PG 9.5开始支持的特性，根据约束，存在则更新，不存在则插入；
使用某字段约束：
INSERT INTO distributors (did, dname) VALUES (8, 'Anvil Distribution')
ON CONFLICT (did) DO UPDATE
SET dname = EXCLUDED.dname注意字段属性必须是UNIQUE，不然会报错：
there is no unique or exclusion constraint matching the ON CONFLICT specification
使用唯一键约束：
INSERT INTO distributors (did, dname) VALUES (9, 'Antwerp Design')
ON CONFLICT ON CONSTRAINT distributors_pkey DO NOTHING