SQL RAND Function

SQL RAND Function

The SQL RAND() function is used to generate some random numbers at run time. Here is the syntax:

Syntax

RAND ( [ seed ] )

Repetitive calls of RAND() with the same seed value return the same results.

For one connection, if RAND() is called with a specified seed value, all subsequent calls of RAND() produce results based on the seeded RAND() call. For example, the following query will always return the same sequence of numbers.

SELECT RAND(100), RAND(), RAND()

Examples:

The following example produces four different random numbers that are generated by the RAND function.

DECLARE @counter smallint
SET @counter = 1
WHILE @counter < 5
BEGIN
SELECT RAND() Random_Number
SET @counter = @counter + 1
END
GO

The following example returns random float numbers based on three different seed values.

CREATE TABLE Random (Seed1 float, Seed5 float, Seed10 float)
INSERT INTO Random Values (RAND(1), RAND(5), RAND(10))
SELECT * FROM Random

The RAND function is a pseudorandom number generator that operates in a manner similar to the C run-time library rand function. If no seed is provided, the system generates its own variable seed numbers. If you call RAND with a seed value, you must use variable seed values to generate random numbers. If you call RAND multiple times with the same seed value, it returns the same generated value. The following script returns the same value for the calls to RAND because they all use the same seed value:

SELECT RAND(159784)
SELECT RAND(159784)
SELECT RAND(159784)

A common way to generate random numbers from RAND is to include something relatively variable as the seed value, such as adding several parts of a GETDATE:

SELECT RAND( (DATEPART(mm, GETDATE()) * 100000 )
+ (DATEPART(ss, GETDATE()) * 1000 )
+ DATEPART(ms, GETDATE()) )

When you use an algorithm based on GETDATE to generate seed values, RAND can still generate duplicate values if the calls to RAND are made within the interval of the smallest datepart used in the algorithm. This is especially likely when the calls to RAND are included in a single batch. Multiple calls to RAND in a single batch can be executed within the same millisecond. This is the smallest increment of DATEPART. In this case, incorporate a value based on something other than time to generate the seed values.

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Tips And Tricks For Advanced MS SQL Server Developers

Tips And Tricks For Advanced MS SQL Server Developers:

  1. Use “TRUNCATE TABLE” statement instead of “DELETE” clause if you want to delete all rows from a table. It is much faster then “DELETE” statement without any conditions. “TRUNCATE TABLE” frees all the space occupied by that table’s data and indexes, without logging the individual row deletes.
  2. Always use owner prefix in T-SQL  queries:

    SELECT mycolumn FROM dbo.mytable

    In this case query optimizer does not have to decide whether to retrieve from dbo.mytable or other owner’s table and avoids recompilation.  Recompilation results in no performance advantages of stored procedures usage.

  3. Don’t use “sp_“ as your prefix for stored procedures – it is a reserved prefix in MS SQL server! MS SQL server searches for a stored procedure with “sp_” prefix in the system procedures first, and only after that looks for them in client procedures.
  4. If you are unable to install MSDE at home because of unknown error – check that you did not stop “Server” system service on you PC…
  5. There are thousands of examples, when developers use “SELECT COUNT(*)” statement. But there is another, much faster way to accomplish the task:
    SELECT rows FROM sysindexes WHERE id = OBJECT_ID('Table_Name') AND indid < 2
  6. Include “SET NOCOUNT ON” statement in your stored procedures to greatly reduce network traffic.
  7. Use the “BETWEEN” clause instead of “IN” for greater performance:
    SELECT productId FROM customer
    WHERE productId BETWEEN 1 AND 9

    Instead of:

    SELECT productId
    FROM customer
    WHERE productId IN (1, 2, 3, 4,5,6,7,8,9)
  8. Use Table variables – new feature of MS SQL 2000 instead of temp tables. Table variables are created in memory, not written to the tempdb database, and therefore they are much faster. However, be careful to use them only with not very huge amount of data that you want to allocate in temp tables, otherwise you can easily get the server down.

