1. Restrictions of my prototype
(1) aggregate functions are avg, sum, count, min, max
(2) argment or sum of avg is bigint var

2. My experiment settings
(1) hardware settings
  cpu:Intel Corei5-9500 processor(3.0 GHz/9MB cache)(6 cores)
  ram:DDR4 3200Mhz 128GB
  storage:512GB SSD(M.2 NVMe)
(2) software
  os: CentOS7
  postgresql source code:14(in development) with commit-id 947456a823d6b0973b68c6b38c8623a0504054e7
  # build by "make world"
  patchs in [1] I applied:
    v17-0001-Introduce-RelInfoList-structure.patch
    v17-0002-Aggregate-push-down-basic-functionality.patch
    v17-0003-Use-also-partial-paths-as-the-input-for-grouped-path.patch
  [1] https://commitfest.postgresql.org/32/
(3) PostgreSQL settings
  max_parallel_workers_per_gather=6
  work_mem=1000000
(4) tables and data
  execute the following commands using postgresql's client tool such as psql
--
create database tmp;
\c tmp;
create table log as select 1::bigint as id, 1::bigint as size, 1::bigint as total from generate_series(1,1000000000) t;

create table master as select t as id, 'hoge' || mod(t, 1000) as name from generate_series(1,1000000) t;
create index master_idx on master(id);

create extension postgres_fdw;
create server local_server foreign data wrapper postgres_fdw OPTIONS (host 'localhost', port '5432', dbname 'tmp');
create user mapping for user server local_server options (user 'postgres', password 'postgres');
create foreign table f_log(id bigint, size bigint, total bigint) server local_server options (table_name 'log');

analyze;

set enable_agg_pushdown=true;
explain (verbose, analyze) select b.name, avg(a.total) from f_log a join master b on a.id = b.id group by b.name;

set enable_agg_pushdown=false;
explain (verbose, analyze) select b.name, avg(a.total) from f_log a join master b on a.id = b.id group by b.name;
--