Tables from earlier joins are read in portions into the join buffer, and then their rows are used from the buffer to perform the join with the current table. Using temporary Query过程中构造一张临时表,常见order by,gr
表示先走二级索引,再走一级索引找到数据 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....
比较常见的范围扫描是带有BETWEEN子句或WHERE子句里有>、>=、<、<=、IS NULL、<=>、BETWEEN、LIKE、IN()等操作符。 代码语言:javascript 代码运行次数:0 运行 AI代码解释 SELECT*FROMtbl_nameWHEREkey_columnBETWEEN10and20;SELECT*FROMtbl_nameWHEREkey_columnIN(10,20,30); 11 index:全索引扫描,和ALL类似,...
Introduced in 7.3 and requires V733_PLANS_ENABLED=TRUE and CBO, may be hinted using INDEX_FFS hint, uses multiblock i/o, can be executed in parallel, can be used to access second column of concatenated indexes. This is because we are selecting all of the index. ...
EXPLAIN语句返回MYSLQ的执行计划,通过他返回的信息,我们能了解到MYSQL优化器是如何执行SQL语句的,通过分析他能帮助你提供优化的思路。 语法 MYSQL 5.6.3以前只能EXPLAIN SELECT; MYSQL5.6.3以后就可以EXPLAIN SELECT,UPDATE,DELETE EXPLAIN 语法例子: mysql> explain select customer_id,a.store_id,first_name,last_na...
Hash joins require more memory to execute than index joins but execute much faster when there are a lot of rows that need to be joined. The hash join operator is multi-threaded in TiDB and executes in parallel. An example of hash join is as follows: EXPLAIN SELECT /*+ HASH_JOIN(t1, ...
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...
2.explain plan for sql语句; select plan_table_output from table(dbms_xplan.display()); 3.通过第3方工具,如plsql developer(f5查看执行计划)、toad等; 1. 2. 3. 4. 5. 6. 7. 三、看懂执行计划 1.执行计划中字段解释 1. SQL> select * from scott.emp a,scott.emp b where a.empno=b.mgr...
The main IndexJoin thread joins each outer row by looking up to the inner rows' hash table. The IndexJoin operator contains the following execution information: inner:{total:4.297515932s, concurrency:5, task:17, construct:97.96291ms, fetch:4.164310088s, build:35.219574ms}, ...
displaying both the planned and actual execution details. It helps in understanding query performance, identifying bottlenecks, and optimizing complex queries. By analyzing the execution plan, database administrators and developers can make informed adjustments to indexes, joins, and other query elements ...