parallel index creation: maintenance_work_mem not honored?

[Fabio Pardi <f.pardi@xxxxxxxxxxxx>]

    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