Expressions are used to define filter logic for passing to aDatasetScanner.
Expression$scalar(x) constructs anExpression which always evaluates tothe provided scalar (length-1) R value.
Expression$field_ref(name) is used to construct anExpression whichevaluates to the named column in theDataset against which it is evaluated.
Expression$create(function_name, ..., options) builds a function-callExpression containing one or moreExpressions. Anything in... thatis not already an expression will be wrapped inExpression$scalar().
Expression$op(FUN, ...) is for logical and arithmetic operators. Scalarinputs in... will be attempted to be cast to the common type of theExpressions in the call so that the types of the columns in theDatasetare preserved and not unnecessarily upcast, which may be expensive.