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i2b2 queries often contain multiple attributes which are combined with AND clauses (e.g. "diagnosis = diabetes AND age <= 18 years"). The i2b2 CRC cell (Clinical Research Chart) queries these attributes consecutively from the OBSERVATION_FACT table, using the result sets of each query as a base for the next query. Thus the amount of patients or encounters that need to be evaluated decrease over each step of the query plan. In order to narrow down the search space quickly, it would be ideal to first query for attributes occurring rarely in the overall dataset. While this optimization is supported by i2b2, the platform needs to know about the frequency of each concept in the overall dataset to make use of it.

The ontology table "I2B2" contains the column "C_TOTALNUM" for this purpose. It provides the count of distinct patients for each concept in the overall dataset. The CRC cell utilizes this column to sort the order of query steps in the overall query plan in order to first query for attributes with low occurence. However, the C_TOTALNUM column is not routinely filled during loading of the OBSERVATION_FACT table, which leads to this optimization not being applied in many cases.

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