| -> Limit: 100 row(s) (cost=14.62 rows=1) (actual time=1.252..1.252 rows=0 loops=1) -> Gather (merge sort; slice: 1; workers: 1; nodes: 2) (cost=14.62 rows=1) (actual time=1.250..1.250 rows=0 loops=1) -> Limit: 100 row(s) (cost=4.52 rows=1) (actual time=0.084,0....
filesort使用的算法是QuickSort,即对需要排序的记录生成元数据进行分块排序,然后再使用mergesort方法合并块。其中filesort可以使用的内存空间大小为参数sort_buffer_size的值,默认为2M。当排序记录太多sort_buffer_size不够用时,mysql会使用临时文件来存放各个分块,然后各个分块排序后再多次合并分块最终全局完成排序。 Us...
The following is a basic example of openCypher explain output. The query is a single-node lookup in the air routes dataset for a node with the airport code ATL that invokes explain using the details mode: ## sample query aws neptune-graph execute-query \ --region <region> \ --graph-...
derivedN: The row refers to the derived table result for the row with an id value of N. A derived table may result, for example, from a subquery in the FROM clause. subqueryN: The row refers to the result of a materialized subquery for the row with an id value of N. partitions (...
The following example returns the query plan for the same query with verbose output: explain verboseselecteventid, eventname,event.venueid, venuenamefromevent, venuewhereevent.venueid = venue.venueid; QUERY PLAN ---{HASHJOIN:startup_cost2.52:total_cost58653620.93:plan_rows8712:plan_width43:best...
Sort Sort is used to sort data in ascending order (ASC) or descending order (DESC), which is usually used with the ORDER BY clause. In this example, data in the l_shipdate column of the TPC-H lineitem table is sorted. EXPLAIN SELECT l_shipdate FROM public.lineitem ORDER BY l_...
This is how many blocks you processed in "consistent read" mode. This will include counts of blocks read from the rollback segment in order to roll back a block. This is the mode you read blocks in with a SELECT, for example. Also, when you do a searched UPDATE/DELETE, you read the...
一.OptimizingQueries with EXPLAIN The EXPLAINstatement can be used either as a way to obtain information about how MySQLexecutes a statement, or as a synonym for DESCRIBE: (1)When youprecede a SELECT statement with the keyword EXPLAIN, MySQL displays informationfrom the optimizer about the query...
Figure 19: Example plan output using SORT MERGE JOIN Beginning with Oracle Database 12c Release 2, band joins available. They make certain merge joins more efficient where there are BETWEEN predicates. 14 THE ORACLE OPTIMIZER EXPLAIN THE EXPLAIN PLAN Cartesian join - The optimizer joins every row...
Figure 7.8 A Visual Explain Example with a Hash Join Graphic Conventions Standard Boxes: tables Rounded boxes: operations such as GROUP and SORT Framed boxes: subqueries Diamonds: joins Textual and Informational Conventions Standard text below boxes: table (or alias) name ...