SQL_Fragmentation_Analyzer-684156.png' alt='Sql Server Update Sequence Number Db2' title='Sql Server Update Sequence Number Db2' />Database SQL Language Reference Contents. Importance of Statistics and How It Works in SQL Server Part 1 Database. Journal. com. Introduction. 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. For example, based on these statistical information SQL Server Query Optimizer might decide whether to use the index seek operator or a more resource intensive index scan operator in order to provide optimal query performance. In this article series, I am going to talk about statistics in detail. The SQL language is subdivided into several language elements, including Clauses, which are constituent components of statements and queries. In some cases, these. Basics of Statistics. 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 cardinality to create a high quality query execution plan. Often statistics are created on a single column but its not uncommon to create statistics on multiple columns. Each statistics object contains a histogram displaying the distribution of values of the column or of the first column in the case of multi column statistics. Multi column statistics also contains a correlation of values among the columns called densities, which are derived from the number of distinct rows or the column values. There are different ways you can view the details of the statistics objects. For example, as shown in the query below, you can use the DBCC SHOWSTATISTICS command. DBCC SHOWSTATISTICS shows the header, histogram, and density vector based on data stored in the statistics object. This shows header, histogram, and density vector based on data stored in the statistics object DBCCSHOWSTATISTICSSales. Order. Detail, NCISales. Order. DetailProduct. ID This only shows histogram based on data stored in the statistics object. DBCCSHOWSTATISTICSSales. Order. Detail, NCISales. Step_2_-_Selecting_schemas_and_tables.gif' alt='Sql Server Update Sequence Number Db2' title='Sql Server Update Sequence Number Db2' />Order. DetailProduct. IDWITHHISTOGRAM You can also view the statistical information by going to the properties page of the statistics object in SQL Server Management Studio as shown below Statistics Properties General. Statistics Properties Details. The value for alldensity 1 number of distinct values for a column ranges from 0. This actually helps SQL Server Query Optimizer to decide whether to use Index Seek or Index Scan. The histogram captures the frequency of occurrence for each distinct value in the first key column of the statistics object. SQL Server Query Optimizer creates the histogram by sorting the column values, computing the number of values that match each distinct column value and then aggregating the column values into a maximum of 2. 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. The lowest of the sorted column values is the upper boundary value for the first histogram step. RANGEHIKEY This is also called a key value and represents the upper bound column value for a histogram step. RANGEROWS This represents the estimated number of rows whose column value falls within a histogram step, excluding the upper bound. DISTINCTRANGEROWS This represents the estimated number of rows with a distinct column value within a histogram step, excluding the upper bound. EQROWS This represents the estimated number of rows whose column value equals the upper bound of the histogram step. AVGRANGEROWS RANGEROWS DISTINCTRANGEROWS for DISTINCTRANGEROWS 0 This represents the average number of rows with duplicate column values within a histogram step, excluding the upper bound. When to Create or Update Statistics. When to Create Statistics. Provider-Properties.png' alt='Sql Server Update Sequence Number Db2' title='Sql Server Update Sequence Number Db2' />Often columns being used in JOIN, WHERE, ORDER BY, or GROUP clauses are good candidate to have up to date statistics on them. Though the SQL Server Query Optimizer creates single column statistics when the AUTOCREATESTATISTICS database property is set to ON or when you create indexes on the table or views statistics are created on the key columns of the indexes, there might be times when you need to create additional statistics using the CREATE STATISTICS command to capture cardinality, statistical correlations so that it enables the SQL Server Query Optimizer to create improved query plans. When you find a query predicate containing multiple columns with cross column relationships and dependencies you should create multi column statistics. These multi column statistics contain cross column correlation statistics, often referred to as densities, to improve the cardinality estimates when query results depend on data relationships among multiple columns. When creating multi column statistics, be sure to put columns in the right order as this impacts the effectiveness of densities for making cardinality estimates. 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. Now if your query uses Name and Salary without using Age, the density is not available for cardinality estimates. When to Update Statistics. Substantial data change operations like insert, update, delete, or merge change the data distribution in the table or indexed view and make the statistics goes stale or out of date, as it might not reflect the correct data distribution in a given column or index. 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. A database table with no rows gets a row. A database table had fewer than 5. A database table had more than 5. You can find when each statistics object of a database table was updated using the below query SELECT        name AS Statistics. Name,    STATSDATEobjectid, statsidAS Statistics. Just Cause 2 Highly Compressed Tpb more. Event ID 8 Failed auto update retrieval of thirdparty root list sequence number from lthttpwww. PLSQL Procedural LanguageStructured Query Language is Oracle Corporations procedural extension for SQL and the Oracle relational database. PLSQL is available in. Sound Blaster Audigy Sb1394 Driver Windows 7 more. Updated. Date. FROMsys. WHEREOBJECTNAMEobjectidSales. Order. HeaderORDERBY name GOYou can also use below query, which uses the dynamic management function sys. SELECT. OBJECTNAMEstats. AS Table. Name. stats. AS Statistics. Name. FROMsys. statsstats. OUTERAPPLYsys. dmdbstatspropertiesstats. WHEREOBJECTNAMEstats. Sales. Order. HeaderORDERBYstats. Importance of Statistics in Query Performance. Lets start understanding this with an example. Execute the below query to create a new database and set its AUTOCREATESTATISTICS and AUTOUPDATESTATISTICS properties to OFF so that automatic statistics creation and updating does not happen on the tables of this database. Next create a table and load data from the Sales. Order. Detail table of the Adventure. Works database. I have loaded ten times of the data so that we can see the differences clearly. CREATEDATABASE Statistics. Test. ALTERDATABASE Statistics. Test SETAUTOCREATESTATISTICSOFFALTERDATABASE Statistics. Test SETAUTOUPDATESTATISTICSOFFALTERDATABASE Statistics. Test SETAUTOUPDATESTATISTICSASYNCOFF. USE Statistics. Test. CREATETABLE Sales. Order. Detail. Sales. Order. ID int NOTNULL. Save The Totem Village Game Walkthrough. Sales. Order. Detail.