What run-time configuration parameters are used for concurrent index creation Suppose I were to run the following. The first line shows the server's default maintenance_work_mem value.

mhildreth=# show maintenance_work_mem;
 maintenance_work_mem
----------------------
 16MB
(1 row)

mhildreth=# set maintenance_work_mem = '256MB';
SET

mhildreth=# CREATE INDEX CONCURRENTLY ON my_table(columns);

mhildreth=# set maintenance_work_mem = DEFAULT;
SET


What would be used for maintenance_work_mem of the index creation?
In your example, the maintenance_work_mem you have set of 256MB should be seen by the subsequent CREATE INDEX CONCURRENTLY command, because you have changed this GUC inside your session.

In fact, the docs suggest bumping up maintenance_work_mem (as you showed) for just such a purpose.