create barcode using vb.net DATA MANIPULATION in Java

Maker Data Matrix ECC200 in Java DATA MANIPULATION

DATA MANIPULATION
Generating Data Matrix 2d Barcode In Java
Using Barcode creation for Android Control to generate, create Data Matrix image in Android applications.
www.OnBarcode.com
Print Barcode In Java
Using Barcode creation for Android Control to generate, create Barcode image in Android applications.
www.OnBarcode.com
All DDL commands (such as CREATE, ALTER, DROP, GRANT, and REVOKE) always imply an implicit COMMIT. To put it another way, each single DDL command is executed as a transaction in itself, consisting of a single command, and is committed immediately.
Painting PDF417 In Java
Using Barcode creation for Android Control to generate, create PDF417 image in Android applications.
www.OnBarcode.com
Code 3 Of 9 Creator In Java
Using Barcode encoder for Android Control to generate, create Code39 image in Android applications.
www.OnBarcode.com
6.6 Locking and Read Consistency
Creating Barcode In Java
Using Barcode creator for Android Control to generate, create Barcode image in Android applications.
www.OnBarcode.com
Printing Barcode In Java
Using Barcode creator for Android Control to generate, create Barcode image in Android applications.
www.OnBarcode.com
Normally, many users and applications access database systems at the same time. This is known as concurrency. The RDBMS must make sure that concurrency is handled properly. The most drastic approach for a RDBMS would be to handle all user transactions one by one, blocking all data exclusively until the end of each transaction. Such a transaction serialization approach would result in unnecessary and unacceptable wait times; the overall system throughput would be very poor. RDBMSs like Oracle control concurrent data access with locking to prevent database users from updating rows with pending (uncommitted) changes from other database users. This section gives some information about how the Oracle RDBMS handles locking and concurrency.
Generate Quick Response Code In Java
Using Barcode creation for Android Control to generate, create QR Code ISO/IEC18004 image in Android applications.
www.OnBarcode.com
Interleaved 2 Of 5 Creation In Java
Using Barcode printer for Android Control to generate, create Uniform Symbology Specification ITF image in Android applications.
www.OnBarcode.com
Locking
Paint Data Matrix In Java
Using Barcode creator for Java Control to generate, create DataMatrix image in Java applications.
www.OnBarcode.com
Data Matrix Encoder In None
Using Barcode drawer for Software Control to generate, create ECC200 image in Software applications.
www.OnBarcode.com
To understand how the Oracle RDBMS handles locking, we need to identify a difference between two categories of database users: Readers: Users retrieving data (issuing SELECT statements) Writers: Users changing data (issuing INSERT, UPDATE, DELETE, and MERGE commands)
Make Code39 In Objective-C
Using Barcode printer for iPad Control to generate, create USS Code 39 image in iPad applications.
www.OnBarcode.com
Code 128C Scanner In VB.NET
Using Barcode scanner for Visual Studio .NET Control to read, scan read, scan image in .NET framework applications.
www.OnBarcode.com
The Oracle RDBMS does not lock any data for retrieval. This means that readers never block readers. Moreover, this also means that writers never need to wait for readers, and vice versa.
Code 3/9 Recognizer In None
Using Barcode scanner for Software Control to read, scan read, scan image in Software applications.
www.OnBarcode.com
Make Code 128 In Visual Basic .NET
Using Barcode creator for VS .NET Control to generate, create Code 128C image in Visual Studio .NET applications.
www.OnBarcode.com
Note The Oracle RDBMS s handling of data locking does not mean that readers and writers do not hinder each other in any way. Readers and writers can cause delays for each other by contending for certain system resources, such as CPU.
