4.3.3 Identifying Query Traps
A query trap refers to a join construction in the physical view that generates query results that might confuse or mislead end users. There are three basic kinds of query traps: loop traps, fan traps, and chasm traps. The following sections discuss these trap types in greater detail.
Star schemas are generally free from query traps because of their uniform 1:n relationship from dimension table to fact table, and location of all measures in fact table.
Style Intelligence automatically detects all of these types of query traps. To avoid fan traps and chasm traps, Style Intelligence detects the cardinality of join relationships, and warns users and developers when they select a subset of data model attributes that would instantiate a potential query trap. In Ad Hoc report creation, attributes that would instantiate a query trap are rendered in grey, indicating that they should not be selected.
In general, because fan and chasm trap detection is handled automatically, you should not need to make explicit modifications. However, if the automatic cardinality detection incorrectly identifies a trap where none exists (which might generate unnecessary warnings for end users), you can override the default cardinality detection through the 'Join Properties' dialog box. See Defining Join Relationships for details.
For loop traps, Style Intelligence highlights the offending joins in the physical view using red color. Loop traps are not corrected automatically, and you should select the appropriate method for resolving loops based on your knowledge of the schema. See Resolving Loop Traps for full details.
Additionally, you will be alerted to the existence of un-joined orphan tables when you save the physical view. In general, you should make sure that every table in a physical view is joined to at least one other table. However, when you are prompted with the warning message, you can choose to force the physical view to be saved with orphaned tables. To correct the view, select 'Cancel' in the warning message, and Style Studio will highlight the detached tables.
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