db file scattered read - Wait Event , Cause, Related SQL queries, Possible Resolution

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The "db file scattered read" wait event occurs when a session is waiting for multiple data blocks to be read from disk into the buffer cache in a scattered manner. This wait event is common when a query or operation requires access to multiple non-adjacent data blocks that are not currently in memory. Explanation: This wait event indicates that a session is waiting for I/O operations to complete, bringing the requested data blocks into memory. It often occurs during full table scans or index range scans where multiple non-contiguous blocks are read.

SELECT SID, SERIAL#, WAIT_CLASS, EVENT, WAIT_TIME, SECONDS_IN_WAIT
FROM V$SESSION
WHERE EVENT = 'db file scattered read';

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SELECT EVENT, TOTAL_WAITS, TIME_WAITED_MICRO/1000000 AS SECONDS_WAITED
FROM V$SYSTEM_EVENT
ORDER BY TOTAL_WAITS DESC;

SELECT EVENT, OBJECT_NAME, OBJECT_TYPE, P1, P2, P3
FROM V$SESSION_WAIT
WHERE EVENT = 'db file scattered read';

Resolution: 

 Optimize Queries: Ensure queries are properly tuned. Minimize full table scans and use appropriate indexes to reduce the occurrence of scattered reads. 

 Table and Index Reorganization: Regularly perform table and index maintenance to minimize fragmentation and optimize data access. 

 Increase Buffer Cache: Enlarge the buffer cache to hold more data blocks in memory, reducing the need for frequent scattered reads. 

 Use Parallelism: For large scans, consider using parallel processing to read multiple blocks simultaneously. 

 Partitioning: Utilize table and index partitioning to reduce scattered reads, especially for large tables. 

 Consider Materialized Views: Materialized views can improve read performance by precomputing and storing results. 

 Storage Optimization: Use faster storage solutions to improve I/O performance. 

 Avoid Over-Indexing: Having too many indexes on a table might lead to excessive scattered reads during data modification. 

 Application Design: Optimize application logic to retrieve only the necessary data, minimizing the need for scattered reads. 

 Monitor and Analyze: Continuously monitor the database performance using tools like Oracle Enterprise Manager or other monitoring solutions to identify and address performance issues. 

 Remember that the solution depends on the specific context of your database and workload. Analyze the root cause and apply appropriate optimizations to reduce the impact of "db file scattered read" wait events.

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