Wait Event: DB File Scattered Read

Description

The DB File Scattered Read wait event indicates that a user process is waiting for multiple database blocks to be read from disk into the System Global Area (SGA). This typically occurs when the database session requests a large amount of data, often due to full table scans or fast full index scans. Scattered reads usually involve multiple block reads and can significantly impact database performance if not managed properly.

Example of DB File Scattered Read Wait Event

Imagine a bicycle retailer’s online store where customers can search for different types of bikes. The store’s database contains a BIKES table that holds details about various bike models.

Scenario:

Full Table Scan Usage: The database is queried without efficient indexing on the columns being searched.

SQL Query: A customer searches for a specific bike model, like ‘XYZ,’ triggering a query like this:

SELECT * FROM BIKES WHERE CATEGORY = 'XYZ';

DB File Scattered Read Occurrence:

1. When this query is executed, the database scans the entire BIKES table to find all entries matching the category ‘XYZ.’
2. Since there is no appropriate index on the CATEGORY column, the database reads multiple blocks of data from disk, resulting in scattered reads.
3. As it waits for the required blocks to be fetched, the session experiences DB File Scattered Read wait events.

Factors

1. Full Table Scans: The absence of indexes forces the database to read many blocks, causing scattered reads.
2. Inefficient Indexing: If an index exists but is not selective enough, it may still lead to many disk reads.
3. Large Data Sets: Queries that retrieve large volumes of data can exacerbate scattered read occurrences.
4. High Concurrency: Many users accessing the same table simultaneously can lead to increased wait times.
5. Data Fragmentation: Over time, data can become fragmented, leading to more blocks being read than necessary.

Solutions

1. Optimize Multi-Block I/O: Adjust the DB_FILE_MULTIBLOCK_READ_COUNT parameter to improve performance for operations requiring multiple blocks.
2. Create More Optimal Indexes: Develop indexes on columns used in the WHERE clause of your queries to enable efficient access paths.
3. Rebuild Fragmented Indexes: Regularly rebuild indexes to ensure they are optimized for performance and reduce unnecessary I/O.
4. Partitioning: Implement partitioning to minimize the number of blocks accessed during queries, which can enhance query performance.
5. SQL Query Optimization: Review and optimize SQL statements to minimize physical and logical reads. Analyze if full scans are necessary or if a more efficient index scan can be used.
6. Update Optimizer Statistics: Ensure that statistics are current so the optimizer can make informed decisions about the best execution plan.
7. Utilize Parallel Execution: If resources allow, consider parallel execution for large queries, which can significantly reduce execution time.
8. Monitor Data Distribution: Regularly check the distribution of data in your tables. If certain queries are consistently slow, it may indicate a need for better indexing strategies or query restructuring.
9. Identify Segments with High Read Activity: If the time spent waiting for multiblock reads is significant, determine which segments Oracle is reading from. Use the following query to check the files where reads are occurring:

SELECT sid, total_waits, time_waited FROM v$session_event WHERE event='db file scattered read' and total_waits>0 ORDER BY 3,2;

You can also look at:

• Statements with high DISK_READS in the V$SQL view.
• Sessions with a high number of table scan blocks in the V$SESSTAT view.

I hope you found this blog helpful! We’d love to hear about your experiences or any questions you might have please share your thoughts in the comments. Your feedback is greatly appreciate