理论上完全可以使用该索引,但由于表xxxx_user_cancellation上仅有1条记录,MySQL查询优化器评估通过JOIN BUFER使用Block Nested Loop算法"能有效提升两表关联的性能",Block Nested Loop算法需要"buffer"外表一部分数据后再与内表进行JOIN操作:
Implicit Batch Sort and Nested Loop operator Let’s examine the Nested Loop operator details. You may notice the Optimized property set to true. That is not very informative, what does it mean –“optimized”. Luckily, we have an explanation in the Craig Freedman’s blog for Nested Loop: W...
in the Nested Loop Join series (Introduction to a Nested Loop Join in SQL Server, Parallel Nested Loop Joins – the inner side of Nested Loop Joins and Residual Predicates) in which we will try to understand the Batch Sort, Explicit Sort and some interesting facts about Nested Loop Join. ...
8.2.1.6 Nested-Loop Join Algorithms MySQL executes joins between tables using a nested-loop algorithm or variations on it. Nested-Loop Join Algorithm Block Nested-Loop Join AlgorithmNested-Loop Join Algorithm A simple nested-loop join (NLJ) algorithm reads rows from the first table in a loop ...
So it is clear that merge join will be better in this case. Estimated subTree cost is less for the query with merge joins, but both this queries are fast enough, there both do only 12 logical reads, so SQL Server decides that NESTED LOOP join is also good solution. Not let...
The optimized nested loop join operator uses memory grant during query execution. This information isn't available in the queryplan. This makes troubleshooting of problems related to query execution memory usage more difficul...
Nested loop一般用在连接的表中有索引,并且索引选择性较好的时候. 步骤:确定一个驱动表(outer table),另一个表为inner table,驱动表中的每一行与inner表中的相应记录JOIN。类似一个嵌套的循环。适用于驱动表的记录集比较小(<10000)而且inner表需要有有效的访问方法(Index)。需要注意的是:JOIN的顺序很重要,驱动表...
produces a Nested Loops Join, as illustrated by the underlying execution plan: Nested Loop (cost=0.56..86.08rows=100 width=36) (actualtime=0.035..0.069rows=100 loops=1) ->IndexScan using idx_post_idonpost p (cost=0.28..8.47rows=10 width=20) ...
TheSQLGuru (1/2/2008) A nested loop query plan is usually optimal when there are small numbers of rows in one table (think 10s to perhaps 100s in most cases) that can be probed into another table that is either very small or has an index that allows a seek for each input. One ...
As part of the new implementation for nested loop joins, two NESTED LOOPS join row sources might appear in the execution plan where only one would have appeared in prior releases. In such cases, Oracle Database allocates one NESTED LOOPS join row source to join the values from the table on...