Depends on the research topic. There are many databases that can claim definitive coverage but there are many more which should be kept in reserve. Just like the internet, a researcher is not expected to look at everything relevant, just enough to get to the solution. Size Global Textline was a database of phenomenal size, indexing text from over a hundred newspapers globally, reaching back many years. Each topic may only include 10 relevant theses over 5 years.
Size is a thus linked to database value.
Searching Global Textline would always turn up leads. AEI will not. Selecting a Database Despite the factual nature of information research, word of mouth appears to be tremendously important in choosing databases. Some guides do describe the quality of various databases, and make valuable suggestions but such guides also age rapidly as new products emerge. A rough understanding may emerge with practice.