The QlikView Cache

[hic]

QlikView has a very efficient, patented caching algorithm that effectively eliminates the calculation time for calculations that have been made before. In other words, if you use the “back” button in the toolbar, or if you happen to make a selection that you have made before, you usually get the result immediately. No calculation is necessary.

But how does it work? What is used as lookup ID?

For each object or combination of data set and selection or data sub-set and expression QlikView calculates a digital fingerprint that identifies the context. This is used as lookup ID and stored in the cache together with the result of the calculation.

Here "calculation" means both the Logical Inference and Chart calculation - or in fact, any expression anywhere. This means that both intermediate and final results of a selection are stored.

There are some peculiarities you need to know about the cache…

• The cache is global. It is used for all users and all documents. A cache entry does not belong to one specific document or one user only. So, if a user makes a selection that another user already has made, the cache is used. And if you have the same data in two different apps, one single cache entry can be used for both documents.
• Memory is not returned, when the document is unloaded. Cache entries will usually not be purged until the RAM usage is close to or has reached the lower working set limit. QlikView will then purge some entries and re-use the memory for other cache entries. This behavior sometimes makes people believe there is a memory leak in the product. But have no fear – it should be this way. So, you do not need to restart the service to clear the cache.
• The oldest cache entries are not purged first. Instead several factors are used to calculate a priority for each cache entry; factors like RAM usage, cost to calculate it again and time since the most recent usage. Entries with a combined low priority will be purged when needed. Hence, an entry that is cheap to calculate again will easily be purged, also if it recently was used. And another value that is expensive to recalculate or just uses a small amount of RAM will be kept for a much longer time.
• The cache is not cleared when running macros which I have seen some people claim.
• You need to write your expression exactly right. If the same expression is used in several places, it should be written exactly the same way – Capitalization, same number of spaces, etc. – otherwise it will not be considered to be the same expression. If you do, there should be no big performance difference between repeating the formula, referring to a different expression using the label of the expression or using the Column() function.

The cache efficiently speeds up QlikView. Basically it is a way to trade memory against CPU-time: If you put more memory in your server, you will be able to re-use more calculations and thus use less CPU-time.

HIC

Further reading on the Qlik engine internals:

Symbol Tables and Bit-Stuffed Pointers

Colors, States and State vectors

Logical Inference and Aggregations

[#2]

That's interesting question. (I mean Chris question: Does this also apply with using variables as Expressions? ) I'd like to ask related question:

Let's say we have to tables: first one shows us absolute numbers and uses $(SalesFigure) as expression, the second one shows per customer values: $(SalesFigure)/Count(distinct customer_id). It is requirement to keep it in separate tables so it impossible to use Column() function.

The question is whether QlikView would be able utilize cache for second table?

[#3]

Johan IdhHenric Cronström

Thanks for both..

[msteedle]

I would be surprised to find out how the same cache entry can be used across documents, considering that QlikView doesn't even seem to reuse the same expression grouped by the same dimension in different chart types within one application. Ex. a straight table with Sum(Sales) by Customer and a bar chart with Sum(Sales) by Customer are calculated independently within an application. At least, this appeared to be the case when I tested this in an earlier version of 11.0.

[hic]

It should use the cache across documents and my experience is that it does.

If you have a case where the same combination of dimension and expression doesn't use the cache, there is probably a reason, e.g. that the expression (or a calculated dimension) is written in different ways in the two places. Either that, or you have found a bug. Because it should re-use the cache entry...

HIC

[hic]

Bill Markham

Create a macro, and run this from a button:

Sub ClearCache

ActiveDocument.ClearCache

End Sub

HIC

[ChristofSchwarz]

It was new to me that QlikView uses cache across Documents. What I cannot imagine is, how can the engine know, if a stored aggregate cache from one document is valid for the other document and therefore can be taken? It could be that common field names are pure coincidence without relevance

[hic]

That's the patent!

The digital fingerprint contains information both on field names and the data set behind. So it does not just use the field name. A different data set will inevitably lead to a different fingerprint, whereas two apps with identical data sets will get the same fingerprint.

HIC

[barryharmsen]

And this digital fingerprint is a hash, right?

[hic]

Yes, a 256-bit hash.

HIC

[ChristofSchwarz]

Is an aggregate cache also able be re-used "in chunks"? Assuming this: User selects countries AT,DE,CH and a chart renders Sum(Sales) over the Dimension Country. Then the user deselects CH, which is a subset of what was previously calculated and cached. What will happen?

Can an aggregate cache be reused if the dimensionality is different (less granular)? I mean If a chart has two dimensions, Year and Country and calculates Sum(Sales) and another chart would use only Year and Sum(Sales), can the calculation of the 2nd build the result out of the cached 1st aggregate and save time?

Thank you