What are QRD´s and how does optimizing them result in better querying on large reports?

Query Result Definitions (QRDs) are used in a report to request the data as it should be laid out. This means for every element on the report page (list, crosstab, chart) the layout components creates a QRD and hand this over to the query engine. The QRD describes in which format the data is expected from the query engine so to fit to the user requested layout. So it could be that a lot of QRDs are executed and takes resources.

The optimization will collect all QRDs and analyse if some parts of QRDs can be reused or equal to reduce work for the query engine and the database. Especially on large reports it is a good opportunity to reuse and pool QRDs.
Also, there is no benefit on small and simple reports unless the different elements uses the same query.
Its not database specific, It´s used for an internal optimization before sending the queries to the database.
Note:
In IBM Cognos 10.1 and later releases QRD optimization is enabled by default and this advanced property has been removed.

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