Postgres 16

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with XML_text(col) as
(
select
'<?xml version="1.0" encoding="UTF-8"?>
<purchasePlan xmlns:ns2="http://zakupki.gov.ru/223fz/purchasePlan/1"
              xmlns="http://zakupki.gov.ru/223fz/types/1"
              xmlns:ns10="http://zakupki.gov.ru/223fz/decisionSuspension/1"
              xmlns:ns11="http://zakupki.gov.ru/223fz/disagreementProtocol/1"
              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
              xsi:schemaLocation="https://zakupki.gov.ru/223/integration/schema/TFF-13.1 https://zakupki.gov.ru/223/integration/schema/TFF-13.1/purchasePlan.xsd">
    <body>
        <item>
            <guid>096c4bf6-d656-4441-9032-0b7c45423af1</guid>
        </item>
    </body>
</purchasePlan>'::xml
)
SELECT r.guid
  FROM XML_text as x,
       XMLTABLE(xmlnamespaces('http://zakupki.gov.ru/223fz/types/1' AS "ns1"),
                '/ns1:purchasePlan/ns1:body/ns1:item' 
                PASSING x.col
                COLUMNS guid varchar(50) path 'ns1:guid'
                ) as r
;

guid
096c4bf6-d656-4441-9032-0b7c45423af1

SELECT 1