presto-main/src/main/java/io/prestosql/sql/planner/LookupSymbolResolver.java/LookupSymbolResolver/getValue
class LookupSymbolResolver:    @Override
    public Object getValue(Symbol symbol)
    {
        ColumnHandle column = assignments.get(symbol);
        checkArgument(column != null, "Missing column assignment for %s", symbol);

        if (!bindings.containsKey(column)) {
            return symbol.toSymbolReference();
        }

        return bindings.get(column).getValue();
    }