Support Spanner's UNION syntax, which needs to be UNION DISTINCT or UNION ALL. (#101768)

* Support Spanner's UNION syntax, which needs to be UNION DISTINCT or UNION ALL.

pkg/services/sqlstore/migrator/dialect.go

@@ -32,6 +32,8 @@ type Dialect interface {
 	BooleanStr(bool) string
 	DateTimeFunc(string) string
 	BatchSize() int
+	UnionDistinct() string // this is the default UNION type
+	UnionAll() string
 
 	OrderBy(order string) string
 
@@ -467,3 +469,11 @@ func (b *BaseDialect) Update(ctx context.Context, tx *session.SessionTx, tableNa
 func (b *BaseDialect) Concat(strs ...string) string {
 	return fmt.Sprintf("CONCAT(%s)", strings.Join(strs, ", "))
 }
+
+func (b *BaseDialect) UnionDistinct() string {
+	return "UNION"
+}
+
+func (b *BaseDialect) UnionAll() string {
+	return "UNION ALL"
+}