Function merge_view_target_path 

pub fn merge_view_target_path(
    services: ViewRewriteContext<'_>,
    plan: &MergePlan,
) -> Result<MergeViewTargetPath, SQLError>

Choose how a MERGE reaches view plan.target, as RewriteQuery and rewriteTargetView do: through the view’s INSTEAD OF triggers when every action has one, and otherwise by rewriting the view onto its base relation, which its query must allow, with an updatable column when an action inserts or updates. When it does not, the first action in WHEN order without a trigger reports why; an automatically updatable view cannot mix the two paths.