Via va Viaindex

[Richard Witkopp]

I can't remember, it's been a long time since I fiddled with
a database very much. But.... if you want it ordered by the
columns in an index, how could that index be anything but the
optimal retrieval method? I guess if you had multiple indexes
using the same columns, but that would make me want to specify
the index even more.

> That may be true, I'm not a guru on all the specific 
> capabilities for each
> database, but remember, the PowerHouse code was written 
> longer ago than
> that, and we try to keep it generic, where possible, so that 
> it is usable on
> multiple databases with few or no changes.
> 
> If you specify the index - does the database use it, even if 
> it's not the
> optimal retreival method?  Or does it just take it as a 
> suggestion?  Most
> databases will, in fact, use the index for retrieval if you 
> specify the
> segments that match an index, and it's the optimal retrieval 
> method, so it
> usually amounts to about the same thing, I would suspect.
> Allison Hamilton
> 
> > ----------
> > From: 	Richard Witkopp[SMTP:witkopp@idt.com]
> > Sent: 	Wednesday, August 11, 1999 1:04 PM
> > To: 	powerh-l@lists.swau.edu
> > Subject: 	RE: Via va Viaindex
> > 
> > Really? That may have been true 5 years ago, but I thought
> > most modern versions of databases would let you specify the
> > index.
> > 
> > > Mark,  
> > > 
> > > Most database optimizers will not quarantee that if you give 
> > > them an Index
> > > to use, they will use it. They may decide that the index is 
> > > not the optimal
> > > method of retrieval. (depending on the amount of data and the 
> > > kind of data
> > > in the table). I believe that some database don't even allow 
> > > you to specify
> > > an index on retrieval.  Because of that PowerHouse doesn't 
> > > pass the VIAINDEX
> > > information to the database at all.  We pass the list of 
> > > segments and the
> > > ordering information instead. This allows the database to 
> > > decide for itself
> > > which path is the optimal one for it to use in each case.  
> > > The ordering
> > > request quarantees that the information is retrieved in a 
> > > consistent order
> > > regardless of how the database retrieved it.  In this way we 
> > > are using the
> > > database as efficiently as we can, letting it use its, hopefully
> > > intelligent, information base to determine how best to get 
> > > you the data you
> > > requested.
> > > 
> > > Syntacticly, use of VIAINDEX, for relational databases is 
> a kind of
> > > shorthand for VIA a,b,c ORDERED, which is the information 
> > > that will be sent
> > > to the database.  With VIAINDEX, you can override the 
> > > defaults by specifying
> > > the VIA segments, but if they are not a sequential subset 
> of the index
> > > segments (a,b is valid, a,c is not, nor is b,c), PowerHouse 
> > > will ignore the
> > > VIAINDEX specification in the more recent versions (PH 
> > > 8.1...). (I'm not
> > > sure how accurate the comparison is for earlier versions.) 
> > > You can also
> > > override the ordering by specifying it.  In this case 
> > > PowerHouse will use
> > > the VIAINDEX segments but apply your specified ordering.
> > > 
> > > Hope this helps
> > > 
> > > Allison Hamilton
> > > My words and ideas are not necessarily my employers.
> > 
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