the MySQL optimizer considers many techniques to efficiently performthe lookups involved in an SQL query. A query on a huge table can be performedwithout reading all the rows; a join involving several tables can be performedwithout comparing every combination of rows. The set of operations...
explain关键字主要用来定性分析索引的使用情况,以及SQL语句的优劣,但是无法知道SQL语句的实际执行情况。 而show profile命令可以做到定量分析SQL语句的执行情况。即使用者可以明确知道一条SQL到底执行了多久。 想要使用这个命令,主要步骤可以分为四步: 首先设置属性, set profileing=1; 1. 开启了这个属性后,再执行SQL语...
<dsql_query>最上層/文件元素。 <sql>回應SQL_statement。 <params>目前不使用此標記。 <materialized_view_candidates> (預覽)包含建議具體化檢視的 CREATE 陳述式,可讓 SQL 陳述式有較好的效能。 <dsql_operations>摘要說明和包含查詢步驟,並包含查詢的成本資訊。 也包含所有<dsql_operation>區塊。 此標記包含...
set profileing=1; 开启了这个属性后,再执行SQL语句,就会记录SQL语句执行各个步骤耗时 接着执行多条SQL语句 select * from emp; select * from dept; 执行结果不重要,主要关注各个SQL语句的执行时间 接下来再执行如下语句,显示统计成功的SQL语句 show profiles; 执行结果 mysql> show profiles; +---+---+...
total_number_operations 是查询的操作总数量 。 并行和在多个节点上运行的操作将计为单个操作。 <dsql_operation>描述查询计划中的单个操作。 <dsql_operation> 标记将操作类型包含为属性: <dsql_operation operation_type=operation_type> operation_type 是在sys.dm_pdw_request_steps (Transact-SQL)中找到的值...
There are a number of different operations that promote sorts: (1)order by clauses (2)group by (3)sort merge join –-这三个会产生排序运算 2.filter --过滤,如not in、min函数等容易产生 Has a number of different meanings, used to indicate partition elimination, may also indicate an actual...
set hive.exec.dynamic.partition=true; ( 2 )设置为非严格模式(动态分区的模式,默认 strict ,表示必须指定至少一个分区为静态分区, nonstrict 模式表示允许所有的分区字段都可以使用动态分区。) set hive.exec.dynamic.partition.mode=nonstrict; ( 3 )在所有执行 MR 的节点上,最大一共可以创建多少个动态分区。
IN ('North America', 'Pacific') 14 AND Gender = 'F' 15 GROUP BY G.StateProvinceName, T.SalesTerritoryGroup 16 ORDER BY AVG(YearlyIncome) DESC</sql> 17 <dsql_operations total_cost="0.926237696" total_number_operations="9"> 18 <dsql_operation operation_type="RND_ID"> 19 <identifier...
finds relevant records based on an Index. Index Scans perform 2 read operations: one to read the index and another to read the actual value from the table. 2、顺序扫描 - Seq Scan Node finds relevant records by sequentially scanning the input record set. When reading from a table, Seq Scan...
finds relevant records based on an Index. Index Scans perform 2 read operations: one to read the index and another to read the actual value from the table. 2、顺序扫描 - Seq Scan Node finds relevant records by sequentially scanning the input record set. When reading from a table, Seq Scan...