B+ Tree 叶子节点中的记录是按照索引列排序的,对于的聚簇索引来说,它对应的 B + 树叶子节点中的记录就是按照 id 列排序的。B + 树矮胖,所以这样根据主键值定位一条记录的速度很快。类似的,我们根据唯一二级索引列来定位一条记录的速度也很快的,比如下边这个查询: eq_ref 在连接查询时,如果被驱动表是通过主...
Oracle索引比较常用的有二种,1.B-TREE索引,B-TREE 适用于值变化较多的列,2.BITMAP索引。BITMAP适用于值变化较少的列(少于300个值),比如:性别这样的列。 有了上述基础就可以开始优化工作了。工具:pl/sql developer。 1.建表 Java代码 create table HEK_TEST_IN ( PID INTEGER primary key, NDATE DATE, N...
DbVisualizer presents the plan either in a tree style format or in a graph, or in a simple text format. What information is shown depends on the database you use. In the tree view, put the mouse pointer on the column header for a tooltip description what that column represents. The fol...
MySQL explain执行计划优化 简介MySQL调优: 先发现问题(慢查询,explain, profile) 对于使用索引和没有使用索引,了解到索引可以快速去查找数据 了解什么是索引(索引是排好序的快速查找的数据结构) 索引的管理(查看,创建,删除) 索引的本身的数据结构(B-TREE结构和聚簇结构) MySQL是如何去使用这些索引的(explain工具),...
ClickHouse是一个用于"联机分析"(OLAP)的列式数据库管理系统(DBMS:Database Managerment System),简称CH ,主要用于 “在线分析处理查询”(OLAP:Online Analytical Processing),能够使用SQL查询"实时生成分析"数据报告 . ClickHouse clickhouse数据库 aes加密 大数据 数据库 数据 服务器 clickhouse 数据库schema clickhouse...
Oracle then locates each selected row in the table based on its rowid and does a row-by-row access. Index unique scan – Only one row will be returned from the scan of a unique index. It will be used when there is an equality predicate on a unique (B-tree) index or an index ...
2 2 2 3 b SQL> explain plan for select * from or1 join or2 on or1.a = or2.a; Explained. SQL> SELECT plan_table_output FROM TABLE(DBMS_XPLAN.DISPLAY('PLAN_TABLE')); PLAN_TABLE_OUTPUT --- Plan hash value: 738363828 --- | ...
Node detailsThe properties, columns, and ordering details for the current statement plan node in the tree. TimeThe time details for the statement. The total time is the planning and execution time of the statement. The execution time is the time it took for the final statement plan to comple...
2 2 2 3 b SQL> explain plan for select * from or1 join or2 on or1.a = or2.a; Explained. SQL> SELECT plan_table_output FROM TABLE(DBMS_XPLAN.DISPLAY('PLAN_TABLE')); PLAN_TABLE_OUTPUT --- Plan hash value: 738363828 --- | ...
You can also roll back an EXPLAIN PLAN statement in order to remove the execution plan from the plan table. The EXPLAIN PLAN statement runs very quickly, even if the statement being explained is a query that might run for hours. This is because the statement is simply parsed and its ...