Analysis of redo log generated by select command

In the near future , A user is doing SQL To optimize the , adopt sqlplus Of set autotrace on Command tracking SQL When implementing , I found this SELECT The command also produced REDO journal ; With what we usually understand DML/DDL Statement REDO There are differences in the understanding of logs ~

Yes ORACLE If you have a clear understanding of the characteristics of the database , Especially data blocks 、 Have a deeper understanding of transaction processing , Will know ORACLE Database transactions 、 Data block ITL Some principles of transaction slot 、 structural information ; You will know a concept called “ Delay block clearing ”--delayed block cleanout, In about 9 Years ago, when I was learning to understand this piece of knowledge , The related concepts and principles are summarized and tested , You can check the BLOG: On the concept and experiment of delayed block clearing _ Not a dizzy blog -CSDN Blog

In short, it is ：

Many data blocks may be manipulated in large transactions ; and BUFFER CACHE The time to write the modified dirty data in the data file is not COMMIT It is DBWR According to the rules ; Therefore, the data blocks related to transactions are brushed out first buffer cache, And then COMMIT Not yet implemented .

When this transaction COMMIT After submission , Business related data block ,undo block Transaction information on , The lock information will not be cleared .
The next time the data block is read buffer cache（ May be SELECT Read or other DML）, oracle Make transaction information when reading such data blocks 、 Clearing of lock information .

Statistics
----------------------------------------------------------
        105  recursive calls
          0  db block gets
    1343685  consistent gets
     550158  physical reads
      28532  redo size   ====>>>> SELECT When the REDO
       2940  bytes sent via SQL*Net to client
        520  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
         11  sorts (memory)
          0  sorts (disk)
         15  rows processed