What are the Differences between connected and unconnected
lookup?
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Answer Posted / balu
Connected and Unconnected Lookups
You can configure a connected Lookup transformation to
receive input directly from the mapping pipeline, or you
can configure an unconnected Lookup transformation to
receive input from the result of an expression in another
transformation.
Table 14-1 lists the differences between connected and
unconnected lookups:
Table 14-1. Differences Between Connected and Unconnected
Lookups
Connected Lookup Unconnected Lookup
Receives input values directly from the pipeline. Receives
input values from the result of a :LKP expression in
another transformation.
Use a dynamic or static cache. Use a static cache.
Cache includes all lookup columns used in the mapping (that
is, lookup source columns included in the lookup condition
and lookup source columns linked as output ports to other
transformations). Cache includes all lookup/output
ports in the lookup condition and the lookup/return port.
Can return multiple columns from the same row or insert
into the dynamic lookup cache. Designate one return port
(R). Returns one column from each row.
If there is no match for the lookup condition, the
Integration Service returns the default value for all
output ports. If you configure dynamic caching, the
Integration Service inserts rows into the cache or leaves
it unchanged. If there is no match for the lookup
condition, the Integration Service returns NULL.
If there is a match for the lookup condition, the
Integration Service returns the result of the lookup
condition for all lookup/output ports. If you configure
dynamic caching, the Integration Service either updates the
row the in the cache or leaves the row unchanged. If there
is a match for the lookup condition, the Integration
Service returns the result of the lookup condition into the
return port.
Pass multiple output values to another transformation. Link
lookup/output ports to another transformation. Pass one
output value to another transformation. The
lookup/output/return port passes the value to the
transformation calling : LKP expression.
Supports user-defined default values. Does not support
user-defined default values.
Connected Lookup Transformation
The following steps describe how the Integration Service
processes a connected Lookup transformation:
1. A connected Lookup transformation receives input
values directly from another transformation in the
pipeline.
2. For each input row, the Integration Service
queries the lookup source or cache based on the lookup
ports and the condition in the transformation.
3. If the transformation is uncached or uses a static
cache, the Integration Service returns values from the
lookup query.
If the transformation uses a dynamic cache, the Integration
Service inserts the row into the cache when it does not
find the row in the cache. When the Integration Service
finds the row in the cache, it updates the row in the cache
or leaves it unchanged. It flags the row as insert, update,
or no change.
4. The Integration Service passes return values from
the query to the next transformation.
If the transformation uses a dynamic cache, you can pass
rows to a Filter or Router transformation to filter new
rows to the target.
Note: This chapter discusses connected Lookup
transformations unless otherwise specified.
Unconnected Lookup Transformation
An unconnected Lookup transformation receives input values
from the result of a :LKP expression in another
transformation. You can call the Lookup transformation more
than once in a mapping.
A common use for unconnected Lookup transformations is to
update slowly changing dimension tables. For more
information about slowly changing dimension tables, visit
the Informatica Knowledge Base at
http://my.informatica.com.
The following steps describe the way the Integration
Service processes an unconnected Lookup transformation:
1. An unconnected Lookup transformation receives
input values from the result of a :LKP expression in
another transformation, such as an Update Strategy
transformation.
2. The Integration Service queries the lookup source
or cache based on the lookup ports and condition in the
transformation.
3. The Integration Service returns one value into the
return port of the Lookup transformation.
4. The Lookup transformation passes the return value
into the :LKP expression.
For more information about unconnected Lookup
transformations, see Configuring Unconnected Lookup
Transformations.
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