这篇文章基于oracle 11g介绍了其执行反馈机制的作用,主要涉及到plan cache这个组件,在oracle中称为cursor。文章中也参考oracle 12c reference的内容。 基本概念 在oracle中,每个处理SQL的用户session会使用private SQL area存储和特定查询相关的信息,包括bind variables的值,执行的计划,记录的实际执行统计等,这样的一个区...
Cardinality values is the only statistics stored/used in MyISAM tables. Cardinality values are used to estimate cost and number of records for "ref" and similar access methods. For "range" access method, MySQL estimates cost and number of records by "probing" the index at range bounds, witho...
the MySQL optimizer considers many techniques to efficiently perform the lookups involved in an SQL query. A query on a huge table can be performed without reading all the rows; a join involving several tables can be performed without comparing every combination of rows. The set of ...
Oracle的cost-based query transformation component 也是依赖于estimator组件组件,对各种逻辑等价的plan进行cost计算和比较,选择最佳。关于cost-based query transformation component ,可见[VLDB 06]Cost-based query transformation in Oracle 论文学习。 代价评估模型中将计算IO、CPU、网络通信方面的代价。对on-disk的行存...
Please note that only the cost-based optimizer has been enhanced; Oracle has not altered the operation of the rule-based optimizer. The optimizer generates an execution plan for SQL queries and DML statements--SELECT, INSERT, UPDATE, or DELETE statements. For simplicity, we describe the ...
The Oracle Query Optimizer The Optimizer is one of the most fascinating components of the Oracle Database, since it is essential to the processing of every SQL statement. The Optimizer determines the most efficient execution plan for each SQL statement based on the structure of the query, the ...
8.8.2 EXPLAIN Output Format 8.8.3 Extended EXPLAIN Output Format 8.8.4 Obtaining Execution Plan Information for a Named Connection 8.8.5 Estimating Query Performance Depending on the details of your tables, columns, indexes, and the conditions in yourWHEREclause, the MySQL optimizer considers many ...
TheEXPLAIN planallows you to view what has been checked on each step of query execution, including optimization approaches Oracle SQL query optimizer used, the number of rows processed, indexes to speed up the query, the amount of time (in units) it is expected to take to run the query, ...
To view a tree-based representation of a query plan Open a script. On theQuerymenu, selectQuery Plan. The Show Plan mode starts. To generate the Show Plan in a separate result window, clickExecute. Viewing a Tree-based, Graphical, or DBMS_XPLAN Query Plan (Oracle) ...
Re: Query execution plan 1308 Rick James May 15, 2012 07:22PM Sorry, you can't reply to this topic. It has been closed. Content reproduced on this site is the property of the respective copyright holders. It is not reviewed in advance by Oracle and does not necessarily represent the op...