为什么PostgreSQL需要Execute脚本而不需要query_to_xml脚本？

Question

I am new to PostgreSQL. I have a doubt about PostgreSQL script .

In my last question i was trying to use "execute" . Then i come to know , If i want to use Execute in query i have to make script (use $$ LANGUAGE ... ).

But one answer from similar questions , use query_to_xml and it not need script . why ?

Answer 1

Unlike SQL Server, Postgres (and many other DBMS like Oracle, DB2, Firebird) makes a clear distinct between procedural code and SQL. Procedural code can only be run in the context of a function (or procedure). A do block is essentially an anonymous function that doesn't return anything.

Dynamic SQL can only be used in procedural code. query_to_xml does exactly that: it uses dynamic SQL.

To count the rows in a table you could also create a function that uses dynamic SQL:

create function count_rows(p_schemaname text, p_tablename text)
  returns bigint
as
$$ 
declare
  l_stmt text;
  l_count bigint;
begin
  l_stmt := format('select count(*) from %I.%I', p_schemaname, p_tablename);
  execute l_stmt
    into l_count;
  return l_count;
end;
$$
language plpgsql;

You can then use:

select schema_name, table_name, count_rows(schema_name, table_name)
from information_schema.tables
where schema_name = 'public';

query_to_xml does essentially the same thing as the function count_rows() - it's just a generic function to run any SQL statement and return that result as XML, rather than a specialized function that only does exactly one thing.