In addition to cardinality in databases, I also want to help simplify what it means to use cardinality in monitoring. If you’ve seen discussions of “high-cardinality dimensions” or “observability requires support for high-cardinality fields,” this is what we’re talking about. So what does...
The terms "highcardinality" and "lowcardinality" are frequently used when discussing table columns. A highcardinalitycolumn means that a column has many unique values. For example, a primary key c Oracle 数据库 原创 maclean_007 2011-08-08 16:15:43 ...
When there are no statistics available or it cannot be used, the optimizer has to guess. One of the most important guesses is inequality (“>” or “<“) with the unknown (local variable, for example). In this case, 30% selectivity guess is made. That means that 30% of table rows ...
SQL Server’s query optimizer using the old CE would choose the first non-clustered index (FILCA~1) to access the table. This means there would be five filters which could be applied since all the five columns of the ~1 index are specified in the query. In the single column...
Theone-to-one (1:1) relationshipdefines the fact that one row in a database table relates to exactly one row in a second table. In an ER diagram, 1:1 means that one occurrence of an entity is related to one event in a second entity. ...
The SQL Server Query Optimizer is a cost-based Query Optimizer. This means that it selects query plans that have the lowest estimated processing cost to execute. The Query Optimizer determines the cost of executing a query plan based on two main factors:...
In the SQL 2014 XML plan, there is a new attribute in StmtSimple called CardinalityEstimationModelVersion. When the value is 120, it means the new cardinality estimator is used. If the value is 70, it means the old cardinality estimator is used. This new XML attribute is only available fo...
In the SQL 2014 XML plan, there is a new attribute in StmtSimple called CardinalityEstimationModelVersion. When the value is 120, it means the new cardinality estimator is used. If the value is 70, it means the old cardinality estimator is used. This new XML at...
First published on MSDN on Jan 16, 2014 IntroductionOne area where a lot of development work went into SQL Server 2014 is the area of query optimization...
In database, probability, may be called as selectivity, means the percentage of the selected database table rows satisfying a certain condition, e.g., having a particular value. The conventional approach to cardinality estimation is to use a histogram to summarize the data. Given a value on ...