Extent sizes for tables in a dbspace

When you create a table, you can specify the size of the first extent, as well as the size of the extents to be added as the table grows. You can also modify the size of the first extent in a table in a dbspace, and you can modify the size of new subsequent extents.

The following sample SQL statement creates a table with a 512-kilobyte initial extent and 100-kilobyte added extents: 
CREATE TABLE big_one (column specifications)
   IN big_space
   EXTENT SIZE 512
   NEXT SIZE 100

The default value for the extent size and the next-extent size is eight times the disk page size on your system. For example, if you have a 2-kilobyte page, the default length is 16 kilobytes.

You can use the ALTER TABLE statement with the MODIFY EXTENT SIZE clause to change the size of the first extent of a table in a dbspace. When you change the size of the first extent, OneDB records the change in the system catalog and on the partition page, but only makes the actual change when the table is rebuilt or a new partition or fragment is created.

You might want to change the size of the first extent of a table in a dbspace in either of these situations:

  • If a table was created with small first extent size and you need to keep adding a lot of next extents, the table becomes fragmented across multiple extents and the data is scattered.
  • If a table was created with a first extent that is much larger than the amount of data that is stored, space is wasted.
The following example changes the size of the first extent of a table in a dbspace to 50 kilobytes: 
ALTER TABLE customer MODIFY EXTENT SIZE 50;

Changes to the first extent size are recorded into the system catalog table and on the partition page on the disk. However, changes to the first extent size do not take effect immediately. Instead, whenever a change that rebuilds the table occurs, the server uses the new first extent size.

For example, if a table has a first extent size of 8 kilobytes and you use the ALTER TABLE statement to change this to 16 kilobytes, the server does not drop the current first extent and recreate it with the new size. Instead, the new first extent size of 16 kilobytes takes effect only when the server rebuilds the table after actions such as creating a cluster index on the table or detaching a fragment from the table.

If a TRUNCATE TABLE statement without the REUSE option is executed before the ALTER TABLE statement with the MODIFY EXTENT SIZE clause, there is no change in the current first extent.

Use the MODIFY NEXT SIZE clause to change the size of the next extent to be added. This change does not affect next extents that already exist.

The following example changes the size of the next extent of a table to 50 kilobytes: 
ALTER TABLE big_one MODIFY NEXT SIZE 50;
The next extent sizes of the following kinds of tables do not affect performance significantly:
  • A small table is defined as a table that has only one extent. If such a table is heavily used, large parts of it remain buffered in memory.
  • An infrequently used table is not important to performance no matter what size it is.
  • A table that resides in a dedicated dbspace always receives new extents that are adjacent to its old extents. The size of these extents is not important because, being adjacent, they perform as one large extent.

Avoid creating large numbers of extents

When you assign an extent size to these kinds of tables, the only consideration is to avoid creating large numbers of extents. A large number of extents causes the database server to spend extra time finding the data. In addition, an upper limit exists on the number of extents allowed. (Considering the upper limit on extents covers this topic.)

Tips for allocating space for table extents

No upper limit exists on extent sizes except the size of the chunk. The maximum size for a chunk is 4 terabytes. When you know the final size of a table (or can confidently predict it within 25 percent), allocate all its space in the initial extent. When tables grow steadily to unknown size, assign them next-extent sizes that let them share the dbspace with a small number of extents each.

Allocating space for table extents

To allocate space for table extents:

  1. Decide how to allocate space among the tables.

    For example, you might divide the dbspace among three tables in the ratio 0.4: 0.2: 0.3 (reserving 10 percent for small tables and overhead).

  2. Give each table one-fourth of its share of the dbspace as its initial extent.
  3. Assign each table one-eighth of its share as its next-extent size.
  4. Monitor the growth of the tables regularly with oncheck.

As the dbspace fills up, you might not have enough contiguous space to create an extent of the specified size. In this case, the database server allocates the largest contiguous extent that it can.