parallel index creation: maintenance_work_mem not honored?

Hi all,

I noticed that after an upgrade from 9.6 to 14.x, Postgres started producing temp files.

I narrowed down the behavior to parallel operations.

I was able to reproduce the following on a test machine running latest Postgres 14.4 on Ubuntu 18.04

CREATE TABLE test_parallel (i int);
INSERT INTO test_parallel VALUES (generate_series(0,10000000));

table size: 346 MB
maintenance_work_mem = '5GB' (but the same would happen if we set much higher values)

CREATE INDEX test_parallel_i_idx ON test_parallel (i);

... LOG:  temporary file: path "base/pgsql_tmp/pgsql_tmp4611.1.sharedfileset/2.0", size 37879808
... STATEMENT:  CREATE INDEX test_parallel_i_idx ON test_parallel (i);
... LOG:  temporary file: path "base/pgsql_tmp/pgsql_tmp4611.1.sharedfileset/0.0", size 59326464
... STATEMENT:  CREATE INDEX test_parallel_i_idx ON test_parallel (i);
... LOG:  temporary file: path "base/pgsql_tmp/pgsql_tmp4611.1.sharedfileset/1.0", size 103194624
... STATEMENT:  CREATE INDEX test_parallel_i_idx ON test_parallel (i);

After disabling parallelism with:

ALTER TABLE test_parallel SET (parallel_workers = 0);

The same 'CREATE INDEX' statement does not produce temp files.

Is this behavior expected? I cannot find mentions on the documentation nor on the literature available to me.

regards,

fabio pardi