**Use case**

I want to estimate better parameters for fulltext (`ngrambf_v1`) index.

**Describe the solution you'd like**

Implement it as SQL function:

`ngrams(string, N)`

that returns array of FixedString.

**Describe alternatives you've considered**

It can be done with less efficient and more cumbersome way:

```
WITH 3 AS n, extractAll(text, '.') AS chars 
SELECT 
  arrayMap(x -> arrayStringConcat(x), 
    arrayFilter(x -> length(x) = n, 
      arrayMap(x, i -> arraySlice(chars, i, n), 
        chars, arrayEnumerate(chars)))) AS ngrams
```

**Additional context**

`ngramsUTF8` can also be implemented to extract ngrams of Unicode code points.
else if (element.function == RPNElement::FUNCTION_AND)
        {
            auto arg1 = rpn_stack.back();
            rpn_stack.pop_back();
            auto arg2 = rpn_stack.back();
            rpn_stack.back() = arg1 & arg2;
        }
        else if (element.function == RPNElement::FUNCTION_OR)
        {
            auto arg1 = rpn_stack.back();
            rpn_stack.pop_back();
            auto arg2 = rpn_stack.back();
            rpn_stack.back() = arg1 | arg2;
        }
        else if (element.function == RPNElement::ALWAYS_FALSE)
        {
            rpn_stack.emplace_back(false, true);
        }
        else if (element.function == RPNElement::ALWAYS_TRUE)
        {
            rpn_stack.emplace_back(true, false);
        }
        else
            throw Exception("Unexpected function type in BloomFilterCondition::RPNElement", ErrorCodes::LOGICAL_ERROR);
    }
    if (rpn_stack.size() != 1)
        throw Exception("Unexpected stack size in BloomFilterCondition::mayBeTrueOnGranule", ErrorCodes::LOGICAL_ERROR);
    return rpn_stack[0].can_be_true;
}
bool MergeTreeConditionFullText::getKey(const std::string & key_column_name, size_t & key_column_num)