The ALTER FRAGMENT statement interface

When the database server executes an ALTER FRAGMENT statement, the database server moves data between existing fragments and also creates a fragment.

The statement in the following figure creates and fragments a jobs table.

Figure 1. SQL to create the fragmented jobs table

CREATE TABLE jobs (sstatus file_ops)
   FRAGMENT BY EXPRESSION
      sstatus > 15 IN fragspace2,
      REMAINDER IN fragspace1
   USING file_am

The statement in the following figure changes the fragment expression for jobs, which redistributes the table entries.

Figure 2. SQL to alter the jobs fragments

ALTER FRAGMENT ON TABLE jobs
   MODIFY fragspace1 TO (sstatus <= 5) IN
fragspace1,
   MODIFY fragspace2 TO 
      (sstatus > 5 AND sstatus <= 10) IN
fragspace2,
   REMAINDER IN fragspace3

For each fragment that the ALTER FRAGMENT statement specifies, the database server performs the following actions:

Executes an access-method scan
Evaluates the returned rows to determine which ones must move to a different fragment
Executes the access method to create a fragment for the target fragment that does not yet exist
Executes the access method to delete rows from one fragment and insert them in another

Figures Figure 3 through Figure 6 show the separate sequences of purpose functions that create the fragments and distribute the data for the SQL ALTER FRAGMENT statement in Figure 2. The database server performs steps 1, 2, and 3 to move fragments from fragspace1 to fragspace2 and then performs steps 1 through 3 to move fragments from fragspace2 to fragspace3.

The following figure shows the sequential scan in step 1, which returns all rows from the fragment because the scan descriptor contains a NULL-valued pointer instead of a pointer to a qualification descriptor.

Figure 3. Getting all the rows in fragment 1

This figure is described in the surrounding text.

In the following figure, the database server returns the row identifiers that the access method should delete from fragspace1 and insert in fragspace2.

Figure 4. Moving rows between fragments

The following figure again shows the sequential scan in step 1. This scan returns all the rows from fragment2.

Figure 5. Getting All the Rows in Fragment 2

The following figure shows steps 3 and 4. The database server returns the row identifiers that the access method should delete from fragspace2 and insert in fragspace3. The database server does not have fragspace3, so it executes am_create to have the access method create a fragment before it executes am_insert.

Figure 6. Adding and filling a fragment