It will check the row count in the J bucket, and if there are 50 rows, it will estimate that when retrieving the rows for this table, it will get 50 rows back. When the query optimizer has a predicate on this column, for instance “Where FirstName = Joe”, it will estimate how many rows in the table will have the name Joe. So the histogram will count the rows that start with the letter A and record that into the A bucket and it will do that for each letter of the alphabet. For instance, if the column holds first names, you may get 26 buckets, one for each letter of the alphabet. The goal is to count the number of rows that are in each bucket. It can be multiple columns, but it is easiest to think about it as a single column. So what is a statistic?Ī statistic is a histogram of a column of data in a table. Predicate expressions return true or false. A predicate is a logic expression in either the where clause or within the “On” expression in a query. Statistics are used by the query optimizer to estimate how many rows will be returned from a table based on the predicates in the query. The optimizer will compare the costs of different query plans together and will select a plan that it deems is acceptable. It also choses which indexes to use based on the columns needed in the query.
#Statistical calculations in sql server how to
The engine has a query optimizer, which is logic that takes a look at the tables in the query, and the predicates to determine which tables to pull data in from first and how to join the data together. When the database engine receives a query it needs to decide how to retrieve the data and return it to the calling program in an acceptably efficient way. I’ll back up a bit and explain how the SQL Server database engine executes a query. So what are statistics and where do they fit in? Statistics in SQL Server play an important and often ignored role in this performance. For all three having SQL Server running optimally will have a real bearing on the performance for end users. Sage 500 does the same, however around 60% of the business logic workload in Sage 500 is also executed within SQL Server. For Sage X3 and Sage 100, their primary use of SQL Server is to store and retrieve data for the transactional workload and retrieve data for reporting workload. (Sage X3 can run on Oracle – but most installs we work with are SQL Server). Sage X3, Sage 500 and Sage 100 Premium all run with their relational database on Microsoft SQL Server. Business / Enterprise Intelligence (24).
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