Database Settings Optimization Tips

Database Settings Optimization Tips

  • You can turn off the ‘auto create statistics’ database option.When this database option is set to true, statistics are automatically created on columns used in a predicate. By default, this database option is set to true. Because auto creation statistics results in some performance degradation, you can turn off this database option and create statistics manually during off-peak times by using the CREATE STATISTICS statement. By the way, in most cases, it will not provide some performance benefits.
  • You can turn off the ‘auto update statistics’ database option.When this database option is set to true, existing statistics are automatically updated when the statistics become out-of-date. By default, this database option is set to true. Because auto update statistics results in some performance degradation, you can turn off this database option and update statistics manually during off-peak times by using the UPDATE STATISTICS statement. By the way, in most cases, it will not provide some performance benefits.
  • Turn off the ‘autoclose’ database option.When this option is turned on, the database’s resources are freed after the last user exits. When the new user will connect to database, the database should be reopened, which takes some time. So, do hot set this database option to true on your production server. By default, this database option is set to true when using SQL Server Desktop Edition, and set to false for all other editions.
  • Turn off the ‘autoshrink’ database option.When this database option is set to true, the database files will be periodically shrink. Autoshrinking results in some performance degradation, therefore you should shrink the database manually or create a scheduled task to shrink the database periodically during off-peak times, rather than set Autoshrink feature to on. By default, this database option is set to true when using SQL Server Desktop Edition, and set to false for all other editions.
  • You can turn on the ‘read-only’ database option to prevent users to modify the database’s data.By default, this database option is set to false. If you have data that should not be modified, you can place it into another database and set for this database the ‘read-only’ option to true. It can increase the speed of your queries. If you need to allow permissions management (for example, prevent some users to select data from some tables), you should create another filegroup and make only this filegroup read-only, because when the ‘read-only’ database option is set to true, the database’s system tables will be also read-only and this will prevent the permissions management.
  • You can turn on the ‘select into/bulkcopy’ database option to allow SELECT INTO statements and nonlogged bulk copies.The nonlogged bulk copy is much faster than logged one, but to use it you must provide all the following conditions:1. The database option ‘select into/bulkcopy’ is set to true.

    2. The target table is not being replicated.
    3. The TABLOCK hint is specified.
    4. The target table has no indexes, or if the table has indexes, it is empty when the bulk copy starts.
    By default, this database option is set to false.

  • You can turn off the ‘trunc. log on chkpt.’ database option to prevent the transaction log from truncating on checkpoint.This option can be set if the transaction log grows very quickly to prevent the transaction log from filling rapidly and running out of disk space. If you set the ‘trunc. log on chkpt.’ database option to true, the transaction log cannot be backed up, so you cannot restore your data to the point of failure (only to the time when the last full backup was made). So, the general recommendation about this option is allow it to be turned off, and make the transaction log backup periodically to truncate the log. By default, this database option is set to true when using SQL Server Desktop Edition, and set to false for all other editions.

Note. You can set the above database options by using the sp_dboption system stored procedure or Enterprise Manager. If you want to set the above database options for the newly created database, you should set these options for the model database.

SQL Server Settings Optimization Tips

SQL Server Settings Optimization Tips:

  1. You can increase the ‘min memory per query’ option to improve the performance of queries that use hashing or sorting operations, if your SQL Server has a lot of memory available and there are many queries running concurrently on the server.
    The SQL Server will automatically allocate, at a minimum, the amount of memory set in this configuration setting. The default ‘min memory per query’ option is equal to 1024 Kb.
  2. You can increase the ‘max async IO’ option if your SQL Server works on a high performance server with high-speed intelligent disk subsystem (such as hardware-based RAID with more than 10 disks).
    This option specifies the maximum number of outstanding asynchronous disk I/O requests that the entire server can issue against a file. By the way, the ‘max async IO’ SQL Server option is no longer supported in SQL Server 2000.
  3. You can change the ‘network packet size’ option to the appropriate value.
    This option can improve performance on networks whose base topology supports larger packets than TCP/IP’s default of 4096 bytes. For example, if client sends or receives large amounts of data, a larger packet size can improve performance, because it results in fewer network reads and writes. The default value for the ‘network packet size’ option is 4096 bytes. Microsoft does not recommend changing this option, because for most applications, the default packet size of 4096 bytes is best.
  4. You can change the ‘fill factor’ option to the appropriate value.

