Fix the BY {REF,VALUE} clause of XMLEXISTS/XMLTABLE

This clause is used to indicate the passing mode of a XML document, but
we were doing it wrong: we accepted BY REF and ignored it, and rejected
BY VALUE as a syntax error.  The reality, however, is that documents are
always passed BY VALUE, so rejecting that clause was silly.  Change
things so that we accept BY VALUE.

BY REF continues to be accepted, and continues to be ignored.

Author: Chapman Flack
Reviewed-by: Pavel Stehule
Discussion: https://postgr.es/m/5C297BB7.9070509@anastigmatix.net

doc/src/sgml/func.sgml

@@ -10583,7 +10583,7 @@ SELECT xmlagg(x) FROM (SELECT * FROM test ORDER BY y DESC) AS tab;
     </indexterm>
 
 <synopsis>
-<function>XMLEXISTS</function>(<replaceable>text</replaceable> PASSING <optional>BY REF</optional> <replaceable>xml</replaceable> <optional>BY REF</optional>)
+<function>XMLEXISTS</function>(<replaceable>text</replaceable> PASSING <optional>BY { REF | VALUE }</optional> <replaceable>xml</replaceable> <optional>BY { REF | VALUE }</optional>)
 </synopsis>
 
     <para>
@@ -10596,7 +10596,7 @@ SELECT xmlagg(x) FROM (SELECT * FROM test ORDER BY y DESC) AS tab;
     <para>
      Example:
      <screen><![CDATA[
-SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY REF '<towns><town>Toronto</town><town>Ottawa</town></towns>');
+SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY VALUE '<towns><town>Toronto</town><town>Ottawa</town></towns>');
 
  xmlexists
 ------------
@@ -10606,14 +10606,25 @@ SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY REF '<towns><town>Tor
     </para>
 
     <para>
-     The <literal>BY REF</literal> clauses have no effect in
-     PostgreSQL, but are allowed for SQL conformance and compatibility
-     with other implementations.  Per SQL standard, the
-     first <literal>BY REF</literal> is required, the second is
-     optional.  Also note that the SQL standard specifies
-     the <function>xmlexists</function> construct to take an XQuery
-     expression as first argument, but PostgreSQL currently only
-     supports XPath, which is a subset of XQuery.
+     The <literal>BY REF</literal> or <literal>BY VALUE</literal> clauses
+     have no effect in <productname>PostgreSQL<productname>, but are allowed
+     for compatibility with other implementations.  Per the <acronym>SQL</acronym>
+     standard, the one that precedes any argument is required, and indicates
+     the default for arguments that follow, and one may follow any argument to
+     override the default.
+     <productname>PostgreSQL</productname> ignores <literal>BY REF</literal>
+     and passes by value always.
+    </para>
+
+    <para>
+     In the <acronym>SQL</acronym> standard, an <function>xmlexists</function>
+     construct evaluates an expression in the XQuery language, allows passing
+     values for named parameters in the expression as well as for the context
+     item, and does not require the passed values to be documents, or even of
+     XML type.
+     In <productname>PostgreSQL</productname>, this construct currently only
+     evaluates an XPath 1.0 expression, and allows passing only one value,
+     which must be an XML document, to be the context item.
     </para>
    </sect3>
 
@@ -10820,7 +10831,7 @@ SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com">test</m
 
 <synopsis>
 <function>xmltable</function>( <optional>XMLNAMESPACES(<replaceable>namespace uri</replaceable> AS <replaceable>namespace name</replaceable><optional>, ...</optional>), </optional>
-          <replaceable>row_expression</replaceable> PASSING <optional>BY REF</optional> <replaceable>document_expression</replaceable> <optional>BY REF</optional>
+          <replaceable>row_expression</replaceable> PASSING <optional>BY { REF | VALUE }</optional> <replaceable>document_expression</replaceable> <optional>BY { REF | VALUE }</optional>
           COLUMNS <replaceable>name</replaceable> { <replaceable>type</replaceable> <optional>PATH <replaceable>column_expression</replaceable></optional> <optional>DEFAULT <replaceable>default_expression</replaceable></optional> <optional>NOT NULL | NULL</optional>
                         | FOR ORDINALITY }
                    <optional>, ...</optional>
@@ -10850,11 +10861,11 @@ SELECT xpath_exists('/my:a/text()', '<my:a xmlns:my="http://example.com">test</m
     <para>
      <replaceable>document_expression</replaceable> provides the XML document to
      operate on.
-     The <literal>BY REF</literal> clauses have no effect in PostgreSQL,
-     but are allowed for SQL conformance and compatibility with other
-     implementations.
      The argument must be a well-formed XML document; fragments/forests
      are not accepted.
+     The <literal>BY REF</literal> or <literal>BY VALUE</literal> clauses are
+     accepted, as described for the <function>xmlexists</function> predicate,
+     but ignored; PostgreSQL currently passes XML by value always.
     </para>
 
     <para>

src/backend/catalog/sql_features.txt

@@ -635,8 +635,8 @@ X204	XMLQuery: initializing an XQuery variable			NO
 X205	XMLQuery: EMPTY ON EMPTY option			NO	
 X206	XMLQuery: NULL ON EMPTY option			NO	
 X211	XML 1.1 support			NO	
-X221	XML passing mechanism BY VALUE			NO	
-X222	XML passing mechanism BY REF			YES	
+X221	XML passing mechanism BY VALUE			YES	
+X222	XML passing mechanism BY REF			NO	parser accepts BY REF but ignores it; passing is always BY VALUE
 X231	XML(CONTENT(UNTYPED)) type			NO	
 X232	XML(CONTENT(ANY)) type			NO	
 X241	RETURNING CONTENT in XML publishing			NO	

src/backend/parser/gram.y

@@ -13951,20 +13951,25 @@ xmlexists_argument:
 				{
 					$$ = $2;
 				}
-			| PASSING c_expr BY REF
+			| PASSING c_expr xml_passing_mech
 				{
 					$$ = $2;
 				}
-			| PASSING BY REF c_expr
+			| PASSING xml_passing_mech c_expr
 				{
-					$$ = $4;
+					$$ = $3;
 				}
-			| PASSING BY REF c_expr BY REF
+			| PASSING xml_passing_mech c_expr xml_passing_mech
 				{
-					$$ = $4;
+					$$ = $3;
 				}
 		;
 
+xml_passing_mech:
+			BY REF
+			| BY VALUE_P
+		;
+
 
 /*
  * Aggregate decoration clauses