set-access-method
errorSET ACCESS METHOD
Why it's unsafe
ALTER TABLE ... SET ACCESS METHOD rewrites the entire table and rebuilds its indexes under an ACCESS EXCLUSIVE lock when the access method changes, blocking all reads and writes for the rewrite.
Safe rewrite
There is no online way to change a table's access method. Do it in a maintenance window, or create a new table with the target access method, copy the data in batches, then swap (expand/contract).
Example
Try it in the playground →Unsafe
ALTER TABLE events SET ACCESS METHOD heap2;