虎搞瞎搞

I don’t remember how many times I was asked about an equivalent term of the infamous “NOLOCK” hint for mysql database server, hence I thought it was worth to write about it here. “WITH (NOLOCK)” is a transaction isolation levels that defines how data is available during an update, or with other words it is a property that defines at what point changes made by an update operation will become available in a row, table or database to other processes.

The official SQL standard defines four isolation levels:

READ COMMITTED
READ UNCOMMITTED
REPEATABLE READ
SERIALIZABLE

When WITH (NOLOCK) is used with SQL Server, the statement does not place a lock nor honor exclusive locks on table. The MySQL equivalent is READ UNCOMMITTED, also known as “dirty read” because it is the lowest level of isolation. If we specify a table hint then it will override the current default isolation level. MySQL default isolation level is REPEATABLE READ which means locks will be placed for each operation, but multiple connections can read data concurrently.

SQL Server WITH (NOLOCK) looks like this:

SELECT * FROM TABLE WITH (nolock)

To achieve the same with MySQL, we change the session isolation mode using the SET SESSIONcommand.

SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED ;
SELECT * FROM TABLE_NAME ;
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ ;

This statement will work similar to WITH (NOLOCK) i.e READ UNCOMMITTED data. We can also set the isolation level for all connections globally:

SET GLOBAL TRANSACTION ISOLATION LEVEL READ UNCOMMITTED ;

In addition, two system variables related to isolation also level exist in MySQL server:

SELECT @@global.tx_isolation; (global isolation level)
SELECT @@tx_isolation; (session isolation level)

Or set the isolation level inside a transaction:

SET TRANSACTION ISOLATION LEVEL REPEATABLE READ
GO

引用:http://articles.itecsoftware.com/mysql/with-nolock-table-hint-equivalent-for-mysql

If you're getting...

"HTTP Error 404.17 - Not Found - The requested content appears to be script and will not be served by the static file handler."

...on Vista while trying to get PHP working under IIS7 with the standard ISAPI "php5isapi.dll" ask yourself, are you running 64-bit? That ISAPI DLL is a 32-bit DLL, so you'll have to either change your default Application Pool to enable 32-bit, or preferably create a separate 32-bit AppPool for your PHP Application.

Right click on the Application Pool and select "Advanced Settings" then "Enable 32-bit Applications."

At this point, you're all set with the standard ISAPI PHP stuff. 

Even better, consider using the FastCGI for IIS component. I'll do a screencast on that soon.

引用:http://www.hanselman.com/blog/HTTPError40417PHPOnIIS7Under64bitVista.aspx?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed:+ScottHanselman+(Scott+Hanselman+-+ComputerZen.com)

一、什麼是索引？
索引用來快速地尋找那些具有特定值的記錄，所有MySQL索引都以B-樹的形式保存。如果沒有索引，執行查詢時MySQL必須從第一個記錄開始掃瞄整個表 的所有記錄，直至找到符合要求的記錄。表裡面的記錄數量越多，這個操作的代價就越高。如果作為搜索條件的列上已經創建了索引，MySQL無需掃瞄任何記錄 即可迅速得到目標記錄所在的位置。如果表有1000個記錄，通過索引查找記錄至少要比順序掃瞄記錄快100倍。