  Dense retrieval (DR) approaches based on powerful pre-trained language models
(PLMs) achieved significant advances and have become a key component for modern
open-domain question-answering systems. However, they require large amounts of
manual annotations to perform competitively, which is infeasible to scale. To
address this, a growing body of research works have recently focused on
improving DR performance under low-resource scenarios. These works differ in
what resources they require for training and employ a diverse set of
techniques. Understanding such differences is crucial for choosing the right
technique under a specific low-resource scenario. To facilitate this
understanding, we provide a thorough structured overview of mainstream
techniques for low-resource DR. Based on their required resources, we divide
the techniques into three main categories: (1) only documents are needed; (2)
documents and questions are needed; and (3) documents and question-answer pairs
are needed. For every technique, we introduce its general-form algorithm,
highlight the open issues and pros and cons. Promising directions are outlined
for future research.