Generating Universal Product Code Version A In Objective-C
Using Barcode generator for iPad Control to generate, create UPC Code image in iPad applications.
www.OnBarcode.com
Data Matrix 2d Barcode Scanner In .NET
Using Barcode scanner for .NET Control to read, scan read, scan image in .NET applications.
www.OnBarcode.com
Multiple database users trying to change the same rows need to wait for each other, so writers may block other writers. Each attempt to change a row tries to acquire the corresponding row-level lock first. If the lock cannot be acquired, you must wait until the pending change is committed or rolled back. All row-level locks are released upon a COMMIT (explicit or implicit) or ROLLBACK. This means that the Oracle DBMS tries to minimize locking overhead and tries to maximize throughput and concurrency.
Recognizing Code39 In VB.NET
Using Barcode reader for Visual Studio .NET Control to read, scan read, scan image in VS .NET applications.
www.OnBarcode.com
Encoding UPC-A Supplement 2 In C#
Using Barcode creation for VS .NET Control to generate, create UPC-A Supplement 2 image in Visual Studio .NET applications.
www.OnBarcode.com
Note Only those rows that are actually being modified are locked. Many separate users and sessions can simultaneously lock rows in a single table.
Draw Barcode In Java
Using Barcode printer for Java Control to generate, create Barcode image in Java applications.
www.OnBarcode.com
Print Code128 In VS .NET
Using Barcode maker for ASP.NET Control to generate, create Code128 image in ASP.NET applications.
www.OnBarcode.com
DATA MANIPULATION
Read Consistency
In a database environment, read consistency is an important concept. Read consistency is a first requirement to guarantee correct query results, regardless of how long it runs and regardless what else happens simultaneously in the database. The Oracle RDBMS must make sure that each SQL query creates a snapshot of the data at the point in time when the query started. It needs this snapshot because a query should never see any uncommitted changes nor any changes that were committed after the query started. Imagine the problems that would occur if one person was updating salaries (even making mistakes that had to be rolled back) while another person was running a payroll report. Without read consistency, the payroll report might include old salaries, new salaries and salary mistakes and there would be no way to know which person was being paid incorrectly. This means that the Oracle RDBMS must be able to reconstruct previous versions of the data in order to process queries. We will not go into technical details here, but the Oracle RDBMS accomplishes this by using information stored in undo segments. One way to think about undo segments is that they contain the before image of the data before any modification, though this is not technically precise. Believe it or not, read consistency is even important in a single-user environment. Suppose that upper management has decided to grant a salary raise of 50% to all employees who currently earn less than the average salary of their department. You might want your salary to be checked last by the UPDATE statement, hoping that earlier salary raises have influenced your department s average salary in such a way that you became entitled to a raise, too. In an Oracle environment, this hope is in vain, because the read consistency mechanism will ensure that the subquery in the UPDATE statement (to derive the average salary of your department) returns the same result, regardless of how often the subquery is reexecuted for the same department, within the scope of that single UPDATE command. But note that because the Oracle RDBMS does not use any locking or other obstructive techniques when you do this, you incur the risk that, at a point in time, the Oracle RDBMS will not be able to reconstruct the desired original data anymore, especially if your query is running a long time. You get the following error message in such situations: ORA-01555: Snapshot too old Oracle will never return data to a query that is inconsistent with the point in time at which the query began. (You could term such data as being read inconsistent). Instead, Oracle terminates the query with the Snapshot too old error. This error simply means that the query is unable to create the before image of the data as of the time the query started. Read consistency and the Oracle mechanisms used to enforce it also insure the integrity of the data in the query. This completes your introduction to data manipulation commands and concepts. You learned about the four DML commands of the SQL language: INSERT, UPDATE, DELETE, and MERGE. Then we discussed transaction processing, using the commands COMMIT, SAVEPOINT, and ROLLBACK. Finally, we briefly discussed read consistency and locking, and introduced the SET TRANSACTION command, which you can use to influence the default read consistency behavior of the Oracle DBMS. Before continuing with 7, which returns to the topic of data definition, make sure that all of your case tables are in their unmodified state. You should have rolled back all of the changes you applied in this chapter. Alternatively, you can re-create the tables before proceeding.
Copyright © OnBarcode.com . All rights reserved.