Sybase 12.4.2 manual Linear joins, Star joins, 165

CHAPTER 4 Adaptive Server IQ Indexes

Linear joins

You can think of a linear join as a tree with no branches. Each table in the hierarchy is related to the table above it, until you reach the top table. In Figure 4-1 on page 153. Tables A, D, and F constitute a linear join hierarchy. Tables C, E, and F form another linear join hierarchy.

In a linear join, each pair of tables represents a one-to-many relationship, in which the lower table of the pair is the “one” side, and the higher table of the pair is the “many” side. Linear join hierarchies can rely on any of the underlying join conditions: key join, natural join, or ON clause join.

Star joins

You can picture a star join as a structure with many branches, in which each branch is directly related to one table in the middle. In Figure 4-1, Tables D, F, and E form a very simple star join. More commonly, Table F would be at the center of many tables, each of which is joined to Table F.

In a star join, multiple tables are related to one table at the center of the join, in

aone-to-many relationship. The one table at the center of the join represents the “many” side of the relationship, while each of the tables around it represent the “one” side of the relationship. Each table on the “one” side holds a set of values with its own unique primary key. A foreign key in the table on the “many” side of the relationship relates that table to the primary key of the table on the “one” side of the relationship.

The “many” table at the center of the star is sometimes called the fact table.

The “one” tables related to it are called the dimension tables.

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