Gbase 8s parallel operation problem scenario description

         The common ways for databases to read data in parallel are 3 A problem scenario ： Dirty reading （Dirty Read）、 It can't be read repeatedly （Non-repeatable Read） And phantom reading （Phantom Read）.

1． Dirty reading problem

         Dirty reading is a transaction T2 Transaction read T1 Results not submitted , The result may be rolled back .

         Examples are as follows ： Business T1 Read records , Then update the record ; Business T2 Read the updated record ; Business T1 Subsequent operations of may fail , Causes the updated record to roll back ; At the same time , Business T2 An inaccurate or uncommitted value is used . If the transaction is serial , The expected situation is ： If T1 Failure , be T2 The value before the update will be applied .

2． Unrepeatable read problem

         It can't be read repeatedly ： If the transaction T2 Multiple read operations in the return different results , It is called non repeatable reading .

Examples are as follows ： Business T2 Read an object ; Business T1 Update the same object ; Business T2 Read the same object again , But read a new modified value ; If the transaction is serial , The expected situation is ： If the transaction is read-only , The result of each reading is consistent .

3． Phantom reading problem

         Phantom reading ： Similar to non repeatable reading , Business T2 Execute multiple times in the same case SELECT The results read are different .

         Examples are as follows ： Business T2 Retrieving records for a specific condition from a table returns m Bar record ; Business T1 To the table insert/delete Other records that meet the same conditions ; Business T2 Retrieve the data of the table again under the same conditions , Will return <>M Bar record ; If the transaction is serial , The expected situation is ： The first time within a transaction 、 The second query should return the same result set .