Postgres: Support XML functions (XMLELEMENT, XMLPI, XMLROOT, XMLSERIALIZE, XMLEXISTS)

[BenSatori]

Adds support for PostgreSQL XML function syntaxes:

• XMLELEMENT(NAME name [, XMLATTRIBUTES(...) ] [, content [, ...]])
• XMLPI(NAME name [, content ])
• XMLROOT(xml, VERSION {text | NO VALUE} [, STANDALONE {YES | NO | NO VALUE} ])
• XMLSERIALIZE({ DOCUMENT | CONTENT } value AS type [ [ NO ] INDENT ])
• XMLEXISTS(text PASSING [BY {REF|VALUE}] xml [BY {REF|VALUE}])

Example

SELECT XMLELEMENT(NAME foo, 'bar'), * FROM customers;
SELECT XMLELEMENT(NAME foo, XMLATTRIBUTES('v' AS attr), 'bar');
SELECT XMLROOT('<a/>'::xml, VERSION '1.0');
SELECT XMLSERIALIZE(DOCUMENT '<a/>'::xml AS TEXT);
SELECT XMLEXISTS('/a' PASSING BY REF '<a/>');

Docs:

• https://www.postgresql.org/docs/current/functions-xml.html
• https://www.postgresql.org/docs/current/datatype-xml.html

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