22 September 2009

Incremental Update Technique(s)     In Cognos Enterprise Business Intelligence Developer Guides mentioned in the previous post is repeated over and over again in the WHERE clause a set of constraints like in the below example(e.g. Accounts Payable Analysis for Oracle e-Business Suite, p. 21), involving more or less than 3 tables: (A.CREATION_DATE > TO_DATE(:FROM_DATETIME,'YYYYMMDDHH24MISS') AND A.CREATION_DATE <= TO_DATE(':TO_DATETIME','YYYYMMDDHH24MISS')) OR (B.CREATION_DATE > TO_DATE(:FROM_DATETIME,'YYYYMMDDHH24MISS') AND B.CREATION_DATE <= TO_DATE(':TO_DATETIME','YYYYMMDDHH24MISS')) OR (C.CREATION_DATE > TO_DATE(:FROM_DATETIME,'YYYYMMDDHH24MISS') AND C.CREATION_DATE <= TO_DATE(':TO_DATETIME','YYYYMMDDHH24MISS'))     For 3 tables it's not a lot of work, though the set of constraints becomes more complicated with the increase in the number of tables. It can be simplified by using the LEAST & GREATEST Oracle functions, resulting the equivalent writting: (LEAST(A.CREATION_DATE, B.CREATION_DATE, C.CREATION_DATE) > TO_DATE(:FROM_DATETIME,'YYYYMMDDHH24MISS') AND GREATEST(A.CREATION_DATE, B.CREATION_DATE, C.CREATION_DATE) <= TO_DATE(':TO_DATETIME','YYYYMMDDHH24MISS'))     The second formulation is simpler and should have the same performance as the first formulation, however it has a small impediment – the LEFT JOINs and LEAST/GREATEST behaviour when working with NULL values. If one of the parameters provided to the two functions is NULL then the result will be NULL, thing which can mess up a query. That’s not difficult to avoid by using for example the NVL function. Supposing that there is a LEFT JOIN between A and two other tables B & C, the above constraints can be rewritten as: (LEAST(A.CREATION_DATE, NVL(B.CREATION_DATE, A.CREATION_DATE), NVL(C.CREATION_DATE, A.CREATION_DATE)) > TO_DATE(:FROM_DATETIME,'YYYYMMDDHH24MISS') AND GREATEST(A.CREATION_DATE, NVL(B.CREATION_DATE, A.CREATION_DATE), NVL(C.CREATION_DATE, A.CREATION_DATE)) <= TO_DATE(':TO_DATETIME','YYYYMMDDHH24MISS'))     One might arrive to write such constraints for example when attempting to do an incremental update in a data warehouse based on the Creation Date from multiple source tables, loading the records having a Creation Date greater than the date of the previous update (of course the records existing already in data warehouse needs to be deleted or updated). The Creation Date works only for transactional data which don’t change after their creation, for the other types of cases it should be considered the Last Update Date (Date of the Last Update for a record). Also this approach has its flows, working well when records are not deleted from the table; this issue could be fixed for example by creating a trigger on deletion and save the ID of the record deleted in a third party table or actually delete the record from the third party database when the deletion happens in the source table.

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