Elsewhere on the forum, a user is asking whether DEVONThink indexes Bear. The answer, as always, is no, because Bear stores documents in SQLite, not text files. And that is perfectly reasonable.
Similarly, I’d love for DEVONThink to be able to work with Drafts, which also stores documents in SQLite.
It just occurred to me: What if DEVONThink could index SQLite? Would it then be able to index Bear and Drafts documents?
What other popular Apple productivity apps use SQLite? Ulysses, Agenda?
SQL files aren’t constructed in a simple to index format. They have a particular schema, defined by the originator, which means indexing for one app has no utility in indexing for other apps. Each would have to be parsed individually.
Also they are built of internal tables, creating relational data connections between them. So parsing individual tables would be of little use compared to making proper SQL queries. But SQL queries are also relative to the tables of one situation. They’re not universal.
The request is noted but highly unlikely. @cgrunenberg could weigh in on this, if he wishes.
SQLite is the basis for many not too sophisticated local relational storage needs on MacOS (plus it’s free, so Apple doesn’t have to pay royalties). But, and that’s a polite way of saying “no way”: As @BLUEFROG noted, there’s no such thing as “Index SQLite”. It is a relational database management system. How it is used, which tables, indices, etc., is completely up to the application that uses it. You might want to get one of the freely available SQLite browsers just to get an idea what Bear does and what possibly Drafts does.
What’s worse: There is no concept of “files” in SQL(ite). A database stores records. If they feel so compelled, they might split up the information over several tables (which is what “relational” is all about). DT (or any other app for that matter) would have to scramble these pieces together, and it would be at the mercy of the other app’s programmers: If they change the database structure, “indexing” in DT would break.
In short: That’s a dead end, in my opinion.
Highly unlikely is a good description 
Oh, well. It was an idea.
Perhaps agreements with key productivity app vendors to index their apps?
And there’s nothing wrong with discussing ideas.
Perhaps agreements with key productivity app vendors to index their apps?