topic join or Mapping ? in QlikView

join or Mapping ?

Thu, 05 Apr 2012 06:33:25 GMT

Hi,

Suppose i want to join two tables each having 2 columns and a column common in each , what should i perform left join or mapping ?

which is better performance wise ,,my feeling is mapping would be better.

join or Mapping ?

Thu, 05 Apr 2012 06:34:26 GMT

sorry not left join but inner join

join or Mapping ?

SunilChauhan — Thu, 05 Apr 2012 06:35:41 GMT

mapping is causing slow performance

i suggest you to use inner join

hope this helps

join or Mapping ?

Thu, 05 Apr 2012 06:43:48 GMT

I read in the manual that after mapping the columns, the mapping table gets deleted by itself. So isnt mapping table better then? also, can u tell me why mapping causes slow performance

join or Mapping ?

vijay_iitkgp — Thu, 05 Apr 2012 06:51:06 GMT

Hi Vikram,

Join and Mapping are two different things. First you need to think what exactly you need in resulting table. If you want a new column then you require Join . but if you want to update existing column mapping will be needed. Both have their own importance.

Hope I have cleared ur doubts.

Regards

VIjay

join or Mapping ?

Thu, 05 Apr 2012 07:13:00 GMT

Hi Vijay,

Can you give me the example of update?that would be helpful as i am under impression that mapping table is only used add a column to the table like join.

join or Mapping ?

SunilChauhan — Thu, 05 Apr 2012 07:15:54 GMT

A left join keeps the load optimized. All tested mapping methods BREAK the optimization. This matters. It turns out that a left join is the fastest option for a small number of mapping tables, at least for my sample data (5,000,000 rows, ID field, 10 other fields with values from 0 to 3).

its better to look at http://community.qlik.com/message/114731

hope this helps

join or Mapping ?

vijay_iitkgp — Thu, 05 Apr 2012 07:18:37 GMT

You can see it in help tab in QlikView. If still doubt then I will create some example for you.

Regards

VIjay

join or Mapping ?

vijay_iitkgp — Thu, 05 Apr 2012 07:22:15 GMT

Also Mapping table can have only 2 columns.

join or Mapping ?

Thu, 05 Apr 2012 07:37:38 GMT

No need dude thanks i got it my mistake ..

join or Mapping ?

Thu, 05 Apr 2012 07:40:07 GMT

I feel that Join and Mapping are essentially different in terms of usage. While the result may be the same, the reason they are used is different.

Mapping should be primarily used when there is a 'if A, then (always) B' relation. For example,

Table 1:

Day, Person

Monday, A

Tuesday, B

Wednesday, C

Thursday, C

Friday, C

Saturday, C

Sunday, B

Table 2:

Person, Number

A, 1

B, 2

C, 3

If we want to get the value of 'Number' in Table 1, then mapping is used, because A always has the value of 1.

I am not sure about the performance of either method though. Maybe someone else can elaborate on that.

join or Mapping ?

Thu, 05 Apr 2012 07:41:44 GMT

I didnt understand that article completely , but from what i inferred from your statement is left join is preferred if less rows are present else mapping

join or Mapping ?

SunilChauhan — Thu, 05 Apr 2012 07:44:15 GMT

according to that post what i understand is mapping load in not optimized where as Join is optimized .