Avoid row chaining in a database
Originally published by Tricore: Dec 21, 2017
Row chaining is a bottleneck that causes disruption in database performance. We should avoid row chaining as much possible. In this blog, we'll discuss row chaining, how to identify it, and how to remove or avoid it completely.
Normally, we encounter row chaining when the size of a database row is larger than the size of the database block that is used for storing it. In this situation, the row is split across more than one database block. When you need to access this row, the system traverses more than one database block, which results in more input/output (I/O) operations, as shown in the following illustration:
Basic assumption to test the scenario
Before we start, we need to alter an initialization parameter. Assuming the default block size is 8 KB. Execute the following code to set this parameter to allocate a memory buffer that is dedicated to store database blocks of different sizes:
ALTER SYSTEM SET DB_16K_CACHE_SIZE =16M SCOPE=BOTH;
Row chaining demonstration
To better understanding row chaining, use the following steps to create a new tablespace that uses a larger block size and move the table into the newly created tablespace to gather the statistics.
Create the table BIG_ROWS with the following command:
CREATE TABLE HR.BIG_ROWS ( Id number not null, Field1 char(2000) default ‘A’ not null, Field2 char(2000) default ‘B’ not null, Field3 char(2000) default ‘B’ not null, Field4 char(2000) default ‘D’ not null, Field5 char(2000) default ‘E’ not null, Constraint pk_big_rows primary key (id));
Populate the table with the following command:
INSERT INTO HR.BIG_ROWS (ID) SELECT ROWNUM FROM SYS.DBA_OBJECTS WHERE ROWNUM<101;
Analyze the table to refresh the statistics with the following command:
ANALYZE TABLE HR.BIG_ROWS COMPUTE STATISTICS;
Check for chained rows with the following command:
SELECT CHAIN_CNT FROM ALL_TABLES WHERE OWNER=’HR’ AND TBALE_NAME=’BIG_ROWS’;
- Create a tablespace with a different block size with the following command:
CREATE TABLESPACE TS\_16K BLOCKSIZE 16K DATAFILE ‘TS\_16K.DBF’ SIZE 30M EXTENT MANAGEMENT LOCAL UNIFORM SIZE 1M;
- Move the table BIG_ROWS to the tablespace that was just created with the following command:
ALTER TABLE HR.BIG\_ROWS MOVE TABLESPACE TS\_16K;
- Rebuild the indexes because they are unusable after the move with the following command:
ALTER INDEX HR.PK\_BIG\_ROWS REBUILD;
- Analyze the table to refresh the statistics with the following command: ANALYZE TABLE HR.BIG\_ROWS COMPUTE STATISTICS;
- Check if row chain still exists with the following command:
SELECT CHAIN\_CNT FROM ALL\_TABLES WHERE OWNER=’HR’ AND TABLE\_NAME=’BIG\_ROWS’;
Index rebuild after moving a table
After moving a table, such as in the preceding example, you should do an index rebuild. An index contains the row IDs of the table rows, and the row IDs identify the position of the row. The position is composed of the objects, the datafile, the block number, and the slot (row) number. When we move a table, the datafile and the block number changes, so we must rebuild the indexes.
Row chaining leads to poor performance because accessing a row in the database requires the system to read more than once DB block, even when accessing the table by the index lookup. When different block sizes are introduced in the database, remember the pros and cons of a larger block size. The larger the block size, the more likely that contention issues occur on the database block.
Using multiple block sizes provides the following advantages:
- Contention reduction
- Reduced chaining
- Faster update
- Reduced pinging
- Less disk space waste
- Less RAM Waste
- Minimum redo-generation
- Faster scan
Chained rows affect index reads and full table scans. Keep teh following points in mind:
- Row chaining is typically caused by insert operations.
- SQL statements that create or query chained rows degrade performance because of the additional I/O operations.
- To diagnose chained or migrated rows, use the analyze command and query the
- To remove chained rows, set a higher
PCTFREEvalue by using the alter table move command.
I/O is the main culprit for Oracle® database performance issues. If a database is having row chaining issues, the process needs to traverse more database blocks to complete the task, which in turns degrades the database performance. By removing row chaining, we can reduce the number of blocks that the Oracle processes need to traverse.
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