CREATE TEMP TABLE test_names (
  id serial primary key,
  name character varying(50),
  age int
);

INSERT INTO test_names(name,age) values ('name1',10),('name2',20);

CREATE TEMP TABLE test_names_details (
  id serial primary key,
  test_names_id int,
  col1 int,
  col2 int,
  col3 int
);

<info>
  <maininfo>
    <pn name="name">name1</pn>
    <agen name="age">10</age>
  </maininfo>
  <data>
    <row>
      <col1 name="col1">2</col1>
      <col2 name="col2">3</col1>
      <col3 name="col3">4</col1>
    </row>
    <row>
      <col1 name="col1">5</col1>
      <col2 name="col2">6</col1>
      <col3 name="col3">7</col1>
    </row>
    <row>
      <col1 name="col1">8</col1>
      <col2 name="col2">9</col1>
      <col3 name="col3">10</col1>
    </row>
  </data>

  <maininfo>
    <pn name="name">name2</pn>
    <agen name="age">20</age>
  </maininfo>
  <data>
    <row>
      <col1 name="col1">20</col1>
      <col2 name="col2">21</col1>
      <col3 name="col3">22</col1>
    </row>
    <row>
      <col1 name="col1">23</col1>
      <col2 name="col2">24</col1>
      <col3 name="col3">25</col1>
    </row>
  </data>

</info>

SELECT
'<info>' || 
    string_agg(
    '<maininfo>' ||
        '<pn name="name">' || name || '</pn>' ||
        '<age name="age">' || age || '</age>' ||
    '</maininfo>' ||
    '<data>' ||
        (SELECT
            string_agg(
        '<row>' ||
            '<col name="col1">'||col1||'</col>' ||
            '<col name="col2">'||col2||'</col>' ||
            '<col name="col3">'||col3||'</col>' ||
        '</row>', '')
        FROM test_names_details WHERE test_names_id = test_names.id
        ) ||
    '</data>'
    , '') ||
'</info>'
FROM test_names

WITH dt(id, col) AS (
	SELECT id, UNNEST(ARRAY[col1, col2, col3])
	FROM test_names_details
),
data AS (
	SELECT
	dt.*,
	tn.*,
	ROW_NUMBER() OVER(PARTITION BY dt.id ORDER BY dt.id) AS n
	FROM dt INNER JOIN test_names tn USING(id)
),
xml AS (
	SELECT CASE n
		WHEN 1 THEN '<maininfo><pn name="name">'||name||'</pn><age name="age">'||age||'</age></maininfo><row>'||
			'<col name="col1">'||col||'</col>'
		WHEN 3 THEN '<col name="col3">'||col||'</col></row>'
		ELSE '<col name="col'||n||'">'||col||'</col>'
		END AS row
	FROM data
)
SELECT '<info>'||string_agg(row, '')||'</info>' FROM xml;