create temp table t(x xml) on commit drop;
insert into t values('<?xml version="1.0" encoding="UTF-8"?>
<E-Document>
  <Document>
    <DocumentInfo>
      <DocumentNum>123</DocumentNum>
    </DocumentInfo>
    <DocumentItem>
      <ItemEntry>
        <SellerItemCode>9999999</SellerItemCode>
        <ItemReserve>
          <LotNum>(1)</LotNum>
          <ItemReserveUnit>
            <AmountActual>3.00</AmountActual>
          </ItemReserveUnit>
        </ItemReserve>
      </ItemEntry>
    </DocumentItem>
  </Document>
</E-Document>
'::xml);

SELECT
        unnest(xpath('/E-Document/Document/DocumentInfo/DocumentNum/text()', x))::text AS tasudok,
        unnest( xpath(
 '/E-Document/Document/DocumentParties/BuyerParty/ContactData/ContactFirstName/text()', x))::text AS ContactFirstName,
        unnest(xpath('/E-Document/Document/DocumentItem/ItemEntry/SellerItemCode/text()', x))::text AS toode
        FROM t