You might be confusing writable and editable. A table with no BTREE index is not editable (and records can't be selected). That's because records can't be singled out in any way. But the table may still be writable (unless it is read-only). That's a different thing. In particular, you can still add fields (including a Geom field), add indexes,* and add records, and you can update a field (for all records) using SQL. (*This has to be the case. If you couldn't add a BTREE index, you couldn't make the table editable.) Grey does not mean read-only. Grey means you can't select or edit individual records. Subtly different perhaps, but completely different. I suspect that the reason you can't add a BTREE index on the PT_ID field is that values in that field are not unique. (As unlikely as that sounds.) You can test that with SQL: SELECT [PT_ID] FROM [Table] GROUP BY [PT_ID] HAVING COUNT([PT_ID]) > 1 ; (No BTREE index is required to run the query.) Even if you can't add a BTREE on PT_ID, you can still add a Geom field and populate it (e.g. from Longitude and Latitude fields).
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