| % Generated by roxygen2: do not edit by hand |
| % Please edit documentation in R/dataset.R |
| \name{Scanner} |
| \alias{Scanner} |
| \alias{ScannerBuilder} |
| \title{Scan the contents of a dataset} |
| \description{ |
| A \code{Scanner} iterates over a \link{Dataset}'s fragments and returns data |
| according to given row filtering and column projection. A \code{ScannerBuilder} |
| can help create one. |
| } |
| \section{Factory}{ |
| |
| \code{Scanner$create()} wraps the \code{ScannerBuilder} interface to make a \code{Scanner}. |
| It takes the following arguments: |
| \itemize{ |
| \item \code{dataset}: A \code{Dataset} or \code{arrow_dplyr_query} object, as returned by the |
| \code{dplyr} methods on \code{Dataset}. |
| \item \code{projection}: A character vector of column names to select |
| \item \code{filter}: A \code{Expression} to filter the scanned rows by, or \code{TRUE} (default) |
| to keep all rows. |
| \item \code{use_threads}: logical: should scanning use multithreading? Default \code{TRUE} |
| \item \code{...}: Additional arguments, currently ignored |
| } |
| } |
| |
| \section{Methods}{ |
| |
| \code{ScannerBuilder} has the following methods: |
| \itemize{ |
| \item \verb{$Project(cols)}: Indicate that the scan should only return columns given |
| by \code{cols}, a character vector of column names |
| \item \verb{$Filter(expr)}: Filter rows by an \link{Expression}. |
| \item \verb{$UseThreads(threads)}: logical: should the scan use multithreading? |
| The method's default input is \code{TRUE}, but you must call the method to enable |
| multithreading because the scanner default is \code{FALSE}. |
| \item \verb{$BatchSize(batch_size)}: integer: Maximum row count of scanned record |
| batches, default is 32K. If scanned record batches are overflowing memory |
| then this method can be called to reduce their size. |
| \item \verb{$schema}: Active binding, returns the \link{Schema} of the Dataset |
| \item \verb{$Finish()}: Returns a \code{Scanner} |
| } |
| |
| \code{Scanner} currently has a single method, \verb{$ToTable()}, which evaluates the |
| query and returns an Arrow \link{Table}. |
| } |
| |