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🦑 Comparison of SQL Server and Oracle to prevent data locking
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1) Database parallel access, that is, two Or two or more users access the same data at the same time, which is also the biggest problem faced by the database engine how to design and implement a moderate response.

2) A well-designed, high-performance database engine can easily serve thousands of users at the same time. The database system with "lack of confidence" will greatly reduce its performance as more users access the system at the same time. In the worst case, the system may even crash.

3) Of course, parallel access is the most important issue for any database solution. Various database systems have proposed various solutions in order to solve the problem of parallel access. SQL Server and Oracle DBMS also use different parallel processing methods. What is the real difference between them?

🦑
1) Parallel access problems
Parallel access problems arise there are several situations. In the simplest case, more than one user may query the same data at the same time. In this case, the operation goal of the database is simple: provide users with fast data access as much as possible. This is not a problem for our common databases: SQL Server and Oracle both use a multi-threaded mechanism, they can of course handle multiple requests at once.

2) SQL Server Methods
Now assume that some people might start modify the data stored on SQL Server, so this piece of data is immediately lock the database. The data lock operation blocks any other connection that accesses the data-even the query operation will not let go. Therefore, this locked data can only accept other access operations after the transaction is submitted or rolled back.

The following uses the pubs sample database that comes with SQL Server as a simple demonstration. Open two windows in Query Analyzer. Execute the following SQL operation statement in the first window to update the price of a book in the pubs database:

use pubs
go
begin tran
update titles
set price = price * 1.05
where

title_id = 'BU2075'

3) Because the commit statement is not executed in the code, Therefore, the data change operation has not actually been completed. Next, execute the following statement in another window to query the titles data table:
select title_id, title, price
from titles
order by title_id.

4) You get nothing. The small globe icon at the bottom of the window will keep turning. Although I only updated one row in the previous operation, the execution object of the select statement happens to contain the row whose data is being modified. Therefore, the above operation will not return any data unless you go back to the first window to submit the transaction or roll back.

5) SQL Server's data locking scheme may reduce the performance and efficiency of the system. The longer the data is locked, or the larger the amount of data locked, the more likely other data access users will have to wait for the execution of their query statements. Therefore, from the programmer's point of view, the transaction code should be designed to be as small and fast as possible when programming SQL Server.

6) The SQL Server solution sounds simple, but in fact, many measures have been taken behind the scenes to provide adequate system performance. For example, if you are modifying multiple rows of data at the same time, SQL Server will increase the data lock range to the page level or even lock the entire data table, so that you do not have to track and maintain individual data locks for each record.

written by undercode
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🦑 Oracle Method

1) Here we look at how Oracle database is to implement similar operations. First, I open an instance of SQLPlus and execute the following query statement (this example can be found in the example in Oracle 9i). This instance is called the query instance:
select first_name, last_name, salary
from hr.employees
where
department_id = 20; The
code returns two rows of data, as shown below:

2) Then, open another instance of SQLPlus-update the instance to execute the following command:
1 SQL> update hr.employees
2 set salary = salary * 1.05
3 where
4 department_id = 20
5 / After the
code is executed, the reply message says that the two lines of data have been updated.

>Note that in the above code, there is code that type "begin tran" as in the SQL Server example. Oracle's SQLPlus implicitly enables transactions (you can also mimic the behavior of SQL Server and set "autocommit to on" to automatically submit transactions). Next, we execute the same select statement as the query instance in the SQLPlus update instance.

3) The results clearly show that the salary of bboth Michael and Pat has increased, but at this time I have not submitted a data change transaction.
Now go to the first SQLPlus query instance and re-run the query


🦑Oracle does not require the user to wait for the operation to be submitted in the data update instance. It directly returns the query information of Michael and Pat, but actually returns the data before the data update begins. Data view!

5) At this time, people who are familiar with SQL Server may say that setting (NOLOCK) in the query can not achieve the same effect? However, for SQL Server, data cannot be obtained before the data image. Specifying (NOLOCK) actually just got the data that was not submitted. Oracle's approach provides a consistent view of the data. All information is transaction-oriented and based on stored data snapshots.

6) If you submit an update transaction in the update instance of SQLPlus, you can see the salary data change in the query instance. If you rerun the previous query statement in the query instance, Oracle will return the new salary value.

🦑 Storing data snapshot
That for a long, displayed to the user in a previous version of the data at the same time, Oracle is how to allow other users to modify data in it? In fact, as soon as a user initiates a transaction to modify data, the previous data image will be written to a special storage area. This "front image" is used to provide a consistent view of the database to any user who queries the data. In this way, when other users are modifying the data, we can see the salary data that has not changed in the above test.

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