Calculating cumulative line length on large dataset

Question

I have a line split into segments that I want to calculate the accumulative length for, like this:

I can achieve this easily on a smaller dataset with this inbuilt function in the field calculator in QGIS:

aggregate('layer', 'sum', "Length", "id"<=attribute(@parent, 'id'))

However, my dataset has hundreds of thousands of lines and this method is far too inefficient.

Is there a more effective way to achieve this?

Answer 1

For a large amount of lines, you may have better efficiency by using a database and running this computation directly in the DB.

Here is a solution using a virtual layer. Go the the menu layer / add layer / add-edit virtual layer and enter the following query.

select *, 
   sum("length") over (order by id) as cumul, 
   sum("length") over (partition by gp order by id) as cumul_by_group
from myLayer

The 1st computation returns the total cumulative length, while the 2nd one groups them (by connected segments maybe?), it's up to you to choose which one to use.