wcag levels

Tags: database acid transaction ISOLATION LEVELDatabase acid PrinciplesA: Atomicity, atomicityC: Consistency, consistencyI: Isolation, isolationD: Persistent, durabilityIn MySQL InnoDB engine, atomicity, consistency, isolation through redo and undo implementation, Redo is ib_logfile physical files, and undo by default in the shared Tablespace ibdata, by setting parameters can be independent.Transaction ISOLATION LEVELREAD UNCOMMITTED the disadvantage of allowing dirty reads, not allowing simulta

1.read UNCOMMITTEDYou can see uncommitted data (dirty reads), for example: You believe what people say, but maybe he just says it and doesn't actually do it.2.read committedReads the submitted data. However, data results that may be read multiple times are inconsistent (non-repeatable read, Phantom Read). The reading and writing view is: Read the row data, can write.3.repeatable Read (MySQL default isolation level)Can be read repeatedly, but with Phantom read. Read-write view: The data rows read

), the index as a condition to update the data, there will be a gap lock gap lock, Row lock, the next key lock problem, thereby locking some rows, if there is no index, the update data will lock the entire table.5. When the transaction isolation level is serialized, the read/write data will lock the entire table6, the higher the isolation level, the more you can ensure the integrity and consistency of data, but the impact on concurrency is also greater, fish and bear paw can not be both. For mos

ODBC imported and text imported in 1.plsql, you can select file import directly. Specific import operation details are not detailed. Large data volume import efficiency is not good;2.sql develop, select the target table, select "Import data" from the right-click menu, save the Excel file in CVS format, browse to select, and follow the wizard. The efficiency is relatively high!380,000 data about 14 minutes to complete the import.Several ways to import 100,000

the blood-light disaster of reducing wages.Summary: While reading the same data guarantees consistency, there is no guarantee that no new data will be inserted　　4.SERIALIZABLE (Serializable)The serializable is the highest isolation level, which is performed serially by forcing transactions (note serial), avoiding the previous phantom read, and because of his large number of locks, which causes a large number of requests to time out, so performance is compared to the bottom, This isolation level

) If another transaction changes this record first and does not commit, the transaction will be stuck and waiting. This transaction continues to run until another transaction has ended. The difference is that this change will affect the record is not 0, that is, the modification will take effect (overwrite the first transaction modification). (second type of update is missing)Test code for MySQL-- ------------------------------Table structure for test-- ----------------------------DROP TABLE IF

= 1While @i = 1BeginIf GETDATE () > ' 2004-07-22 14:23 '--set a time to execute the UPGETBH stored procedure at the same timeSet @i = 0EndEXEC UP_GETBHThen, running each connection in succession, at the moment of 2004-7-22 14:23, each connection runs UP_GETBH at the same time. From the running results you can see that the connection sequence appears with a 18001-digit number, and there is no sign of the number or the phenomenon of missing.Analysis: Because the SELECT statement in UP_GETBH uses

, granularity is finer and concurrency can be achieved, but the implementation mechanism is more complex(MySQL server supports table-level locks only, row locks need to be completed by the storage engine) Table lock Row lock Isolation Level : from low to high, the higher the isolation level, the lower the concurrency capability READ UNCOMMITTED: Reads uncommitted: All transactions can see the results of other transactional operations, various chaotic Read COMMITTED

2015-11-20RMAN>Sql'ALTER DATABASE open Resetlogs'; SQL statement:Alter Database OpenResetlogsrman-00571:===========================================================RMAN-00569:===============ERROR MESSAGE STACK follows===============RMAN-00571:===========================================================RMAN-03009: Failure ofSQL command on defaultChannel at One/ -/ - -: -: toRMAN-11003: Failure during parse/Execution ofSQL statement:Alter Database OpenResetlogsora-01152:file 1Was notRestored fromA

