Uma rápida pesquisa na internet trouxe esses documentos, acho o primeiro muito interessante:

• http://raghavt.blogspot.com/2011/ 04/postgresql-90-architecture.html
• http://cisc322.files.wordpress.com/2010/ 10/conceptual_architecture_of_postgresql.pdf
• http://vibhork.blogspot.com/ 2011/04/postgresqlpostgres-plus-advanced-server.html
• http://www.interdb.jp/pg/pgsql02.html
• https://www.slideshare.net/EnterpriseDB/overviewutilityprocesses-finalaug222013