    The ‘fill factor’ option specifies how full SQL Server will make each index page. When there is no free space to insert new row on the index page, SQL Server will create new index page and transfer some rows from the previous page to the new one. This operation is called page splits. You can reduce the number of page splits by setting the appropriate fill factor option to reserve free space on each index page. The fill factor is a value from 1 through 100 that specifies the percentage of the index page to be left empty. The default value for fill factor is 0. It is treated similarly to a fill factor value of 100, the difference in that SQL Server leaves some space within the upper level of the index tree for FILLFACTOR = 0. The fill factor percentage is used only at the time the index is created. If the table contains read-only data (or data that very rarely changed), you can set the ‘fill factor’ option to 100. When the table’s data modified very often, you can decrease the ‘fill factor’ option to 70 percent, for example.

  5. You can increase the ‘recovery interval’ value.
    The ‘recovery interval’ option specifies the maximum number of minutes per database that SQL Server needs to complete its recovery procedures. The default value of this option is 0. It means that SQL Server will automatically configure this option. SQL Server issues a checkpoint using the ‘recovery interval’ option. Microsoft does not recommend changing this option in general case, but sometimes you can improve performance by changing this option. You can monitor disk-write activity on the data files, and if you see periodic spikes that send disk utilization to 100 percent, you can increase the recovery interval. In this case, Microsoft suggests setting the ‘recovery interval’ option to 5 and continuing monitoring.
  6. You can set the ‘priority boost’ SQL Server options to 1.
    You can set this option to 1, if you want from SQL Server to work with a higher priority than other processes on the same computer. The default value is 0. Setting ‘priority boost’ to 1 can degrade the performance of other applications running on the same computer with SQL Server. So, you should set the ‘priority boost’ SQL Server options to 1 only if you have dedicated server to SQL Server. In other case, do not change this option.
  7. Set the ‘max worker threads’ options to the maximum number of the user connections to your SQL Server box.
    The default setting for the ‘max worker threads’ option is 255. If the number of user connections will be less than the ‘max worker threads’ value, a separate operating system thread will be created for each client connection, but if the number of user connections will exceed this value the thread pooling will be used. For example, if the maximum number of the user connections to your SQL Server box is equal to 50, you can set the ‘max worker threads’ options to 50, this frees up resources for SQL Server to use elsewhere. If the maximum number of the user connections to your SQL Server box is equal to 500, you can set the ‘max worker threads’ options to 500, this can improve SQL Server performance because thread pooling will not be used.
  8. You can specify the ‘min server memory’ and ‘max server memory’ options.
    These options can be used to specify the fixed amount of memory to allocate to SQL Server. In this case, you should set the ‘min server memory’ and ‘max server memory’ to the same value (equal to the maximum amount of physical memory that SQL Server will use), and set the ‘set working set size’ SQL Server option to 1. This can improve performance because SQL Server will not dynamically allocate memory. You can also change these options when SQL Server works on the same computer with other applications. In this case, the ‘min server memory’ options is used to allow SQL Server works when other applications pretend to use all available memory, and the ‘max server memory’ options is used to allow other applications work when SQL Server tried to use all available resources.
  9. You can specify the ‘set working set size’ SQL Server option to reserve the amount of physical memory space for SQL Server.
    Unlike SQL Server 6.5, SQL Server 7.0/2000 can automatically allocate memory (can take more memory if SQL Server need it, and can give memory back to operation system). This is one of the main advantages in comparison with previous versions, but dynamic memory allocation takes some time. If you know the maximum amount of physical memory that SQL Server will use, you can specify this amount by setting ‘min server memory’ and ‘max server memory’ to the same value (equal to the maximum amount of physical memory that SQL Server will use) and set the ‘set working set size’

Basic Definitions

Definition of a Database

A database is a collection of related information, accessed and managed by its DBMS. After experimenting with hierarchical and networked DBMSs during the 1970’s, the IT industry became dominated by relational DBMSs (Or Object-Relational Database Management System) such as Informix database, Oracle, Sybase, and, later on, Microsoft SQL Server and the like.

In a strictly technical sense, for any database to be defined as a “Truly Relational Model Database Management System,” it should, ideally, adhere to the twelve rules defined by Edgar F. Codd, pioneer in the field of relational databases. To date, while many come close, it is admitted that nothing on the market adheres 100% to those rules, any more than they are 100% ANSI-SQL compliant.

