[Monetdb-developers] Question about milprint_summer

Keulen, M. van (Maurice) m.vankeulen at utwente.nl
Fri Apr 20 15:31:53 CEST 2007

Thanks for your info! Much clearer now. I added the comments to the
mentioned Wiki-page for future reference.

Peter Boncz wrote:
> Hi Maurice,
>> For my research on probabilistic XML, I need some additional 
>> functions in XQuery implemented in mil. By means of copying 
>> code from existing functions in milprint_summer, I've been 
>> able to get some things working, but I still do not have the 
>> feeling that I understand the meaning and purpose of all the 
>> bats. For example, what's this ipik bat? There seem to be two 
>> interfaces (NORMAL and VALUES) that pass data from one 
>> subexpression to the next. My question is (I guess): which bats 
>> are involved in each of the interfaces and which conditions 
>> hold and do I need to adhere to wrt these bats? Is there some
>> documentation around about this? I checked the pathfinder Wiki, 
>> but it, for example, gives no result for a search for "ipik".
> Jan wrote the following indications for writing functions in mps:
> http://www.pathfinder-xquery.org/wiki/index.php/Function_implementation
> The 'ipik' thing is not described there, because it is newer. It stands for
> iter-pos-item-kind, the four bats crucial to the relational sequence
> representation, which contain BAT[void,T] values.
> At some point, Stefan and me introduced support for constants. Since then,
> each variable (item-pos-item-kind) can possibly be a MIL constant. However,
> one of them *must* be still a BAT. But which? The purpose of the 'ipik' is
> to identify that BAT variable. That is, it is assigned to either iter,pos,
> item or kind.
> Then, the hairy part is whether your MIL code is constant-resistant.
> Obviously, if you can operate on the constants directly, the query tends to
> be faster. We provide support for constants in many pf_support.mx MIL procs
> (e.g. get_container()), as wel as MIL maps (e.g. [+](1,2) just works, i.e.
> multiplex allows constant-only parameters). A part of the MIL bat algebra
> (join, min, max, texist, etc)  is supported on constants as well, see
> MonetDB4/src/modules/plain/constant.mx
> If you do not want to bother optimizing your code for constants, you can
> always use the 'materialize' command to inflate any constant back into a
> bat:
> # enforce all variables to be bats (no effort if already so)
> iter := iter.materialize(ipik);
> pos  :=  pos.materialize(ipik);
> item := item.materialize(ipik);
> kind := kind.materialize(ipik);
> As for the two "rc" modes in mps (VALUES, NORMAL) the following:
> - NORMAL should be used always for heterogeneously typed sequences. The
> 'item'  is a BAT[void,oid], where the tail OID refers to a value container
> (e.g. str_values). Booleans and nodes do not have a container, BTW. Thus, we
> have str_values/int_values and dbl_values (= dec_values).
> - the VALUES representation uses 'item_str', 'item_int' resp 'item_dbl'
> variables instead. It can be used for homogeneous sequences (typically,
> 'kind' is a constant then), and those values directly contain values of the
> desired type, e.g. BAT[void,lng] for integers (yes: XQ integers are longs!).
> - translate2MIL() can be called with the request VALUES. It then *may*
> produce a result in VALUES representation, returning the type in that case.
> E.g. for integer-typed results in VALUES representation, it would return
> INT. But, even if you request VALUES, translate2MIL may still return NORMAL.
> In case translate2MIL is called with rc==NORMAL, it must produce normal.
> - if you have the VALUES representation, and want to go to NORMAL, use the
> MIL proc addValues(), generated in mps by the convenience function
> addValues.
> Good luck,
> Peter
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Dr.Ir. M. van Keulen - Assistant Professor, Data Management Technology
Univ. of Twente, Dept of EEMCS, POBox 217, 7500 AE Enschede, Netherlands
Email: m.vankeulen at utwente.nl, Phone: +31 534893688, Fax: +31 534892927
Room: ZI 3039, WWW: http://www.cs.utwente.nl/~keulen

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