1、 打开熟悉的查看工具:PL/SQL Developer。 在PL/SQL Developer中写好一段SQL代码后,按F5,PL/SQL Developer会自动打开执行计划窗口,显示该SQL的执行计划。 2、 查看总COST,获得资源耗费的总体印象 一般而言,执行计划第一行所对应的COST(即成本耗费)值,反应了运行这段SQL的总体估计成本,单看这个总成本没有实际...
Usually, it's not recommended that you use loops in SQL unless you need to. You should use set-based queries instead. However, if you need to, there are many ways to loop, one of them is using cursors. For example, let's say that you have multiple DBs and you need to select coun...
SQL>select count(*) from TRAN_HIST_TOPIC where SOC_NO='123'; T1 COUNT(*) --- 229194 2. SQL>explain plan for select /*+ index(T1 XPK_TRAN_HIST_TOPIC) use_nl(T1,T2) leading(T1) */ 2 SUM(T1.LOCAL_AMOUNT) AS TOTALLOCALAMOUNT, 3 SUM(T1.USD_AMOUNT) AS TOTALUSDAMOUNT FROM TRAN...
从很多网页上都看到,SQL Server有三种Join的算法, nested loop join, merge join, hash join. 其中最常用的就是nested loop join. 在介绍nested loop join的很多文章里,都提到如果两个表做nested loop join,取行数较小的表作为外循环表,行数较多的表作为内循环表, join的效率会比较高. 其中之一的原因是如果...
Oracle SQL 性能调优:使用Hint固定执行计划1(Hash Join) Nested Loop Join 指定时用到的 Hint 和Hash Join 相对应的,通常,利用索引时一般会用到 Nested Loop Join。 下面我们来继续看看如何控制 Nested Loop Join 的使用,以及 Nested Loop Join 的顺序。
Nested Loop 对于被连接的数据子集较小的情况，Nested Loop 是个较好的选择。Nested Loop 就是扫描一个表（外表），每读到一条记录，就根据 Join 字段上的索引去另一张表（内表）里面查找，若 Join 字段上没有索引查询优化器一般就不会选择 Nested Loop。在 Nested Loop 中，内表（一般是带索引的大表）被...
DO $$DECLAREv_ts TIMESTAMP;v_repeat CONSTANT INT := 25;rec RECORD;BEGIN-- Repeat the whole benchmark several times to avoid warmup penaltyFOR r IN 1..5 LOOPv_ts := clock_timestamp();SET enable_memoize = OFF;FOR i IN 1..v_repeat LOOPFOR rec IN (SELECT t.*FROM t JOIN u ON...
DO$$DECLAREv_tsTIMESTAMP;v_repeatCONSTANTINT:=25;recRECORD;BEGIN--Repeat the whole benchmark several times to avoid warmup penaltyFORrIN1..5LOOPv_ts:=clock_timestamp();SETenable_memoize=OFF;FORiIN1..v_repeatLOOPFORrecIN(SELECTt.*FROMtJOINuONt.j=u.j)LOOPNULL;ENDLOOP;ENDLOOP;RAISEINFO'...
假如下⾯的 sql 语句中表 T1 和 T2 的连接⽅式是循环嵌套连接, T1 是驱动表 select * from T1, T2 where T1.id = T2.id and T1.name = 'David';那么将上述 sql 语句翻译为伪码应该如下所⽰:1for each row in (select*from T1 where name ='David') loop 2for (select*from T2 where T...
Note In SQL Server 2014 SP2, the MemoryFractions showplan XML attribute is updated for the optimized nested loop join operator to reflect the memory that's required to run it. Status Microsoft has confirmed that this is a problem in the Microsoft products that are listed in th...