How do underivable rules affect parsing? - parsing

Question

When writing an XText grammar for a simple SQL dialect, I found out, that apparently rules that cannot be derived from the start symbol affect parsing.
E.g. given the following (very simplified) extract of my grammar which should be able to parse expressions like FROM table1;:
Start:
subquery ';';
subquery:
/*select=select_clause */tables=from_clause;
from_clause:
'FROM' tables;
tables:
tables+=table (',' tables+=table)*;
table:
name=table_name (alias=alias)?;
table_name:
prefix=qualified_name_prefix? name=qualified_name;
qualified_name_prefix:
ID'.';
qualified_name :
=>qualified_name_prefix? ID;
alias returns EString:
'AS'? alias=ID;
with_clause :
'WITH' elements+=with_list_element (',' elements+=with_list_element)*;
with_list_element :
name=ID (column_list_clause=column_list_clause)? 'AS' '(' subquery=subquery ')';
column_list_clause :
'(' names+=ID+ ')';
When trying to parse the string FROM table1;, I get the following error:
'no viable alternative at input ';'' on EString
If I remove rule with_clause, the error is gone and the string is parsed properly. How is this possible even though with_clause cannot be derived from Start?

Answer 1

the problem is that the predicate (=>) covers an ambiguity
maybe you can pull together prefix and name
Table_name:
name=Qualified_name;
Qualified_name :
(ID '.' (ID '.')?)? ID;
or you try something like
Table_name:
((prefix=ID ".")? =>name=Qualified_name);
Qualified_name :
=>(ID '.' ID) | ID;