-- Migration 0107: Sub-issues / epics hierarchy
-- Adds a nullable self-referencing foreign key to `issues` so any issue
-- can be a child of another issue (the parent becomes an "epic").

ALTER TABLE issues
  ADD COLUMN parent_issue_id uuid
    REFERENCES issues(id)
    ON DELETE SET NULL;

CREATE INDEX issues_parent ON issues (parent_issue_id)
  WHERE parent_issue_id IS NOT NULL;