Efficient way to merge multiple large XML files on android

Efficient way to merge multiple large XML files on android

Use Case: Think of an application that performs a daily backup of your
call log or messages and stores it into an XML file.
Now, I want to implement the ability to merge two or more files and create
one XML file. Some of the entries may be duplicate, some added, some
deleted.
I could create a Map and put all entries into it with "id" being the key
(which will be unique for each row). But this approach may cause memory
issues if the number of rows run into several thousands.
Can anybody suggest a better way of implementing this merge feature?