd35_index_on_assignment_title_is_not_null.py

@@ -0,0 +1,32 @@
+"""Index on assignment.title is not null."""
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = "b39108c0cd35"
+down_revision = "3d0c022c716c"
+
+
+def upgrade() -> None:
+    # CONCURRENTLY can't be used inside a transaction. Finish the current one.
+    op.execute("COMMIT")
+
+    op.create_index(
+        "ix__assignment_title_is_not_null",
+        "assignment",
+        ["title"],
+        unique=False,
+        postgresql_where=sa.text("title IS NOT NULL"),
+        postgresql_concurrently=True,
+    )
+
+
+def downgrade() -> None:
+    op.execute("COMMIT")
+
+    op.drop_index(
+        "ix__assignment_title_is_not_null",
+        table_name="assignment",
+        postgresql_where=sa.text("title IS NOT NULL"),
+        postgresql_concurrently=True,
+    )

lms/models/assignment.py

@@ -25,9 +25,6 @@ class Assignment(CreatedUpdatedMixin, Base):
     """
 
     __tablename__ = "assignment"
-    __table_args__ = (
-        sa.UniqueConstraint("resource_link_id", "tool_consumer_instance_guid"),
-    )
 
     id: Mapped[int] = mapped_column(autoincrement=True, primary_key=True)
 
@@ -91,6 +88,15 @@ class Assignment(CreatedUpdatedMixin, Base):
 
     course: Mapped[Course | None] = relationship(Course)
 
+    __table_args__ = (
+        sa.UniqueConstraint("resource_link_id", "tool_consumer_instance_guid"),
+        sa.Index(
+            "ix__assignment_title_is_not_null",
+            "title",
+            postgresql_where=title.is_not(None),
+        ),
+    )
+
     def get_canvas_mapped_file_id(self, file_id):
         return self.extra.get("canvas_file_mappings", {}).get(file_id, file_id)