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Answer Posted / tom
There are at least three of the answers that are right. As
stated above when a query needs to go through two or more
fact tables to pick up all the columns requested then a
stitch query is generated.
Consider: Fact A with two dimensions D1 and D2. Fact B with
two Dimensions D2 and D3. D2 is common to both facts i.e. a
conformed dimension. Report requires a column from D1 and
D3. Cognos generates a query that joins D1 to Fact A, Fact
A to D2, D2 to Fact B and finally Fact B to D3. It is quite
possible that Fact A will have a links to a set of records
in D2 that are different to the set of records linked to
from Fact B. The sets can some members in common or one set
could be a subset of the other. Cognos does not know which
is true or how it should be approached so it writes a query
that does not lose any records i.e. a full outer join (never
heard it called a RIGHT full outer join)
Many data bases will support FOJ so the term stitch query
described above is understood in DB circles. In Cognos it
has an additional meaning. To perform the stitch Cognos
sends 2 or more queries to the database and 'stitches' the
results together on the Application Server i.e not in a DB.
The Cognos SQL will have the FOJ / coalesce statements but
the Native SQL will not. The down side of this is that you
can't put filters in the query to remove rows where the FOJ
has delivered a null. Any filters you add will be applied
to one of the native queries BEFORE it is sent to the DB.
Filtering has to be done in the report. Most people, and
nearly all report writers, don't understand stitch queries
so it's best to avoid them if possible.
Personally I think Cognos should provide the option of
specifying the type of join between the facts
Tom Spillane, Sydney March/2010
| Is This Answer Correct ? | 26 Yes | 1 No |
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