PostgreSQL如何判断字段是否存在

select count(*) from information_schema.columns WHERE table_schema = 'table_schema'

     and table_name = 'table_name' and column_name = 'column_name';

select * from information_schema.columns WHERE table_schema = 'table_schema'

    and table_name = 'table_name' and column_name = 'column_name';

select count (*) from information_schema.tables WHERE table_schema = 'table_schema'

    and table_name = 'table_name';

select * from information_schema.tables WHERE table_schema = 'table_schema'

    and table_name = 'table_name';