| star transformation [message #93166] |
Sat, 17 May 2003 13:08  |
kochunni
Messages: 17
Registered: May 2003
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Junior Member |
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An article on star query and star transformation says the following:
"Where the Star Query execution goes to the Fact table last in its execution, the Star Transformation Query goes to the Fact table first via bitmap indexes, then joins to the Dimension tables. This method does not limit the query to use an ordered composite index and doesn’t default to a Cartesian product joins of the dimensions. The bitmap
indexes reduce storage space requirements and provide flexibility for complex star schemas. This method is identified as a “transformation” because the CBO actually rewrites the query as a series of sub queries on the joining dimension tables."
My question is how will the CBO know which Bitmap index is to us if the query starts from Facts. My belief is a query always start from a dimension table, identify the unique identifier for the dimension attribute, then read the bitmap index on the fact table for that particular dimension key.
I may be wrong. Please tell me how CBO identify which Bitmap index to use without reading a dimension table first, because the bitmap on Fact table are created on each distinct value of dimension foreign keys.
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