A custom index would allow an index on arrays and other multi-key value types as well as allowing custom index implementations.
The virtual table table interface does not easily allow the user to create an index on a table after the table has been created, especially a composite key index. It does not support arbitrary values using the current method of `INSERT INTO vtab(vtab, col1, col2) VALUES ('create index', ..., ...)`. This syntax is invalid and does not support extra user defined columns for the index and really should just be `CREATE INDEX name ON vtab(...)` and have an overloaded function in the virtual table handle it.