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From Alan Gates <ga...@yahoo-inc.com>
Subject Re: Getting query information while loading data
Date Mon, 04 Feb 2008 21:42:15 GMT

I apologize, I got busy and let this thread drop.  Comments inlined below.

Charlie Groves wrote:
> On Jan 18, 2008, at 11:45 AM, Alan Gates wrote:
>> We're definitely interested.
> Excellent!
>> Our thinking of how to provide field metadata (name and eventually 
>> types) for pig queries was to allow several options:
>>    1) AS in the LOAD, as you can currently do for names.
>>    2) using an outside metadata service, where we would tell it the 
>> file name and it would tell us the metadata.
>>    3) Support self describing data formats such as JSON.
>> You're suggestion for a very simple schema provided in the first line 
>> of the file falls under category 3.  The trick here is that we need 
>> to be able to read that metadata about the fields at parse time 
>> (because we'd like to be able to do type checking and such).  So in 
>> addition to the load function itself needing to examine the tuples, 
>> we need a way for the load function to read just enough of the file 
>> to tell the front end (on the client box, not on the map-reduce 
>> backend) the schema.  Maybe the best way to implement this is to have 
>> an interface that the load function would implement that lets the 
>> parser know that the load function can discover the metadata for it, 
>> and then the parser could call that load function before proceeding 
>> to type checking.
>> We're also interested in being able to tell the load function the 
>> fields needed in the query.  Even if you don't have field per file 
>> storage (aka columnar storage) it's useful to be able to immediately 
>> project out fields you know the query won't care about, as you can 
>> avoid translation costs and memory storage.
>> It's not clear to me that we need another interface to implement 
>> this.  We could just add a method "void neededColumns(Schema s)" to 
>> PigLoader.  As a post parsing step the parser would then visit the 
>> plan, as you suggest, and submit a schema to the PigLoader function.  
>> It would be up to the specific loader implementation to decide 
>> whether to make use of the provided schema or not.
> I don't see the use for the first new function in addition to the 
> second.  If a schema is required by the query, the loader must be able 
> to produce data matching that schema.  If the loader can figure out an 
> internal schema, it can make that check that you describe in function 
> 1 in addition to structuring its data correctly as in function 2.  If 
> it can't determine its internal schema until it loads data, then it 
> can do neither and we have to wait until runtime to see if it 
> succeeds.  What about making the call "Schema neededColumns(Schema s) 
> throws IOException"?  The returned Schema is the actual Schema that 
> will be loaded which must be a superset of the incoming Schema.  If 
> the loader is unable to create the needed schema, an IOException is 
> thrown.
> Is the necessary Schema known somewhere in the parser, or will I have 
> to figure that out from the Schemas available at each step?  I haven't 
> seen anything like that.
> Charlie
I'm not sure I understand what you're proposing.  I was trying to say 
that we need two separate things from the load function:
1) A way to discover the schema of the data at parse time for type 
checking and query correctness checking (e.g. the user asked for field 
5, is there a field 5?)  This is needed for metadata option 3, where the 
metadata is described by the data (as in JSON) or where the metadata is 
located in a file associated with the data.  We want to detect these 
kinds of errors before we submit to the backend (i.e. Hadoop) so that we 
can give the earliest possible error feedback.
2) A way to indicate to the load function the schema it needs to load, 
as a way to support columnar storage schemes (such as you propose) or 
pushing projection down into the load.

Were you saying that you didn't think one of those is necessary, or are 
you saying that you think we can accomplish both with one function being 
adding to the load function?


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