While IBM and Oracle technically were the earliest on the RDBMS scene, many others have followed, and while it is unlikely that miniSQL still exist in their original form, Monty’s MySQL is still extant and thriving, along with the Ingres-descended PostgreSQL. Microsoft Access – the 1995+ versions, not the prior versions – were, despite various limitations, technically the closest thing to being ‘Truly Relational’ DBMS’s for the desktop PC, with Visual FoxPro, and many other desktop products marketed at that time far less compliant with Codd’s Rules.

A relational DBMS manages information about types of real-world things (entities) in the form of tables that represent the entities. A table is like a spreadsheet; each row represents a particular entity (instance—), and each column represents a type of information about the entity (domain). Sometimes entities are made up of smaller related entities, such as orders and order lines; and so one of the challenges of a multi-user DBMS is provide data about related entities from the standpoint of an instant of logical consistency.

Properly managed relational databases minimize the need for application programs to contain information about the physical storage of the data they access. To maximize the isolation of programs from data structures, relational DBMSs restrict data access to the messaging protocol SQL, a nonprocedural language that limits the programmer to specifying desired results. This message-based interface was a building block for the decentralization of computer hardware, because a program and data structure with such a minimal point of contact become feasible to reside on separate computers.

Recoverability

Recoverability means that, if a data entry error, program bug or hardware failure (Vista) occurs, the DBA can bring the database backward in time to its state at an instant of logical consistency before the damage was done. Recoverability activities include making database backups and storing them in ways that minimize the risk that they will be damaged or lost, such as placing multiple copies on removable media and storing them outside the affected area of an anticipated disaster. Recoverability is the DBA’s most important concern.

The backup of the database consists of data with timestamps combined with database logs to change the data to be consistent to a particular moment in time. It is possible to make a backup of the database containing only data without timestamps or logs, but the DBA must take the database offline to do such a backup.

The recovery tests of the database consist of restoring the data, then applying logs against that data to bring the database backup to consistency at a particular point in time up to the last transaction in the logs. Alternatively, an offline database backup can be restored simply by placing the data in-place on another copy of the database.

If a DBA (or any administrator) attempts to implement a recoverability plan without the recovery tests, there is no guarantee that the backups are at all valid. In practice, in all but the most mature RDBMS packages, backups rarely are valid without extensive testing to be sure that no bugs or human error have corrupted the backups.

Security
Security means that users’ ability to access and change data conforms to the policies of the business and the delegation decisions of its managers. Like other metadata, a relational DBMS manages security information in the form of tables. These tables are the “keys to the kingdom” and so it is important to protect them from intruders.

Performance

Performance means that the database does not cause unreasonable online response times, and it does not cause unattended programs to run for an unworkable period of time. In complex client/server and three-tier systems, the database is just one of many elements that determine the performance that online users and unattended programs experience. Performance is a major motivation for the DBA to become a generalist and coordinate with specialists in other parts of the system outside of traditional bureaucratic reporting lines.

Techniques for database performance tuning have changed as DBA’s have become more sophisticated in their understanding of what causes performance problems and their ability to diagnose the problem.

In the 1990s, DBAs often focused on the database as a whole, and looked at database-wide statistics for clues that might help them find out why the system was slow. Also, the actions DBAs took in their attempts to solve performance problems were often at the global, database level, such as changing the amount of computer memory available to the database, or changing the amount of memory available to any database program that needed to sort data.

DBA’s now understand that performance problems initially must be diagnosed, and this is best done by examining individual SQL statements, table process, and system architecture, not the database as a whole. Various tools, some included with the database and some available from third parties, provide a behind the scenes look at how the database is handling the SQL statements, shedding light on what’s taking so long.Having identified the problem, the individual SQL statement can be clarify

Development/Testing Support

Development and testing support is typically what the database administrator regards as his or her least important duty, while results-oriented managers consider it the DBA’s most important duty. Support activities include collecting sample production data for testing new and changed programs and loading it into test databases; consulting with programmers about performance tuning; and making table design changes to provide new kinds of storage for new program functions.

