Table fragmentation and data storage

The fragmentation feature gives you more control over where the database stores data. You are not limited to specifying the locations of individual tables and indexes. You can also specify the location of table and index fragments, which are different parts of a table or index that are on different storage spaces.

You can fragment a table in the following ways:
  • Fragment a table over more than one dbspace. However, you cannot put fragments into dbspaces that have different page sizes. All fragments must have the same page size.
  • Create multiple partitions of a fragmented table within a single dbspace if the fragmented table uses an expression-based or round-robin distribution scheme.
You can fragment the following storage spaces:
  • Dbspaces
  • Sbspaces
Usually you fragment a table when you initially create it. The CREATE TABLE statement takes one of the following forms:
CREATE TABLE tablename ... FRAGMENT BY ROUND ROBIN IN dbspace1,
 dbspace2, dbspace3;

CREATE TABLE tablename ...FRAGMENT BY EXPRESSION 
   <Expression 1> in dbspace1,
   <Expression 2> in dbspace2,
   <Expression 3> in dbspace3;

The FRAGMENT BY ROUND ROBIN and FRAGMENT BY EXPRESSION keywords refer to two different distribution schemes. Both statements associate fragments with dbspaces.

If you set the AUTOLOCATE configuration parameter or session environment variable to a positive integer, and you do not specify a location for the table, new tables are fragmented in round-robin order in dbspaces that are chosen by the database server.

When you fragment a table, you can also create multiple partitions of the table within the same dbspace, as shown in this example:
CREATE TABLE tb1(a int) 
   FRAGMENT BY EXPRESSION
         PARTITION part1 (a >=0 AND a < 5) in dbs1,
         PARTITION part2 (a >=5 AND a < 10) in dbs1
          ...
       ;
The following figure illustrates the role of fragments in specifying the location of data.
Figure 1. Dbspaces that link logical units (including table fragments) and physical units of storage
The figure shows an arrow pointing from the system catalog to Dbspace 1 and an arrow pointing from Table 1 to Dbspace 2. For Table 2, the figure shows an arrow pointing from Fragment 1 to Dbspace 1, an arrow pointing from Fragment 2 to Dbspace 2, and an arrow pointing from Fragment 3 to Dbspace 3.

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