Updating multiple rows in sql server
For example, a statistic created on these columns and in order - Name, Age, and Salary.In this case, the statistics object will have densities for the following column prefixes: (Name), (Name, Age), and (Name, Age, Salary).SQL Server Query Optimizer identifies these stale statistics before compiling a query and before executing a cached query plan.The identification of stale statistics are done by counting the number of data modifications since the last statistics update and comparing the number of modifications to a threshold as mentioned below.DBCC SHOW_STATISTICS shows the header, histogram, and density vector based on data stored in the statistics object.The value for all_density (1 / number of distinct values for a column) ranges from 0.0 to 1, which indicates selectivity (duplicates), 0.0 being highly selective with less duplicates whereas 1 is less selective with a high number of duplicates; as a rule of thumb, the lesser the better.The lowest of the sorted column values is the upper boundary value for the first histogram step.
Each histogram step includes a range of column values followed by an upper bound column value, which includes all possible column values between boundary values (excluding the boundary values themselves).Statistics refers to the statistical information about the distribution of values in one or more columns of a table or an index.The SQL Server Query Optimizer uses this statistical information to estimate the cardinality, or number of rows, in the query result to be returned, which enables the SQL Server Query Optimizer to create a high-quality query execution plan.SQL Server Query Optimizer uses statistics to estimate the distribution of values in one or more columns of a table or index views, and the number of rows (called ) to create a high-quality query execution plan.Often statistics are created on a single column but it’s not uncommon to create statistics on multiple columns.
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Next create a table and load data from the Sales Order Detail table of the Adventure Works database.