CDS View Substring wrong Char length

Hey!

I used this SUBSTRING: substring (comm_product.product_id, 23,18) as matnr,

What I get in the abap view is a CHAR(30) type from table DDDDLCHARTYPES. (Sure, CHAR(30) is the closest charlength to 18 at DDDDLCHARTYPES).

What I need is CHAR(18).

Should I try to add a new entry to the table DDDDLCHARTYPES like CHAR(18) ?

Or any other solutions?

Thanks!