Row chaining in oracle and how to avoid row chaining

Row chaining in Oracle occurs when a row is too large to fit into a single database block. As a result, the row is divided and stored in multiple blocks, causing a performance overhead because accessing the row requires multiple I/O operations.

Causes of Row Chaining

1. Large Columns: Tables with columns that have large data types (e.g., CLOB, BLOB, VARCHAR2, LONG, etc.).
2. High Row Size: Rows with many columns or columns with large sizes.
3. Frequent Updates: Updates that increase the size of a row, causing it to exceed the block size.

Identifying Row Chaining

To identify row chaining, you can query the DBA_TABLES view and check the CHAIN_CNT column, which shows the number of chained rows for each table.

SELECT OWNER, TABLE_NAME, CHAIN_CNT FROM DBA_TABLES WHERE CHAIN_CNT > 0;

Avoiding Row Chaining

1. Proper Block Size:

   • Choose an appropriate block size for your tablespace. A larger block size can reduce the likelihood of row chaining, especially for tables with large rows.

2. Column Splitting:

   • If a table has columns with large sizes or many columns, consider normalizing the table to split it into smaller, related tables.

3. Row Migration and Updates:

   • Avoid frequent updates that increase the size of rows significantly. If unavoidable, reorganize the table regularly to minimize row chaining.

4. Storage Parameters:

• Set appropriate storage parameters such as PCTFREE and PCTUSED to provide sufficient space for row expansion. PCTFREE specifies the percentage of space reserved for updates to the rows in each block.
• ALTER TABLE your_table_name PCTFREE 20;
5. LOB Storage:
• Store large objects (LOBs) in dedicated tablespaces and use the ENABLE STORAGE IN ROW clause only when necessary. This can help to manage large data more efficiently.
6. Regular Maintenance:
• Regularly analyze tables and rebuild or reorganize them if necessary to address chaining issues.
• ANALYZE TABLE your_table_name LIST CHAINED ROWS;

7. Fixing Existing Row Chaining

   1. Move Table:
   • Moving the table to another tablespace can help. This operation can compact the rows and reduce chaining.
   • ALTER TABLE your_table_name MOVE TABLESPACE new_tablespace;
   2. Export and Import:
      • Exporting the table and then importing it back can help in reorganizing the data and reducing chaining
      • -- Export the table
      • expdp user/password DIRECTORY=dir DUMPFILE=table_name.dmp TABLES=your_table_name
      • -- Import the table
      • impdp user/password DIRECTORY=dir DUMPFILE=table_name.dmp TABLES=your_table_name
      • 
        
      • By following these practices, you can minimize the occurrence of row chaining in Oracle databases and improve the performance of your queries and transactions.