Mastering the Art of Oracle Redo Queries: Tips and Tricks（oracle查询redo）

Oracle is one of the most prominent databases used in the world of information technology. One of its key features is the redo log which keeps track of all changes made to the database. This information can be incredibly useful for troubleshooting and optimizing database performance. However, it can also be overwhelming to navigate and query. Here are some tips and tricks to master the art of Oracle redo queries.

First, it is important to have a basic understanding of the redo log. The redo log records all changes to the database made since the last backup. It is a critical component of the database because it allows for recovery in the event of a crash or other failure. The redo log consists of one or more redo log files, which are typically stored on disk. When an operation modifies the database, it generates a redo entry which is then written to the redo log.

To access and query the redo log, Oracle provides a set of views and functions. The primary view is V$LOG which shows information about the redo log files, including their size, status, and sequence number. V$LOG_HISTORY shows a history of changes made to the redo log files. DBMS_LOGMNR is a PL/SQL package that provides more advanced access to the redo log. It allows for queries across multiple redo log files and can be used to recover data lost due to user error or other issues.

One useful query to run is to see how much redo is being generated by the database. This can help identify potential performance issues or excessive activity. The following query will show the amount of redo generated in the last hour:

SELECT ROUND((SUM(BYTES)/1024/1024),2) AS "Redo Generated (MB)"
FROM V$LOG_HISTORY
WHERE FIRST_TIME > SYSDATE - 1/24;

Another useful query is to see how much space is being used by the redo logs. This information can help identify when to add or remove redo log files to ensure adequate space for the database. The following query will show the amount of space used by the redo logs:

SELECT ROUND((SUM(BYTES)/1024/1024),2) AS "Redo Log Space Used (MB)"
FROM V$LOG;

DBMS_LOGMNR can also be used to query the redo log in more advanced ways. One useful feature is the ability to filter by specific table changes. For example, the following code would retrieve all changes made to the EMP table in the last day:

DECLARE 
v_start_scn NUMBER;
v_end_scn NUMBER;
BEGIN
SELECT MIN(START_SCN), MAX(END_SCN) 
INTO v_start_scn, v_end_scn 
FROM V$LOGMNR_CONTENTS 
WHERE SEG_OWNER = 'SCOTT' AND TABLE_NAME = 'EMP' AND TIMESTAMP > SYSDATE - 1;
DBMS_LOGMNR.START_LOGMNR(STARTSCN=>v_start_scn,ENDSCN=>v_end_scn,OPTIONS=>DBMS_LOGMNR.DICT_FROM_ONLINE_CATALOG+DBMS_LOGMNR.CONTINUOUS_MINE);
END;
/
SELECT SQL_REDO 
FROM V$LOGMNR_CONTENTS 
WHERE SEG_OWNER = 'SCOTT' AND TABLE_NAME = 'EMP';

In conclusion, the redo log is a crucial component of the Oracle database and mastering the art of querying it can lead to better performance and troubleshooting. Using the views and functions provided by Oracle, as well as the advanced queries available with DBMS_LOGMNR, can help uncover insights into the database and ensure smooth operations.