R : readBin 和 writeBin（用于存储/检索 MySQL BLOB 或 LONGBLOB）

Question

我正在使用 readBin 函数将文件保存为 MySQL BLOB，如本文所述（http://www.r-bloggers.com/save-r-plot-as-a-blob/）

plot_binary <- paste(readBin("temp.png", what="raw", n=1e6), collapse="")

我的问题是：一旦这是在数据库中，我如何将其转储回文件？

> f = file ( "backIntoFile.png", "wb")
> writeBin(object = plot_binary, con = f ) 
> close(f)

这不起作用；该文件似乎不是有效的 png ；

干杯！

Answer 1

这是迄今为止我找到的最佳解决方案。DbDownloadImages 函数的执行时间非常短（实际上几乎不需要时间）。

# Helper function I use
Paste <- function(string, vals)
{
    string <- gsub(x = string, pattern = '\\?', replacement = "%s")
    result <- do.call(sprintf, as.list(c(string,vals)))
    return(result)
}
# conn is a RMySQL connection object
DbInsertImage <- function( conn, link.to.file ) 
{

        binary = readBin ( link.to.file , what = "raw", n = 1e6 ) 
        binary.str = paste ( binary, collapse = "" ) 
        statement = Paste ( "CALL InsertImage('?')" , c(binary.str))
        dbSendQuery(conn, statement )
        return(GetIdentity(conn)) # one of my helper functions ; 
            # it returns the "LAST INSERT ID" 
}

#conn is a RMySQL connection object 
DbDownloadImage <- function( conn, id, destination) 
{

    query = "SELECT Data FROM Image WHERE Id = ? LIMIT 1" 
    query = Paste( query, c( id ) )
    result = dbGetQuery(conn, query )$Data[1]

    sst <- strsplit(result, "")[[1]]
    result <- paste0(sst[c(TRUE, FALSE)], sst[c(FALSE, TRUE)])
    result <- as.raw ( as.hexmode ( result ) ) 

    f = file ( destination, "wb")

    writeBin(object = result, con = f ) 
    close(f)
}

另请参阅： 如何将字符串拆分为给定长度的子字符串？