We're planning to start with filtering on submission date and submitter. (@dmenne, you're right that this submission metadata is easier to access.) However, we've discussed expanding to filtering on any form field, though there's no timeline right now. It's very useful to learn more about your use cases and the ways in which this functionality would be helpful.
@dmenne, I think there are a few sources of complexity here:
- Parsing the OData filter expression
- Selecting the field
- Indexing the field so that filtering is faster than pulling all rows
@issa discusses (1) a little here.
You're right that (2) is more challenging for form fields than it is for submission metadata like submission date and submitter. I think you're right that the Postgres xpath() function isn't designed for text values, but Postgres also allows conversion between text and xml, and we could maybe also alter the type of that column.
@dmenne, for your example 2, isn't filtering on submission date what you need there?