Speeding up SQL queries by orders of magnitude using UNION(foxhound.systems)

    SELECT
        meal_items.*,
        customer_id,
        customers.employee_id
    FROM customers
        INNER JOIN customer_orders
            ON customers.id = customer_orders.customer_id
        INNER JOIN customer_order_items
            ON customer_orders.id = customer_order_items.customer_order_id
        INNER JOIN meal_items
            ON customer_order_items.meal_item_id = meal_items.id

  SELECT vegetable_id, SUM(price) as price, SUM(weight) as weight
  FROM
  (
    SELECT vegetable_id, price, NULL as weight
    FROM prices
    UNION ALL
    SELECT vegetable_id, NULL as price, weight
    FROM weights
  )
  GROUP BY vegetable_id

  SELECT vegetable_id, price, weight
  FROM prices 
    JOIN weights 
      ON price.vegetable_id = weights.vegetable_id

  SELECT meal_items.*, employee_markouts.employee_id, customer_orders.customer_id
  FROM meal_items
  LEFT JOIN employee_markouts ON employee_markouts.meal_item_id = meal_items.id
  LEFT JOIN employees ON employees.id = employee_markouts.employee_id
  LEFT JOIN customer_order_items ON customer_order_items.meal_item_id = meal_items.id
  LEFT JOIN customer_orders ON customer_orders.id = customer_order_items.customer_order_id
  LEFT JOIN customers ON customers.id = customer_orders.customer_id
  WHERE (employees.store_id = 250 AND 
         employee_markouts.created >= '2021-02-03' AND
         employee_markouts.created < '2021-02-04') OR
        (customers.store_id = 250 AND
         customer_orders.created >= '2021-02-03' AND
         customer_orders.created < '2021-02-04')

    ON (customer_order_items.meal_item_id = meal_items.id
        OR employee_markouts.meal_item_id = meal_items.id
       )

    ON meal_items.id = COALESCE( employee_markouts.meal_item_id, customer_order_items.meal_item_id )