DO {{dollar_delimiter}}
BEGIN
  IF NOT EXISTS (
    SELECT 1 FROM information_schema.table_constraints
    WHERE table_schema = {{quote .SchemaName}}
      AND table_name = {{quote .TableName}}
      AND constraint_name = {{quote .ConstraintName}}
  ) THEN
    ALTER TABLE {{qual_table .SchemaName .TableName}} ADD CONSTRAINT {{quote_ident .ConstraintName}} UNIQUE {{if .UsingIndex}}USING INDEX {{quote_ident .UsingIndex}}{{else}}({{.Columns}}){{end}};
  END IF;
END;
{{dollar_delimiter}};