Spark Dataframe count pushdown #29
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Here, I was not sure of `putIfAbsent` is call by name or not. That is, the create connection should be lazy and should not be evaluated if key already exists.
If there is a `.count` action after loading the Exasol dataframe, then we should not shuffle all those data through network just for counting afterwards. We detect `df.count()` operation when the list of column names (`requiredColumns`) is empty. In such cases, perform a single count star query on Exasol and create Spark RDD of empty rows as many as resulted count. Fixes #24
jpizagno
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Dec 4, 2018
Spark Dataframe count action pushdown
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Fix for #24