Microsoft SQL Server Query Hints UNION hints
Example
When you use UNION operator on two query results, Query optimizer (QO) can use following operators to create a union of two result sets:
- Merge (Union)
- Concat (Union)
- Hash Match (Union)
You can explicitly specify what operator should be used using OPTION() hint:
select OrderID, OrderDate, ExpectedDeliveryDate, Comments
from Sales.Orders
where OrderDate > DATEADD(day, -1, getdate())
UNION
select PurchaseOrderID as OrderID, OrderDate, ExpectedDeliveryDate, Comments
from Purchasing.PurchaseOrders
where OrderDate > DATEADD(day, -1, getdate())
OPTION(HASH UNION)
-- or OPTION(CONCAT UNION)
-- or OPTION(MERGE UNION)
