Updating one table from another sql server
The query optimizer, for the most part, takes care of generating and updating the distribution statistics, creating them on certain columns within a table or indexed view.Each set of statistics is stored as a table-level object that tracks the distribution of values across the first column in any given set.Microsoft recommends that you create statistics under the following circumstances: clause, which specifies that all rows should be scanned when creating the statistics.However, you can instead specify that a percentage of the data be sampled, rather than every row.I created the examples on a local instance of SQL Server 2014, but you should be able to apply the principles we cover here to any version of SQL Server from 2008 onward, as well as to Azure SQL Database, Azure SQL Data Warehouse, and Parallel Data Warehouse.SQL Server generates a statistics object automatically when you create an index on a table or indexed view. Because the column is defined as the primary key, SQL Server automatically created a clustered index on that column and generated the associated statistics object.In this case, the histogram is broken into 184 steps, based on ranges of data in the first indexed column, column shows the average number of rows for each distinct value, based on the formula range_rows/distinct_range_rows, once again excluding the upper end rows.
The statistics provide the optimizer with clues on the best ways to process the query and consequently maximize performance.For example, the statistics might lead the optimizer to choose an index seek over an index scan, a potentially more resource-intensive operation.Without statics, it would be difficult for the optimizer to determine the best processing strategy in advance.(The optimizer rounds the figure up to 1.49351.) Although this is not a big deal when returning one row, it could make a difference when returning multiple rows from a large data set, leading you to consider creating statistics or updating them.(More on that in a bit.) Let’s look at one more example.