Indexing Service

What is Indexing Service?

Indexing Service is a base service for Microsoft® Windows® 2000 or later that extracts content from files and constructs an indexed catalog to facilitate efficient and rapid searching.

Indexing Service can extract both text and property information from files on the local host and on remote, networked hosts. The files can be simply members of a selected file system or part of a virtual Web hosted by, for example, Internet Information Services (IIS).

Indexing Service extracts the content by filtering—using filter components that understand a file’s format. The format could include multi-language features such as international languages and locales. A filter component implements the IFilter interface, which supplies methods to read a file to extract text and properties. Windows 2000 and Microsoft Windows XP supply filters for Microsoft Office files, Hypertext Markup Language (HTML) files, Multipurpose Internet Mail Extension (MIME) messages, and plain-text files.

Indexing Service then merges the extracted information into catalogs of indexes for efficient searches. Indexing is the overall process of filtering, creating index entries, and merging them into catalogs.

The final step in the indexing process is creation of a catalog that contains a master index (and any temporary word lists and shadow indexes) storing words and their locations within a set of indexed documents. Subsequently, searching, or querying, the catalogs for particular word combinations uses the master index as well as word lists and shadow indexes to execute queries quickly and efficiently.

Windows 2000 and Windows XP include basic facilities for querying the Indexing Service catalog and for managing the state and properties of Indexing Service itself. These facilities include:

  • When Indexing Service is running, Start/Search/For Files or Folders uses the Indexing Service catalog.
  • The Indexing Service snap-in for the Microsoft Management Console (MMC) provides the means to start, stop, and pause Indexing Service, and to administer many of its properties, such as those defining its catalogs.
  • The Platform Software Development Kit (SDK) provides additional versatile and flexible facilities for programmatically interacting with Indexing Service. These facilities include:
  • Admin and Query Helper objects and ActiveX® Data Object (ADO) methods for use with Microsoft Visual Basic®, Microsoft Visual Basic Scripting Edition (VBScript), Microsoft Visual J++® and Microsoft JScript® development software.
  • ISAPI Extensions for use in .idq, .ida, and .htx files.
  • OLE DB Helper functions for use with Microsoft Visual C++® development system.
  • OLE DB Provider for Indexing Service interfaces for use with Visual C++.
  • IFilter interface for use with Visual C++

Source: MSDN

SQL Performance Tuning using Indexes

SQL Performance Tuning using Indexes

This article looks at general guidelines to creating effective indexes using short keys, distinct keys, covering indexes and clustered indexes.

Effective indexes are one of the best ways to improve performance in a database application. Without an index, the SQL Server engine is like a reader trying to find a word in a book by examining each page. By using the index in the back of a book, a reader can complete the task in a much shorter time. In database terms, a table scan happens when there is no index available to help a query. In a table scan SQL Server examines every row in the table to satisfy the query results. Table scans are sometimes unavoidable, but on large tables, scans have a terrific impact on performance.

One of the most important jobs for the database is finding the best index to use when generating an execution plan. Most major databases ship with tools to show you execution plans for a query and help in optimizing and tuning indexes. This article outlines several good rules of thumb to apply when creating and modifying indexes for your database. First, let’s cover the scenarios where indexes help performance, and when indexes can hurt performance.

Useful Index Queries

Just like the reader searching for a word in a book, an index helps when you are looking for a specific record or set of records with a WHERE clause. This includes queries looking for a range of values, queries designed to match a specific value, and queries performing a join on two tables. For example, both of the queries against the Northwind database below will benefit from an index on the UnitPrice column.

DELETE FROM Products WHERE UnitPrice = 1

SELECT * FROM PRODUCTS
WHERE UnitPrice BETWEEN 14 AND 16

Since index entries are stored in sorted order, indexes also help when processing ORDER BY clauses. Without an index the database has to load the records and sort them during execution. An index on UnitPrice will allow the database to process the following query by simply scanning the index and fetching rows as they are referenced. To order the records in descending order, the database can simply scan the index in reverse.

SELECT * FROM Products ORDER BY UnitPrice ASC

Grouping records with a GROUP BY clause will often require sorting, so a UnitPrice index will also help the following query to count the number of products at each price.

