improve store costing of fetch of single row where not all columns are fetched.
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Key: DERBY-1508
URL: http://issues.apache.org/jira/browse/DERBY-1508
Project: Derby
Type: Improvement
Components: Store
Reporter: Mike Matrigali
Priority: Minor
Currently HeapCostController ignores information about what subset of columns
is being requested. For instance
in getFetchFromRowLocationCost() validColumns argument is unused. In
getScanCost() , scanColumnList is unused.
Mostly this probably does no matter as the cost of getting the row dominates
the per column subset cost. The area
where this matters is the case of long columns. The cost of fetching multiple
row keyed by a row location with a 2 gigabyte column that is not in
the select list is currently way smaller than the cost of doing the same query
by scanning the table for the same set of rows.
Currently the heap estimate associates cost with total average row length,
gotten by using the # of rows and the amount of space in the
container. It does not have a statistic available of the average size of each
column. At it's level all column lengths are variable and could
possibly be > 2 gig.
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