There is a good blog regarding key user extensibility tool in S4 written by Thomas Schneider.

One of supported feature is customer can easily add new fields to extensible CDS view without knowing technical detail.
For example, if a given CDS view is marked as “UI Reports” extensible in extensibility registration tcode SCFD_REGISTRY,

it means this CDS view would be visible in key user extensibility tool in S4, in tab “UIs and Reports”, so that end user can simply click “Enable Usage” button to add extension field to the view. In the screenshot below, it shows my Extension field “JDK Minimum version” has already been extended to view I_PRODUCTWD.

Now, just use the single line below, and you can read all fields of this view, standard fields and extension fields:
SELECT SINGLE * INTO @DATA(ls_data) FROM i_productwd WHERE
product = 'JAVA'.

So this CDS view extensibility is really easy to use for application developer.
However, how all these whole scenario work under the hood?
When trying to search the source code of extended CDS view by fragment of extension field name JDK, nothing found. This makes sense since none of SAP extensibility tool will directly MODIFY standard object.

Hover the mouse to this small icon in CDS view and you can see lots of extension view, which holds extension fields created by key user tool.
However how could I quickly locate which extension view holds my extension field “JDK Minimum Version”?
Open the SQL view of CDS view I_PRODUCTWD, it is IPRODUCTWD in SE11.
Search by keyword JDK and I find the extension field is added to this SQL view via append ZZ1_2CC44DDD3F1C.
Double click this append, then you can find the name of CDS extension view which holds this extension field: ZZ1_PRE47GXHDI6P2ZLO3ADGY4HHFE
Now go back to ABAP studio, locate CDS extension view ZZ1_PRE47GXHDI6P2ZLO3ADGY4HHFE and open it:
Here you can see that in this automatically generated CDS extension view, my extension field are there. The standard view I_PRODUCTWD is extended by extension view ZZ1_PRE47GXHDI6P2ZLO3ADGY4HHFE via key word “extend view”, as a result the extension field ZZ1_JDKMinimumversion_PRD in ZZ1_PRE47GXHDI6P2ZLO3ADGY4HHFE will also be visible in view ZZ1_PRE47GXHDI6P2ZLO3ADGY4HHFE.

Last question, how and when the extension view ZZ1_PRE47GXHDI6P2ZLO3ADGY4HHFE is generated?
We can simply do testing by disable the usage of my extension field on view I_PRODUCTWD by click button “Disable Usage”.

Once done, the extension field disappears from view I_PRODUCTWD,

This observation proves that the extension view will only be generated after we click “Enable Usage” and publish the change.
When we click publish button, there are totally 10 enhancements to be generated for my extension field:
The first three rows are CDS extension view to be generated. Double click each and write down detail:

And extension view ZZ1_PRE47GXHDI6P2ZLO3ADGY4HHFE will be generated to extend I_PRODUCTWD:
The left code is responsible to generate extension view which I am not intrested in. What I am curious about is how the internal table mt_enhancements of CL_CFD_ENHANCEMENT_ITERATOR is filled.
It is filled in method determine_generation_scope:
In this method, the task to generate CDS extension view for view I_PRODUCTWD is added to enhancement scope, which will be executed later.

All these ten instances will be generated, to make the whole CDS view extensibility work.

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