+ {
+ /* the last field of list can be star too */
+ Assert(IsA(field2, A_Star));
+
+ /*
+ * In this case, the field1 should be variable name. But
+ * direct unboxing of composite session variables is not
+ * supported now, and then we don't need to try lookup
+ * related variable.
+ *
+ * Unboxing is supported by syntax (var).*
+ */
+ return InvalidOid;
+ }
I don't fully understand the above comments,

The parser allows only two syntaxes - identifier.identifier or identifier.star. Second
syntax is not supported by session variables, and then I didn't try to search for the variable.
Some users can be confused by similar syntaxes identifier.* and (identifier).* Only
second syntax is composite unboxing, and only second syntax is supported for
session variables.

I changed this comment to

<--><--><--><--><-->/*
<--><--><--><--><--> * The syntax ident.* is used only by table aliases,
<--><--><--><--><--> * and then this identifier cannot be a reference to
<--><--><--><--><--> * session variable.
<--><--><--><--><--> */