SELECT ENAME FROM EMP WHERE EMP_TYPE = 'A' AND EMP_CLASS = 'X'; 优化器会注意到EMP_TYPE上的索引并使用它. 这是目前唯一的选择. 如果,一段时间以后,另一个非唯一性建立在EMP_CLASS上,优化器必须对两个索引进行选择,在通常情况下,优化器将使用两个索引并在他们的结果集合上执行排序及合并. 然而,如果...
AI代码解释 --使用连接代替INSELECTA.id,A.nameFROMClass_AAINNERJOINClass_BBONA.id=B.id; 用到了 「id」列上的索引,而且由于没有子查询,也不会生成临时表 2、避免排序 SQL 是声明式语言,即对用户来说,只关心它能做什么,不用关心它怎么做。这样可能会产生潜在的性能问题:排序,会产生排序的代表性运算有下...
The goal of this SQL tuning class is not to just show you how to perform some tuning techniques but have you learn all the details about how queries perform, what the reasons for poorly performing queries are and how to solve them. You will also learn Oracle Database architecture from the...
sql.log.queriesSlowerThan Allows you to specify SQL query execution time threshold in milliseconds (defaults to 10000), which if exceeded query is identified as "slow" and may be logged under specific logging category. See DataSource.logSlowSQL for more details....
Insert queries where the ORDER clause is compatible with an index. ORDER BY clauses that are compatible with the ORDER clause. Aggregates, where GROUP BY is compatible with ORDER clause. DISTINCT aggregates where the distinct columns are compatible with the ORDER clause. The ORDER clause doesn'...
Represents a parameter to a SqlCommand and optionally its mapping to DataSet columns. This class cannot be inherited. For more information on parameters, see Configuring Parameters and Parameter Data Types.
This method can be used to stop long running or runaway queries. If there are statements executing when the cancel() method is called, this method aborts the statements' operations and any incomplete updates or transactions are rolled back. If there are no statements currently executing, ...
To define a table with a security label column, the primary authorization ID of the statement must have a valid security label, and the RACF SECLABEL class must be active. In addition, the following conditions are also required: The data type of the column must be CHAR(8). The subtype ...
db=SQLAlchemy(app)classRole(db.Model):#定义表名__tablename__='roles'#定义列对象id = db.Column(db.Integer, primary_key=True) name= db.Column(db.String(64), unique=True) us= db.relationship('User', backref='role')#repr()方法显示一个可读字符串def__repr__(self):return'Role:%s'%se...
// File: ConnectorSplit.java // package: io.prestosql.spi.connector // Class: ConnectorSplit public interface ConnectorSplit { // 这个信息定义了Split可以从哪些节点访问到(这些节点,并不需要是Presto集群的节点,例如对于计算存储分离的情况,大概率Presto的节点与数据源Split所在的节点不是相同的) List<Host...