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From Mridul Muralidharan <mrid...@yahoo-inc.com>
Subject Re: optimizer hints in Pig
Date Sat, 14 Nov 2009 23:17:29 GMT

This should be interesting to pursue.
For non-trivial plans, it can be used to model how join's are to be 
optimized for example - instead of having users specify it, infer based 
on the hints from various stages.
Similarly, an oft-repeated request is making pig infer the value of 
PARALLEL - which can also be influenced by this :

a-> load, filter1, project
b-> load, filter2, project, filter3.
join a, b.

Now we can estimate 'sizes' of a and b based on input and arrive at 
things like : reducers to be used for a, b ; what kind of joint to use 
(traditional, fragment replicate, etc), and so on.

This should be interesting if it scales to slightly more complex scripts.

The example generator gets into some basic aspects of this iirc - though 
not from a costing function point of view (atleast I remember discussing 
it with Shubham).


Ashutosh Chauhan wrote:
> Hi All,
> We would like to know what Pig devs feel about optimizer hints.
> Traditionally, optimizer hints have been received with mixed reactions
> in RDBMS world.  Oracle provides lots of knobs[1][2] to turn and tune,
> while postgres[3][4] have tried to stay away from them. Mysql have few
> of them (e.g., straight_join). Surajit Chaudhary [5] (Microsoft) is
> making case in favor of them.
> More specifically, I am talking of hints like following
> a = filter 'mydata' by myudf ($1) with "selectivity 0.5";
> // This is letting user to tell Pig that  myudf filters out nearly
> half of tuples of 'mydata'.
> c = join a by $0, b by $0 with "selectivity a.$0 = b.$0, 0.1";
> // This is letting user to tell Pig that only 10% of keys in a will
> match with those in b.
> Exact syntax isn't important it could be adapted. But, question is
> does it seem to be  a useful enough idea to be added in Pig Latin.
> Pig's case is slightly different from other sql engines in that while
> other systems treats them as "hints" and thus are free to ignore them
> Pig treats hints as commands in a sense that it will fail even if it
> can figure out that hint will result in failure of query. Perhaps, Pig
> can interpret "using" as command and "with" as hint.
> Thoughts?
> Ashutosh
> [1] http://www.dba-oracle.com/art_otn_cbo_p7.htm
> [2] http://www.dba-oracle.com/oracle11g/oracle_11g_extended_optimizer_statistics.htm
> [3] http://archives.postgresql.org/pgsql-hackers/2006-10/msg00663.php
> [4] http://archives.postgresql.org/pgsql-hackers/2006-08/msg00506.php
> [5] portal.acm.org/ft_gateway.cfm?id=1559955&type=pdf

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