Pig does exactly what you're describing in the second part, but only if you reuse the same relation. It doesn't load the file into an interim job, although I'm not a pro at that stage. But in MR, the map output files are stored to disk, and then the reducers go. In this case, it is smart enough to have the one map stage, and then the 3 different reducers. 2011/10/3 Something Something <[email protected]>
> Let me ask the question differently. Let's say I was not using Pig. I > wanted to do this just using Java MapReduce. The input file is HUGE. One > obvious way to do this would be to write 3 different MR jobs. But that > means this huge file be read 3 times which is what I am trying to avoid. > > Is there a way to write a Mapper that will read this file only once, and > then write to 3 different Reducers with different keys? > > Going back to Pig, when I LOAD this file & then later 'group by' 3 > different > keys, how does Pig do this? Does it "LOAD" this input file into some > interim file & call 3 different Map Reduce jobs? > > If this makes no sense, please ignore it. I will try to use 'Explain', > 'Describe' to learn the internals. Thanks. > > > On Mon, Oct 3, 2011 at 6:04 PM, Jonathan Coveney <[email protected]> > wrote: > > > If you want to know more about the internals, I'd check out the paper > Yahoo > > put out on the topic (or, of course, buy the book Programming Pig). > > > > The answer to this is pretty simple: if you load a file multiple times > into > > different relations, then it will be scanned multiple times. So... > > > > a = load 'thing'; > > b = load 'thing; > > > > {..stuff using a..} > > {..stuff using b..} > > > > would load 'thing' twice. This is done for joins and whatnot -- there are > > cases when you need to load the same file separately, twice. What happens > > is > > essentially that you're going to load and scan the data twice. > > > > However, as in your case, if you instead combine the load, then you'd > have > > > > a = load 'thing'; > > {..stuff using a..} > > {..stuff using a (which previously used b)..} > > > > Now it will just scan a once, and then go into each of the pipelines you > > defined. > > > > Obviously it's more complex than that, but that's the general gist. > > > > 2011/10/3 Something Something <[email protected]> > > > > > I have 3 Pig scripts that load data from the same log file, but filter > & > > > group this data differently. If I combine these 3 into one & LOAD only > > > once, performance seems to have improved, but now I am curious exactly > > what > > > does LOAD do? > > > > > > How does LOAD work internally? Does Pig save results of the LOAD into > > some > > > separate location in HDFS? Someone please explain how LOAD relates to > > > MapReduce? Thanks. > > > > > >
