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From Apache Wiki <wikidi...@apache.org>
Subject [Pig Wiki] Update of "AvoidingSedes" by ThejasNair
Date Mon, 28 Jun 2010 23:42:43 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Pig Wiki" for change notification.

The "AvoidingSedes" page has been changed by ThejasNair.
http://wiki.apache.org/pig/AvoidingSedes?action=diff&rev1=4&rev2=5

--------------------------------------------------

  == Delaying/Avoiding deserialization at runtime ==
  These approaches (except 5) does not involve major changes to core pig code. Load functions,
or serialization between map and reduce can be separately changed to improve performance.
   1. '''!LoadFunctions make use of public interface !LoadPushDown.pushDownProjection.'''
Don't deserialize columns not that are not in required . This should always improve performance.
!PigStorage indirectly works this way, if a column is not used, the optimizer removes the
casting(ie deserialization) of the column from the type-casting foreach statement which comes
after the load.
-  1. '''!LoadFunction return a custom tuple, which deserializes fields only when tuple.get(i)
is called.''' </b> This can be useful if the first operator after load is a filter operator
- the whole filter expression might not have to be evaluated and that deserialization of all
columns might not have to be done. Assuming the first approach is already implemented, then
this approach is likely to have some overhead with queries where all tuple.get(i) is called
on all columns/rows.
+  1. '''!LoadFunction returns a custom tuple, which deserializes fields only when tuple.get(i)
is called.'''  This can be useful if the first operator after load is a filter operator -
the whole filter expression might not have to be evaluated and that deserialization of all
columns might not have to be done. Assuming the first approach is already implemented, then
this approach is likely to have some overhead with queries where all tuple.get(i) is called
on all columns/rows.
   1. '''!LoadFunction delays deserialization of map and bag types until a member function
of java.util.Map or !DataBag is called. ''' The load function uses subclass of Map and DataBag
which holds the serialized copy. This will help in delaying the deserialization further. This
can't be done for scalar types because the classes pig uses for them are final; even if that
were not the case we might not see much of performance gain because of the cost of creating
an copy of the serialized data might be high compared to the cost of deserialization. This
will only delay serialization up to the MR boundaries. 
  {{{
  Example of query where this will help -

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