Home | About | Sematext search-lucene.com search-hadoop.com
 Search Hadoop and all its subprojects:

Switch to Threaded View
Pig, mail # dev - Early projection and lazy casting


Copy link to this message
-
Re: Early projection and lazy casting
Jie Li 2011-12-03, 02:42
Sure. The two lines in bold are just dropping out non-necessary fields.
Without them Pig would not project, especially for the table lineitem.

lineitem = load '$input/lineitem' USING PigStorage('|') as
(l_orderkey:long, l_partkey:long, l_suppkey:long, l_linenumber:long,
l_quantity:double, l_extendedprice:double, l_discount:double, l_tax:double,
l_returnflag:chararray, l_linestatus:chararray, l_shipdate:chararray,
l_commitdate:chararray, l_receiptdate:chararray,l_shippingstruct:chararray,
l_shipmode:chararray, l_comment:chararray);

part = load '$input/part' USING PigStorage('|') as (p_partkey:long,
p_name:chararray, p_mfgr:chararray, p_brand:chararray, p_type:chararray,
p_size:long, p_container:chararray, p_retailprice:double,
p_comment:chararray);

*lineitem = foreach lineitem generate l_partkey, l_quantity,
l_extendedprice ;*
part = FILTER part BY p_brand == 'Brand#23' AND p_container == 'MED BOX';
*part = foreach part generate p_partkey;*

COG1 = COGROUP part by p_partkey, lineitem by l_partkey;
COG1 = filter COG1 by COUNT(part) > 0;
COG2 = FOREACH COG1 GENERATE COUNT(part) as count_part, FLATTEN(lineitem),
0.2 * AVG(lineitem.l_quantity) as l_avg;

COG3 = filter COG2 by l_quantity < l_avg;
COG = foreach COG3 generate (l_extendedprice * count_part) as l_sum;

G1 = group COG ALL;

result = foreach G1 generate SUM(COG.l_sum)/7.0;

On Fri, Dec 2, 2011 at 9:16 PM, Dmitriy Ryaboy <[EMAIL PROTECTED]> wrote:

> Can you provide a script that shows projection not happening? We've
> observed the opposite (and use that fact extensively)
>
> D
>
> On Fri, Dec 2, 2011 at 4:05 PM, Jie Li <[EMAIL PROTECTED]> wrote:
> > Hi all,
> >
> > We just figured out Pig 0.9.1 doesn't drop those non-necessary fields
> asap,
> > which really affects the performance. Though
> >
> http://ofps.oreilly.com/titles/9781449302641/load_and_store_funcs.html#loadfunc_loaderpushdownsaid
> > that "As part of its optimizations Pig analyzes Pig Latin scripts and
> > determines what fields in an input it needs at each step in the script.
> It
> > uses this information to aggressively drop fields it no longer needs."
> >
> > We also found that Pig casts the data into the types defined in the
> schema,
> > which is usually unnecessary, as most of them will be soon dropped.
> >
> > To work around these, we have to manually drop those fields and remove
> the
> > types in the schema, which are really not interesting.
> >
> > Jie
>
>