Ko'pchilik DBMSlar client-server (server bu yerda database) communication uchun alohida protocollar ishlab chiqqan.
Ya'ni networking uchun ularning alohida protocoli bor.
E'tiborli jihati, ko'p DBMSlarda bu communication davomida va ayniqsa connection ochish vaqtida juda ko'p metadata almashiladi (o'zim PostgreSQL va MongoDBda kuzatdim).

Bu databaselar bilan ishlayotganda connection ochish ancha "qimmat"ga tushadi degani. Bitta connectionni uzoqroq ishlating (albatta, butunligiga ishonch hosil qilib) yoki connection pool ishlating.

P.S. Har bitta query uchun alohida connection ochadigan tanishlarga yuboramiz ))
P.S.2. PHPchilarga yuboramiz ))