https://pgecons-sec-tech.github.io/tech-report/presentation/PGECons_20181018_parallel.pdf
https://www.postgresql.jp/document/11/html/runtime-config-resource.html#GUC-MAX-PARALLEL-WORKERS
https://www.depesz.com/2018/02/12/waiting-for-postgresql-11-support-parallel-btree-index-builds/
https://www.cybertec-postgresql.com/en/postgresql-parallel-create-index-for-better-performance/