How inefficient is this technique?
Since you can’t query a database from a field formula, but you can follow relations…
Have a “all data” table with 1 row and a column per other linked table
Have a “all data” many-to-one column in each linked table, with every row populated
Have a rule for each table to set the column whenever a new row is added
Access all your data from any formula (including fields)
It’s not a 1, but it’s not god-worrying 
To be honest, I find an easier method is to add a formula field (called “X” or whatever) which is just the value true.
Then create a self-relation (many-to-many) which uses automatic linking where X = X. Now, every entity is linked to every other entity in its own database, and you can do formula queries on the whole db easily.
nice to know that link is persistent, I assumed it was a one off thing to populate existing rows
Does “X” need to be a formula, or is that only to prevent accidentally changing a value so it no longer matches all?
It’s a formula to ensure that it’s read-only (even though it is a static value)
