Dirty reading

Dirty reading is also called invalid data reading , In database access , Business T1 Modify a value , Then transaction T2 Read the value , thereafter T1 Undo the change to the value for some reason , And that leads to this T2 The data read is invalid , It is worth noting that , Dirty reading is generally aimed at update Operation of the .

Dirty reading is when a transaction is accessing data , And the data has been modified , This modification has not yet been committed to the database , At this time , Another transaction also accesses this data , And then I used this data . Because this data is not submitted yet , Then the data read by another transaction is dirty data , Operations based on dirty data may not be correct .

Fantasy reading

Fantasy reading （Phantom Read）, A phenomenon that occurs when a transaction is not executed independently .

Business A Read several lines that match the search criteria . Business B Modify transactions by inserting or deleting rows A The result set , Then submit .
Unreal reading refers to a phenomenon that occurs when a transaction is not executed independently , For example, the first transaction modifies the data in a table , For example, this modification involves “ All data lines ”. meanwhile , The second transaction also modifies the data in this table , This change is to insert... Into the table “ A new line of data ”. that , Later, the user who operates on the first transaction will find that there are still unmodified data rows in the table , It's like an illusion . The general solution to unreal reading is to add range locks RangeS, Lock search scope to read only , This avoids unreal reading .

It can't be read repeatedly

It can't be read repeatedly , In database access , Two identical queries within a transaction scope return different data .

This is caused by the commit of other transaction modifications in the system during query . For example, affairs. T1 Read a data , Business T2 Read and modify the data ,T1 Read the data again in order to verify the read value , And we get different results .
A more understandable statement is ： In a business , Read the same data multiple times . Before the end of the business , Another transaction also accesses the same data and modifies the data . that , Between the two readings of the first transaction . Due to the modification of another transaction , It's impossible to read the same data for the first two transactions , This happens that the data read twice in a transaction is different , So it's called unrepeatable reading , That is, the original read is not repeatable .