How to Build an XML Document Incrementally in PL/pgSQL - sql

Question

What's the best way to incrementally build an XML document/string using PL/pgSQL? Consider the following desired XML output:
<Directory>
<Person>
<Name>Bob</Name>
<Address>1234 Main St</Address>
<MagicalAddressFactor1>3</MagicalAddressFactor1>
<MagicalAddressFactor2>8</MagicalAddressFactor2>
<MagicalAddressFactor3>1</MagicalAddressFactor3>
<IsMagicalAddress>Y</IsMagicalAddress>
</Person>
<Person>
<Name>Joshua</Name>
<Address>100 Broadway Blvd</Address>
<MagicalAddressFactor1>2</MagicalAddressFactor1>
<MagicalAddressFactor2>1</MagicalAddressFactor2>
<MagicalAddressFactor3>4</MagicalAddressFactor3>
<IsMagicalAddress>Y</IsMagicalAddress>
</Person>
</Directory>
Where:
Person name and address is based on a simple person table.
MagicalAddressFactor 1, 2, and 3 are all based on some complex links and calculations to other tables from the Person table.
IsMagicalAddress is based on the sum of the three MagicalAddressFactors being greater than 10.
How could I generate this with PL/pgSQL using XML functions to ensure a well-formed XML element? Without using XML functions the code would look like this:
DECLARE
v_sql text;
v_rec RECORD;
v_XML xml;
v_factor1 integer;
v_factor2 integer;
v_factor3 integer;
v_IsMagical varchar;
BEGIN
v_XML := '<Directory>';
v_sql := 'select * from person;'
FOR v_rec IN v_sql LOOP
v_XML := v_XML || '<Name>' || v_rec.name || '</Name>' ||
'<Address>' || v_rec.Address || '</Address>';
v_factor1 := get_factor_1(v_rec);
v_factor2 := get_factor_2(v_rec);
v_factor3 := get_factor_3(v_rec);
v_IsMagical := case
when (v_factor1 + v_factor2 + v_factor3) > 10 then
'Y'
else
'N'
end;
v_XML := v_XML || '<MagicalAddressFactor1>' || v_factor1 || '</MagicalAddressFactor1>' ||
'<MagicalAddressFactor2>' || v_factor2 || '</MagicalAddressFactor2>' ||
'<MagicalAddressFactor3>' || v_factor3 || '</MagicalAddressFactor3>' ||
'<IsMagicalAddress>' || v_IsMagical || '</IsMagicalAddress>';
v_XML := v_XML || '</Person>'
END LOOP;
v_XML := v_XML || '</Directory>'
END;

