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02.08.2008 at 02:09AM PST, ID: 23147279

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8.8

People who use OLAP - how much should they know?

Zones: OLAP, SQL Server 2005

Hi all,

Previously I was a VB programmer and as such generally tried to program so the user couldn't break it.

I have noticed that creating OLAP cubes seems to throw up a few instances where the user can crash Excel, create reports that take ages to run or return data that doesn't make sence.

Am I supposed to be making it so this doesn't happen or is this an accepted point in OLAP and people should know enough not to make it happen? As i'm really struggling to make my cube so none of that happens.

Examples:
I have 2 fact tables. If I bring in a measure from fact 2 and dims from fact 1, the dims get listed and the measure values are split accordingly.
If I bring in a measure from fact 1 and a dim from fact 2, the measure has the same value for every single one of the dim members. i.e.
Issue Type Number of subscribers.
FREE      1944424
OTHER      1944424
PAID      1944424
A subscriber does not have an issue type! Therefore it isn't relevant. Should the user know this and therefore I don't have to code round it. Or should I be making it show blanks if irrelevant or something?

I have a Dimension with 207000 members. If the user filters on the member in the filters its fine. If the user tries to filter on a dimension in the Row Labels Excel crashes.
Should the user know that you cant filter on a 207000 member dimension in that way or should it not be crashing?

If the user tries to display measures for all campaigns split out by country it will take ages to run. Should it? From an SQL user point of view, trying to count 207000 campaigns split by 289 countries should take ages and the user should either expect it or not really want to do that kind of count. Is it accepted to create a cube with calculations that take ages or should everything in the cube be quick?

My final question being; should a user know they can do things to make it crash/take ages or is their horror/disappointment when excel locks up and bombs out or takes ages valid and it should not be doing that?

Regards,
Kinton

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Question Stats

Zone: Database

Question Asked By: kinton

Solution Provided By: PFrog

Participating Experts: 1

Solution Grade: A

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02.08.2008 at 02:31AM PST, ID: 20849020

Rank: Wizard

PFrog:

This really depend on who's going to be using it.
If it is used as a technical tool by a few people who know what they're doing, then you can afford to leave stuff up to the user.
It sounds like your system will be used by non-techy users who don't really know what they're doing, in which case you have two options:
1) Build in as many safeguards as you can
2) listen to the users slate you and your cube because the darn thing gives rubbish data
!!!

However, there are some things that you just can't cope with. Such as
If the user selects 207000 individual members to search on, it will cause problems - but you try running a SQL query will 207000 where clauses... I'm guessing you'll get a similar experience. The only trick here is to limit the user's access to such large attributes. Can you add a hierarchy so that they see smaller, more manageable lists? Or remove this option entirely from their perspective.

Regarding the measure from 1 and attribute from 2, you can protect against this if you want by using calculated members. Rename your "Subscribers" measure to "Subscribers_" and make it invisible. Then create a calculated member "Subscribers" that checks whether any appropriate dimensions have been selected, and return 0 if they haven't. i.e.
iif([dim].[att].CurrentMember=[dim].[att].[All], 0, [Measure].[Subscribers_])
which will only show the number of subscribers if [dim].[att] is included in the rows or columns of the query

I suppose the best way of thinking about it is that you could try and train your users to use it properly, however the more time and effort you put in now to make it foolproof, the less you're going to have to deal with fools using it in the future!

Please shout it you want me to elaborate on anything...

Accepted Solution

02.08.2008 at 02:56AM PST, ID: 20849101

kinton:

Thanks PFrog.

You are right, we have people who are practically computer illiterate (by that I mean sales people or account executives as they now like to be know :) ) using it (not all of them, but thats the extreme).

I have broken the 207000 into slightly shorter lists but for some reason if I filter on it, it crashes (see other post http://www.experts-exchange.com/Database/OLAP/Q_23147246.html) I mentioned that its broken down by first letter, but its also broken down by last years campaigns, this years and a few others like it, so once i've fixed my filtering problem I guess thats just a case of the user knowing not to be an idiot.

The thing with the measures could be useful, though i'll have to look into that. Theres 29 dimensions relevant to the subsctiptions fact and currently 3 relevant to issues fact. However I know we're supposed to be adding in another fact table at some point so I may have to leave that one until everything is in.
I think the Adventure works cube has the same "problem" so I guess its normal practice.

At the moment I have more problems with product expectation than I do with the technical side of it. People have an expectation of how long a cube query should take to run and they have an expectation of what a cube should and shouldn't be able to do. They don't seem to have anything to back up their expectations and I dont have anything to prove or disprove them. I think the bods at microsoft need to make a few more than the Adventure works one, specially ones that we don't necessarily have background access to i.e. make a load and host them theirselves then make the https path to msmdpump.dll available so we can connect to them and see what should and shouldnt happen, and people who will use our cubes can do the same.

Thanks for your help again PFrog

Regards,
Kinton

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