SQL语句与relational algebra之间的对应 (二) Basic SQL Query: Processing steps SELECT[DISTICNT]target-listFROMrelation-listWHEREqualification 1. 从FROM开始,先计算relation-list的笛卡尔积(在哪些relation中查询) 2. 接下来是WHERE, 删除不满足qualification条件的叉积的行 (满足的什么条件) 3. 最后是SELECT, ...
If there is a fresh query and its hash code does not exist in shared pool then that query has to pass through from the additional steps known as hard parsing otherwise if hash code exists then query does not passes through additional steps. It just passes directly to execution engine (refer...
An SQLstatement is thus nothing more than an object model diagram of sorts. In that regard, I recommend the following sequence of four simple steps when building any SQL query statement.Paul D ShermanSan Jose
"expanded_query": "/* select#1 */ select `user`.`id` AS `id`,`user`.`name` AS `name`,`user`.`age` AS `age`,`user`.`code` AS `code` from `user` where (`user`.`age` = 21)" } ] } }, { "join_optimization": { "select#": 1, "steps": [ { "condition_processing":...
steps": [ ] /* steps */ } /* join_execution */ } ] /* steps */ } MISSING_BYTES_BEYOND_MAX_MEM_SIZE: 0 INSUFFICIENT_PRIVILEGES: 0 1 row in set (0.00 sec) ERROR: No query specified第二章 索引的使用索引是数据库优化最常用也是最重要的手段之一, 通过索引通常可以帮助用户解决大多数的...
The basic steps that SQL Server uses to process a single SELECT statement include the following:The parser scans the SELECT statement and breaks it into logical units such as keywords, expressions, operators, and identifiers. A query tree, sometimes referred to as a sequence tree, is...
"steps": [ { "expanded_query": "/* select#1 */ select `e`.`emp_no` AS `emp_no`,`e`.`birth_date` AS `birth_date`,`e`.`first_name` AS `first_name`,`e`.`last_name` AS `last_name`,`e`.`gender` AS `gender`,`e`.`hire_date` AS `hire_date`,`s`.`emp_no` AS `...
The Query Optimizer analyzes different ways the source tables can be accessed. It then selects the series of steps that return the results fastest while using fewer resources. The query tree is updated to record this exact series of steps. The final, optimized version of the query tree is ca...
"steps": [ { "expanded_query": "/* select#1 */ select `rental`.`rental_id` AS `rental_id` from `rental` where ((1 = 1) and (`rental`.`rental_date` >= '2005-05-25 04:00:00') and (`rental`.`rental_date` <= '2005-05-25 05:00:00') and (`rental`.`inventory_id` ...
clustered index, meaning the data rows are not stored in any particular order. This is an important feature for decision support queries. Such disk-order scans can also be employed for clustered and nonclustered indexes if the index's sort order is not required in subsequent processing steps. ...