All these discussions should be in the bug that Chris created rather than on the mailing list.
Olga -----Original Message----- From: Amir Youssefi [mailto:[EMAIL PROTECTED] Sent: Thursday, December 13, 2007 11:02 AM To: [email protected] Subject: RE: Case for Optimization For starter we can give ability to manually specify (or suggest) execution plan i.e. adding Hints. Next step would be for Pig to guess optimizations. Cardinality hints can be used as manually hinting on statistics. Another example I ran into was type of join. Pig keeps one set of data in memory which is good for Nested Loop join. We can hint Pig on how we think it should do it depending on relation of data sizes etc. Perhaps we can let user provide optional Metadata and Statistics (at least cardinality). On theoretical/academic side there seems to be a lot ahead of us on optimization front. I'm looking forward to contributing on that side. -Amir -----Original Message----- From: Chris Olston [mailto:[EMAIL PROTECTED] Sent: Wednesday, December 12, 2007 8:56 PM To: [email protected] Subject: Re: Case for Optimization Yup. It would be great to sprinkle a little relational query optimization technology onto Pig. Given that query optimization is a double-edged sword, we might want to consider some guidelines of the form: 1. Optimizations should always be easy to override by the user. (Sometimes the system is smarter than the user, but other times the reverse is true, and that can be incredibly frustrating.) 2. Only "safe" optimizations should be performed, where a safe optimization is one that with 95% probability doesn't make the program slower. (An example is pushing filters before joins, given that the filter is known to be cheap; if the filter has a user- defined function it is not guaranteed to be cheap.) Or perhaps there is a knob that controls worst-case versus expected-case minimization. We're at a severe disadvantage relative to relational query engines, because at the moment we have zero metadata. We don't even know the schema of our data sets, much less the distributions of data values (which in turn govern intermediate data sizes between operators). We have to think about how to approach this that is compatible with the Pig philosophy of having metadata always be optional. It could be as simple as (fine, if the user doesn't want to "register" his data with Pig, then Pig won't be able to optimize programs over that data very well), or as sophisticated as on-line sampling and/or on-line operator reordering. -Chris On Dec 12, 2007, at 7:10 PM, Amir Youssefi wrote: > Comparing two pig scripts of join+filter and filter+join I see that > pig has an optimization opportunity of first doing filter by > constraints then do the actual join. Do we have a JIRA open for this > (or other optimization scenarios)? > > > > In my case, the first one resulted in OutOfMemory exception but the > second one runs just fine. > > > > -Amir > -- Christopher Olston, Ph.D. Sr. Research Scientist Yahoo! Research
