The delta index is deactivated by default. You can activate it using the TREX admin tool. You activate it per index, not globally.
The best time for activating it depends on your indexing process.
SAP recommends the following:
Activate the delta index after the initial indexing run. If you do not do this, the delta index grows too quickly and you have to integrate it into the main index earlier than you would wish. This means that you need twice the indexing time: Firstly to index the documents in the delta index, and then to integrate the delta index into the main index.
Monitor the size of the main index during routine operation. Activate the delta index if the main index reaches 100,000 to 1,000,000 documents or 500 MB.
Procedure