Answer 1

For OP and future readers, consider a general purpose language whenever needed to migrate database content to XML documents. Simply connect via ODBC/OLEDB drivers, retrieve query, and output to XML document. Using OP's needs, calculations can be incorporated into one select query or a stored procedure that returns a resultset and have coding language import records for document building.
Below are open-source solutions including Java where each connect using corresponding PostgreSQL drivers (requiring installation). SQL queries assumes get_factor1(), get_factor2(), get_factor3() are inline database functions and Persons maintain a unique ID in first column.
Java (using the Postgre JDBC driver)
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerException;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;
import javax.xml.transform.OutputKeys;
import java.sql.* ;
import java.util.ArrayList;
import java.io.IOException;
import java.io.File;
import org.w3c.dom.Attr;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
public class SQLtoXML {
public static void main(String[] args) {
String currentDir = new File("").getAbsolutePath();
try {
String url = "jdbc:postgresql://localhost/test";
Properties props = new Properties();
props.setProperty("user","sqluser");
props.setProperty("password","secret");
props.setProperty("ssl","true");
Connection conn = DriverManager.getConnection(url, props);
String url = "jdbc:postgresql://localhost/test?user=sqlduser&password=secret&ssl=true";
Connection conn = DriverManager.getConnection(url);
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("SELECT name, address, "
+ "get_factor_1(v_rec) As v_factor1, "
+ "get_factor_2(v_rec) As v_factor2, "
+ "get_factor_3(v_rec) As v_factor3, "
+ " CASE WHEN (get_factor_1(v_rec) + "
+ " get_factor_2(v_rec) + "
+ " get_factor_3(v_rec)) > 10 "
+ " THEN 'Y' ELSE 'N' END As v_isMagical "
+ " FROM Persons;");
// Write to XML document
DocumentBuilderFactory docFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder docBuilder = docFactory.newDocumentBuilder();
Document doc = docBuilder.newDocument();
// Root element
Element rootElement = doc.createElement("Directory");
doc.appendChild(rootElement);
// Export table data
ResultSetMetaData rsmd = rs.getMetaData();
int columnsNumber = rsmd.getColumnCount();
while (rs.next()) {
// Data rows
Element personNode = doc.createElement("Person");
rootElement.appendChild(personNode);
Element nameNode = doc.createElement("name");
nameNode.appendChild(doc.createTextNode(rs.getString(2)));
personNode.appendChild(nameNode);
Element addressNode = doc.createElement("address");
addressNode.appendChild(doc.createTextNode(rs.getString(3)));
personNode.appendChild(addressNode);
Element magicaladd1Node = doc.createElement("MagicalAddressFactor1");
magicaladd1Node.appendChild(doc.createTextNode(rs.getString(4)));
personNode.appendChild(magicaladd1Node);
Element magicaladd2Node = doc.createElement("MagicalAddressFactor2");
magicaladd2Node.appendChild(doc.createTextNode(rs.getString(5)));
personNode.appendChild(magicaladd2Node);
Element magicaladd3Node = doc.createElement("MagicalAddressFactor3");
magicaladd3Node.appendChild(doc.createTextNode(rs.getString(6)));
personNode.appendChild(magicaladd3Node);
Element isMagicalNode = doc.createElement("IsMagicalAddress");
isMagicalNode.appendChild(doc.createTextNode(rs.getString(7)));
personNode.appendChild(isMagicalNode);
}
rs.close();
stmt.close();
conn.close();
// Output content to xml file
TransformerFactory transformerFactory = TransformerFactory.newInstance();
Transformer transformer = transformerFactory.newTransformer();
transformer.setOutputProperty(OutputKeys.INDENT, "yes");
transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "2");
DOMSource source = new DOMSource(doc);
StreamResult result = new StreamResult(new File(currentDir + "\\PostgreXML_java.xml"));
transformer.transform(source, result);
System.out.println("Successfully created xml file!");
} catch (ParserConfigurationException pce) {
System.out.println(pce.getMessage());
} catch (TransformerException tfe) {
System.out.println(tfe.getMessage());
} catch (SQLException err) {
System.out.println(err.getMessage());
}
}
}
Python (Using the Psycopg module)
import psycopg2
import os
import lxml.etree as ET
cd = os.path.dirname(os.path.abspath(__file__))
# DB CONNECTION AND QUERY
db = psycopg2.connect("dbname=test user=postgres")
cur = db.cursor()
cur.execute("SELECT name, address, \
get_factor_1(v_rec) As v_factor1, \
get_factor_2(v_rec) As v_factor2, \
get_factor_3(v_rec) As v_factor3, \
CASE WHEN (get_factor_1(v_rec) + \
get_factor_2(v_rec) + \
get_factor_3(v_rec)) > 10 \
