通过对比 Query 1 Test 3 和 Query 1 Test 4 可以看出，Merge Join 的主要开销是排序开销，如果能通过建立聚簇索引（如果 Query 必须显示排序），可以极大提高 Merge Join 的性能。从这两个实验可以看出，创建聚簇索引后，查询时间从 4956.768 ms 缩减到了 1815.238 ms。在两表上同时创建聚簇索引 Query 1 ...
It is important to understand that a joinis not a physical entityin other words, it does not exist in the actual database tables. A join is created by MySQL as needed, andit persists for the duration of the query execution. - maintainingreferential integrity 是说 MySQL 只允许合法的数据(fo...
Query 2 Test 1:强制使用Hash Join,失败 本次实验通过设置enable_hashjoin=true,enable_nestloop=false,enable_mergejoin=false来试图强制使用Hash Join,但是失败了。
The nested loops join cannot directly support full outer join. However, we can transform “T1 full outer join T2” into “T1 left outer join T2 UNION T2 left anti-semi-join T1.” Basically, this transforms the full outer join into a left outer join – which includes all pairs of rows ...
Consider this query: select * from Sales S inner join Customers C on S.Cust_Id = C.Cust_Id option(loop join) I’ve added a “loop join” hint to force the optimizer to use a nested loops join. We get this plan which I captured by running the query with “set statistics profile ...
Query 1 Test 1:查询优化器自动选择Nested Loop,耗时784.845 ms 如下图所示,执行器将小表mse_test_test作为外表(驱动表),对于其中的每条记录,通过大表(nbar_test)上的索引匹配相应记录。 Query 1 Test 2:强制使用Hash Join,耗时1731.836ms 如下图所示,执行器选择一张表将其映射成散列表,再遍历另外一张表并从...
option(loop join) I’ve added a “loop join” hint to force the optimizer to use a nested loops join. We get this plan which I captured by running the query with “set statistics profile on”: Kembangkan jadual The outer table in this plan is Customers while the inner table is Sales....
If I join main table #a with only one secondary table, there will be Merge Join in query plan. select * from #a a inner join #b b on a = b ; But if I join main table #a with both secondary tables, there will be only Nested Loops. select * from #a a inner join ...
The query, NESTED, in the Solution project combines information from the customer, orders, and orditems tables using two inner join conditions. One join condition is between customer and orders; the other between orders and orditems. Each result record has fields for the cust_id and company fr...
left join (select Start_Object_ID, End_Object_ID from t_connector where Stereotype = 'deriveReqt') t on o.Object_ID = t.End_Object_ID where o.Object_Type = 'Requirement' Unfortunately, it ends with an SQL error message. I believe, that there could be some restrictions about the nest...