Storing multiple named fragments in a single dbspace

For fragmented tables that use expression-based, interval, list, or round-robin distribution schemes, you can create named fragments that can be located within a single dbspace.

Storing multiple table or index fragments in a single dbspace improves query performance over storing each fragment in a different dbspace and simplifies management of dbspaces.

Suppose you are creating a fragmented table using an expression-based distribution scheme in which each expression specifies the data sets that are placed in particular fragments. You might decide to separate the data in the table with data from one month in one dbspace and data from the next 11 months in 11 other dbspaces. However, if you want to use only one dbspace for all the yearly data, you can create named fragments so the data for each month is stored in one dbspace.

If you create a fragmented table with named fragments, each row in the sysfragments system catalog table contains a fragment name in the partition column. If you create a fragmented table without named fragments, the name of the dbspace is in the partition column. The flags column in the sysfragments system catalog table tells you if the fragmentation scheme has named fragments.

You can create tables and indexes with named fragments, and you can create, drop, and alter named fragments using the PARTITION keyword and the fragment name.

To create a fragmented table with named fragments, use SQL syntax as shown in the following example:
CREATE TABLE tb1(a int) 
   FRAGMENT BY EXPRESSION
     PARTITION part1 (a >=0 AND a < 5) IN dbspace1,
     PARTITION part2 (a >=5 AND a < 10) IN dbspace1
          ...
       ;
If you created a table or index fragment containing named fragments, you must use syntax containing the fragment name when you use the ALTER FRAGMENT statement, as shown in the following examples:
ALTER FRAGMENT ON TABLE tb1 INIT FRAGMENT BY EXPRESSION
    PARTITION part_1 (a >=0 AND a < 5) IN dbspace1,
    PARTITION part_2 (a >=5 AND a < 10) IN dbspace1;
ALTER FRAGMENT ON INDEX ind1 INIT FRAGMENT BY EXPRESSION 
    PARTITION part_1 (a >=0 AND a < 5) IN dbspace1,
    PARTITION part_2 (a >=5 AND a < 10) IN dbspace1;
You can use the PARTITION BY EXPRESSION keywords in place of the FRAGMENT BY EXPRESSION keywords in the CREATE TABLE, CREATE INDEX, and ALTER FRAGMENT ON INDEX statements, as shown in this example:
ALTER FRAGMENT ON INDEX idx1 INIT PARTITION BY EXPRESSION 
           PARTITION part1   (a <= 10) IN idxdbspc1,
           PARTITION part2   (a <= 20) IN idxdbspc1,
           PARTITION part3   (a <= 30) IN idxdbspc1;
Use ALTER FRAGMENT syntax to change fragmented tables and indexes that do not have named fragments into tables and indexes that have named fragments. The following syntax shows how you might convert a fragmented table with multiple dbspaces into a fragmented table with named fragments:
CREATE TABLE t1 (c1 int) FRAGMENT BY EXPRESSION
     (c1=10) IN dbs1,
     (c1=20) IN dbs2;
ALTER FRAGMENT ON TABLE t1 MODIFY dbs2 TO PARTITION part_3 (c1=20)
 IN dbs1
The following syntax shows how you might convert a fragmented index into an index that contains named fragments:
CREATE TABLE t1 (c1 int) FRAGMENT BY EXPRESSION
     (c1=10) IN dbs1, (c1=20) IN dbs2, (c1=30) IN dbs3
CREATE INDEX ind1 ON t1 (c1) FRAGMENT BY EXPRESSION
     (c1=10) IN dbs1, (c1=20) IN dbs2, (c1=30) IN dbs3
ALTER FRAGMENT ON INDEX ind1 INIT FRAGMENT BY EXPRESSION
     PARTITION part_1 (c1=10) IN dbs1, PARTITION part_2 (c1=20) IN dbs1, 
     PARTITION part_3 (c1=30) IN dbs1,

See the HCL Informix® Performance Guide for more information about fragmentation, including fragmentation guidelines, procedures for fragmenting indexes, procedures for creating attached and detached indexes with named fragments, and examples of SQL statements used to create attached and detached indexes containing named fragments.

See the HCL Informix Guide to SQL: Syntax for more syntax details, including information about named fragments in the GRANT FRAGMENT and REVOKE FRAGMENT statements, and details for using the DROP, DETACH, and MODIFY clauses of the ALTER FRAGMENT statement.


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