Via va Viaindex

[Hamilton, Allison]

My recollection is that the cases would be rare - but if you had a small
amount of data it might be faster to just read the data in and order it
rather than use the index, or if the database knows how the data is stored
(it seems to) it might know that the storage order matches the requested
order anyway, so it might not need to use the index... As you say, it's a
very long time since I was in this kind of stuff.  But I remember
discussions about it at one point when I was first learning about relational
databases and how they optimize data access.  I'm not a relational guru,
however.
Allison Hamilton
> My words and ideas are not necessarily my employers.
> 
> ----------
> From: 	Richard Witkopp[SMTP:witkopp@idt.com]
> Sent: 	Wednesday, August 11, 1999 4:39 PM
> To: 	powerh-l@lists.swau.edu
> Subject: 	RE: Via va Viaindex
> 
> 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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