SELECT Count(*), UnitPrice FROM Products
GROUP BY UnitPrice

By retrieving the records in sorted order through the UnitPrice index, the database sees matching prices appear in consecutive index entries, and can easily keep a count of products at each price. Indexes are also useful for maintaining unique values in a column, since the database can easily search the index to see if an incoming value already exists. Primary keys are always indexed for this reason.

Index Drawbacks

Indexes are a performance drag when the time comes to modify records. Any time a query modifies the data in a table the indexes on the data must change also. Achieving the right number of indexes will require testing and monitoring of your database to see where the best balance lies. Static systems, where databases are used heavily for reporting, can afford more indexes to support the read only queries. A database with a heavy number of transactions to modify data will need fewer indexes to allow for higher throughput. Indexes also use disk space. The exact size will depends on the number of records in the table as well as the number and size of the columns in the index. Generally this is not a major concern as disk space is easy to trade for better performance.

Building The Best Index

There are a number of guidelines to building the most effective indexes for your application. From the columns you select to the data values inside them, consider the following points when selecting the indexes for your tables.

Short Keys

Having short index is beneficial for two reasons. First, database work is inherently disk intensive. Larger index keys will cause the database to perform more disk reads, which limits throughput. Secondly, since index entries are often involved in comparisons, smaller entries are easier to compare. A single integer column makes the absolute best index key because an integer is small and easy for the database to compare. Character strings, on the other hand, require a character by character comparison and attention to collation settings.

Distinct Keys

The most effective indexes are the indexes with a small percentage of duplicated values. As an analogy, think of a phone book for a town where almost everyone has the last name of Smith. A phone book in this town is not very useful if sorted in order of last name, because you can only discount a small number of records when you are looking for a Smith.

An index with a high percentage of unique values is a selective index. Obviously, a unique index is highly selective since there are no duplicate entries. Many databases will track statistics about each index so they know how selective each index is. The database uses these statistics when generating an execution plan for a query.


Covering Queries

Indexes generally contain only the data values for the columns they index and a pointer back to the row with the rest of the data. This is similar to the index in a book: the index contains only the key word and then a page reference you can turn to for the rest of the information. Generally the database will have to follow pointers from an index back to a row to gather all the information required for a query. However, if the index contains all of he columns needed for a query, the database can save a disk read by not returning to the table for more information.

Take the index on UnitPrice we discussed earlier. The database could use just the index entries to satisfy the following query.

SELECT Count(*), UnitPrice FROM Products
GROUP BY UnitPrice

We call these types of queries covered queries, because all of the columns requested in the output are covered by a single index. For your most crucial queries, you might consider creating a covering index to give the query the best performance possible. Such an index would probably be a composite index (using more than one column), which appears to go against our first guideline of keeping index entries as short as possible. Obviously this is another tradeoff you can only evaluate with performance testing and monitoring.

Clustered Indexes

Many databases have one special index per table where all of the data from a row exists in the index. SQL Server calls this index a clustered index. Instead of an index at the back of a book, a clustered index is closer in similarity to a phone book because each index entry contains all the information you need, there are no references to follow to pick up additional data values.

As a general rule of thumb, every non-trivial table should have a clustered index. If you only create one index for a table, make the index a clustered index. In SQL Server, creating a primary key will automatically create a clustered index (if none exists) using the primary key column as the index key. Clustered indexes are the most effective indexes (when used, they always cover a query), and in many databases systems will help the database efficiently manage the space required to store the table.

When choosing the column or columns for a clustered index, be careful to choose a column with static data. If you modify a record and change the value of a column in a clustered index, the database might need to move the index entry (to keep the entries in sorted order). Remember, index entries for a clustered index contain all of the column values, so moving an entry is comparable to executing a DELETE statement followed by an INSERT, which can obviously cause performance problems if done often. For this reason, clustered indexes are often found on primary or foreign key columns. Key values will rarely, if ever, change.

Conclusion

Determining the correct indexes to use in a database requires careful analysis, benchmarking, and testing. The rules of thumb presented in this article are general guidelines. After applying these principals you need to retest your specific application in your specific environment of hardware, memory, and concurrent activity. See my previous article: SQL Server Indexes, for a more thorough introduction.