THEN 'Y' ELSE 'N' END As v_isMagical \
FROM Persons;")
# WRITING XML FILE
root = ET.Element('Directory')
for row in cur.fetchall():
personNode = ET.SubElement(root, "Person")
ET.SubElement(personNode, "Name").text = row[1]
ET.SubElement(personNode, "Address").text = row[2]
ET.SubElement(personNode, "MagicalAddressFactor1").text = row[3]
ET.SubElement(personNode, "MagicalAddressFactor2").text = row[4]
ET.SubElement(personNode, "MagicalAddressFactor3").text = row[5]
ET.SubElement(personNode, "IsMagicalAddress").text = row[6]
# CLOSE CURSOR AND DATABASE
cur.close()
db.close()
# OUTPUT XML
tree_out = (ET.tostring(root, pretty_print=True, xml_declaration=True, encoding="UTF-8"))
xmlfile = open(os.path.join(cd, 'PostgreXML_py.xml'),'wb')
xmlfile.write(tree_out)
xmlfile.close()
print("Successfully migrated SQL to XML data!")
PHP (using Postgre PDO Driver)
<?php
$cd = dirname(__FILE__);
// create a dom document with encoding utf8
$domtree = new DOMDocument('1.0', 'UTF-8');
$domtree->formatOutput = true;
$domtree->preserveWhiteSpace = false;
# Opening db connection
$host="root";
$dbuser = "*****";
try {
$dbh = new PDO("pgsql:dbname=$dbname;host=$host", $dbuser, $dbpass);
$dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$sql = "SELECT name, address,
get_factor_1(v_rec) As v_factor1,
get_factor_2(v_rec) As v_factor2,
get_factor_3(v_rec) As v_factor3,
CASE WHEN (get_factor_1(v_rec) +
get_factor_2(v_rec) +
get_factor_3(v_rec)) > 10
THEN 'Y' ELSE 'N' END As v_isMagical
FROM Persons;";
$STH = $dbh->query($sql);
$STH->setFetchMode(PDO::FETCH_ASSOC);
}
catch(PDOException $e) {
echo $e->getMessage();
exit;
}
/* create the root element of the xml tree */
$xmlRoot = $domtree->createElement("Directory");
$xmlRoot = $domtree->appendChild($xmlRoot);
/* loop query results through child elements */
while($row = $STH->fetch()) {
$personNode = $xmlRoot->appendChild($domtree->createElement('Person'));
$nameNode = $personNode->appendChild($domtree->createElement('Name', $row['name']));
$addNode = $personNode->appendChild($domtree->createElement('Address', $row['address']));
$magadd1Node = $personNode->appendChild($domtree->createElement('MagicalAddressFactor1', $row['v_factor1']));
$magadd2Node = $personNode->appendChild($domtree->createElement('MagicalAddressFactor2', $row['v_factor2']));
$magadd3Node = $personNode->appendChild($domtree->createElement('MagicalAddressFactor3', $row['v_factor3']));
$ismagicalNode = $personNode->appendChild($domtree->createElement('IsMagicalAddress', $row['v_isMagical']));
}
file_put_contents($cd. "/PostgreXML_php.xml", $domtree->saveXML());
echo "\nSuccessfully migrated SQL data into XML!\n";
# Closing db connection
$dbh = null;
exit;
?>
R (using the RPostgreSQL package)
library(RPostgreSQL)
library(XML)
#setwd("C:/path/to/working/folder")
# OPEN DATABASE AND QUERY
drv <- dbDriver("PostgreSQL")
conn <- dbConnect(drv, dbname="tempdb")
df <- sqlQuery(conn, "SELECT name, address,
get_factor_1(v_rec) As v_factor1,
get_factor_2(v_rec) As v_factor2,
get_factor_3(v_rec) As v_factor3,
CASE WHEN (get_factor_1(v_rec) +
get_factor_2(v_rec) +
get_factor_3(v_rec)) > 10
THEN 'Y' ELSE 'N' END As v_isMagical
FROM Persons;")
close(conn)
# CREATE XML FILE
doc = newXMLDoc()
root = newXMLNode("Directory", doc = doc)
# WRITE XML NODES AND DATA
for (i in 1:nrow(df)){
personNode = newXMLNode("Person", parent = root)
nameNode = newXMLNode("name", df$name[i], parent = personNode)
addressNode = newXMLNode("address", df$address[i], parent = personNode)
magicaladdress1Node = newXMLNode("MagicalAddressFactor1", df$v_factor1[i], parent = personNode)
magicaladdress2Node = newXMLNode("MagicalAddressFactor2", df$v_factor2[i], parent = personNode)
magicaladdress3Node = newXMLNode("MagicalAddressFactor3", df$v_factor3[i], parent = personNode)
ismagicalNode = newXMLNode("IsMagicalAddress", df$v_isMagical[i], parent = personNode)
}
# OUTPUT XML CONTENT TO FILE
saveXML(doc, file="PostgreXML_R.xml")
print("Successfully migrated SQL to XML data!")

Answer 2

Your code has three issues:
FOR IN variable LOOP doesn't work - if you really needs dynamic SQL then you have to use form FOR IN EXECUTE variable, but better is directly write the SQL query,
But, it cannot by fast, if persons are more than few
iteration over expensive cycle body is slow,
string concatenation is expensive
The output XML can be wrong, because you are missing escaping.
Last two points are solved pretty well by SQL/XML functions - I'll write only simple example - but it is really pretty strong ANSI/SQL feature (supported by Postgres).
SELECT xmlelement(NAME "Directory",
xmlagg(xmlelement(NAME "Person",
xmlforest(name AS "Name",
address AS "Address"))))
FROM persons;