Clear weight vector after search
Support index section by index in match_columns

It supports:

    select Documents \
      --match_columns "Lexicon.index[0] * 10 || Lexicon.index[1]" \
      --query "QUERY" \
      --output_columns "content, _score"
    # -> _score = (# of matches against section 1) * 10 + (# of matches against section 2)