Set the system run levelRunLevel: A state identifier for the Linux runtime that is typically represented by a number,0 Shutdown Status1 Single User status (usage: Password loss can be retrieved by entering a single user level)2 Multi-user state, but no NFS network file system3 text mode, full command line mode (focus) mode of operation4 unused system is unused, reserved for use by users5 graphical desktop mode, X11 (server side does not need to use)6 Restart ModeCommon operating

1. PrefaceHotspot has adopted a accessibility analysis algorithm to determine whether an object can be used by a GC, whether it is a reference or an accessibility analysis algorithm to determine whether an object exists or not. If the value stored in the data of type reference represents the starting address of another piece of memory, it is said that this memory represents a reference. To enrich the description of the relationship between objects and objects, in order to implement the system ca

The following article describes the prevention of four levels of attack to ensure the security of Linux servers, if you are against four levels of attack to ensure that the Linux server security is curious, the following article will uncover its mystery. With the continuous expansion of Linux enterprise applications. has a large number of network servers that are using the Linux operating system. The secu

++, while thread B is querying cars_beijing and Cars_ Shanghai value, isolation to ensure that thread B can only query to the transaction is completely not executed or completely successful execution of the value, do not allow thread B query to only execute cars_beijing– without executing cars_shanghai++ value4. Persistence (durability), the better understanding is that once a transaction is committed, the modified data is persisted, even if the power is not lost. Isolation is an intermediate va

Label:There are four database isolation levels, as described in the "high-performance MySQL" Book:Then talk about how to modify the transaction isolation level:1. Global modification, modify the Mysql.ini configuration file, at last add1 #可选参数有: read-uncommitted, read-committed, Repeatable-read, serializable.2 [mysqld]3 transaction-isolation = Repeatable-readHere the global default is Repeatable-read, in fact, MySQL is the default is this level2. Modi

transaction two times the number of data pens inconsistent, for example, one transaction queried several columns (row) of data, while another transaction at this time inserted a new column of data, the previous transaction in the next query, you will find that there are a few columns of data that it did not previously.III. four isolation levels for transactionsThe SQL standard defines a Class 4 isolation level, which includes specific rules to define

not present in the first query, or the data that appears in the first query is missing (the same SQL statement that does not require two queries). This is due to the fact that another transaction was inserted into the data during the two queries. 2. Database things isolation level To avoid the above scenarios, in the standard SQL specification, 4 transaction Isolation levels are defined, with different isolation

One, read uncommitted (not submitted)At the READ uncommitted level, modifications in a transaction are visible to other transactions, even if they are not committed. Transactions can read uncommitted data, which is also known as dirty reads (Dirty read). This level can cause a lot of problems. In terms of performance, READ uncommitted is not much better than the other levels, but lacks many of the benefits of other

Three levels of front-end engineers: My Advanced Path and triple advanced ! Export v2-fd52450adf6c98b518618bdc74f1520e_r.png] (http://upload-images.jianshu.io/upload_images/8373224-e56f02b3d4e813e2.png? ImageMogr2/auto-orient/strip % 7CimageView2/2/w/1240)The purpose of this article is to build a cognitive model about the growth process of front-end engineers and analyze the relationship between the Capability Structure of Programmer, Developer, and E

Print logs (C language) in different levels according to requirements. When a program encounters a problem in debugging, it sometimes needs to print some intermediate variables. After the debugging is complete, you need to comment out the printing, which is very troublesome. So I wrote a small program that can set the log printing level. Set the Log Level debug to be printed in the main program, and all the logs of the corresponding level are display

']. Astype(np. Datetime64) For data aggregation, I tested Dataframe.groupby and dataframe.pivot_table as well as Pandas.merge, GroupBy 98 million rows x 3 columns for 99 seconds, a connection table of 26 seconds, and a faster generation of pivot tables. It takes only 5 seconds.Df.GroupBy([' NO ',' Time ',' SVID ']).Count() # GroupFulldata=Pd.Merge(df, Trancodedata) [[ ' NO ' , ' SVID ' , ' time ' ,, ' TYPE ' ]] # connection actions = Fulldata.pivot_table ( ' SVID ' , columns = ' T