生成SQL的执行计划是Oracle在对SQL做硬解析时的一个非常重要的步骤,它制定出一个方案告诉Oracle在执行这条SQL时以什么样的方式访问数据:索引还是全表扫描,是Hash Join还是Nested loops Join等。 比如说某条SQL通过使用索引的方式访问数据是最节省资源的,结果CBO作出的执行计划是全表扫描,那么这条SQL的性能必然是比较...
Example: int a,b,c; In this statement,comma is a separator and tells to the compiler that these (a, b, and c) are three different variables. 2) Comma (,) as an operator Sometimes we assign multiple values to a variable using comma, in that case comma is known as operator. ...
TheEXPLAINPLANresults let you determine whether the optimizer selects a particular execution plan, such as, nested loops join. It also helps you to understand the optimizer decisions, such as why the optimizer chose a nested loops join instead of a hash join, and lets you understand the perfor...
[转]Oracle执行计划（ExplainPlan）说明 [转]Oracle执⾏计划（ExplainPlan）说明 原⽂地址:http://langgufu.iteye.com/blog/2158163 如果要分析某条SQL的性能问题，通常我们要先看SQL的执⾏计划，看看SQL的每⼀步执⾏是否存在问题。如果⼀条SQL平时执⾏的好好的，却有⼀天突然性能很差，如果排除了...
Example: Execute the following statement to check the execution plan of a query on a partitioned parent table with one partition scanned. EXPLAIN SELECT * FROM public.hologres_parent; The following figure shows the returned result. Query data by using a foreign table If you query data by usi...
Q2. Write the syntax of a while statement and explain with an example. WHILE Loops WHILE Loops execute while a condition is still TRUE. If the test expression is TRUE, the body of the WHILE loop is executed. The test expression is evaluated again...
Nested LoopnloopLeast optimal join; mainly used for cross-joins (Cartesian products; without a join condition) and some inequality joins. Hash JoinhjoinAlso used for inner joins and left and right outer joins and typically faster than a nested loop join. Hash Join reads the outer table, hash...
FAST'SLOW' indicates that the scan is expected to progress slowly over the table. For example, if the scan is the outer of a nested loop join). 'FAST' indicates that the scan is expected to progress with higher speed. This information is used to group scans together for efficient sharing...
EXPLAIN returnsa row of information for each table used in the SELECT statement. It lists thetables in the output in the order that MySQL would read them while processingthe statement. MySQL resolves all joins using a nested-loop join method. Thismeans that MySQL reads a row from the first...
SELECT STATEMENT [CHOOSE] Cost=1 FILTER *** This is like a bounded nested loops TABLE ACCESS FULL EMP [ANALYZED] FILTER *** This filter is introduced by the min SORT GROUP BY NOSORT INDEX FULL SCAN BE_IX This example is also interesting in that it has a NOSORT function. The group...