如何在 Google BigQuery 中执行三元组运算？

Question

我确实使用 PostgreSQL 中的 pg_trgm 模块来使用三元组计算两个字符串之间的相似度。特别是我使用：

similarity(text, text)

其中 returns returns 一个数字，表示两个参数的相似程度（在 0 和 1 之间）。

如何在 Google BigQuery 上执行相似函数（或等效函数）？

Answer 1

在下面试试。至少作为增强的蓝图

SELECT text1, text2, similarity FROM 
JS(
// input table
(
  SELECT * FROM 
  (SELECT 'mikhail' AS text1, 'mikhail' AS text2),
  (SELECT 'mikhail' AS text1, 'mike' AS text2),
  (SELECT 'mikhail' AS text1, 'michael' AS text2),
  (SELECT 'mikhail' AS text1, 'javier' AS text2),
  (SELECT 'mikhail' AS text1, 'thomas' AS text2)
) ,
// input columns
text1, text2,
// output schema
"[{name: 'text1', type:'string'},
  {name: 'text2', type:'string'},
  {name: 'similarity', type:'float'}]
",
// function
"function(r, emit) {

  var _extend = function(dst) {
    var sources = Array.prototype.slice.call(arguments, 1);
    for (var i=0; i<sources.length; ++i) {
      var src = sources[i];
      for (var p in src) {
        if (src.hasOwnProperty(p)) dst[p] = src[p];
      }
    }
    return dst;
  };

  var Levenshtein = {
    /**
     * Calculate levenshtein distance of the two strings.
     *
     * @param str1 String the first string.
     * @param str2 String the second string.
     * @return Integer the levenshtein distance (0 and above).
     */
    get: function(str1, str2) {
      // base cases
      if (str1 === str2) return 0;
      if (str1.length === 0) return str2.length;
      if (str2.length === 0) return str1.length;

      // two rows
      var prevRow  = new Array(str2.length + 1),
          curCol, nextCol, i, j, tmp;

      // initialise previous row
      for (i=0; i<prevRow.length; ++i) {
        prevRow[i] = i;
      }

      // calculate current row distance from previous row
      for (i=0; i<str1.length; ++i) {
        nextCol = i + 1;

        for (j=0; j<str2.length; ++j) {
          curCol = nextCol;

          // substution
          nextCol = prevRow[j] + ( (str1.charAt(i) === str2.charAt(j)) ? 0 : 1 );
          // insertion
          tmp = curCol + 1;
          if (nextCol > tmp) {
            nextCol = tmp;
          }
          // deletion
          tmp = prevRow[j + 1] + 1;
          if (nextCol > tmp) {
            nextCol = tmp;
          }

          // copy current col value into previous (in preparation for next iteration)
          prevRow[j] = curCol;
        }

        // copy last col value into previous (in preparation for next iteration)
        prevRow[j] = nextCol;
      }

      return nextCol;
    }

  };

  var the_text1;

  try {
    the_text1 = decodeURI(r.text1).toLowerCase();
  } catch (ex) {
    the_text1 = r.text1.toLowerCase();
  }

  try {
    the_text2 = decodeURI(r.text2).toLowerCase();
  } catch (ex) {
    the_text2 = r.text2.toLowerCase();
  }

  emit({text1: the_text1, text2: the_text2,
        similarity: 1 - Levenshtein.get(the_text1, the_text2) / the_text1.length});

  }"
)
ORDER BY similarity DESC

这是基于@thomaspark

https://storage.googleapis.com/thomaspark-sandbox/udf-examples/pataky.js 的轻微修改

Answer 2

我did it喜欢这样：

CREATE TEMP FUNCTION trigram_similarity(a STRING, b STRING) AS (
  (
    WITH a_trigrams AS (
      SELECT
        DISTINCT tri_a
      FROM
        unnest(ML.NGRAMS(SPLIT(LOWER(a), ''), [3,3])) AS tri_a
    ),
    b_trigrams AS (
      SELECT
        DISTINCT tri_b
      FROM
        unnest(ML.NGRAMS(SPLIT(LOWER(b), ''), [3,3])) AS tri_b
    )
    SELECT
      COUNTIF(tri_b IS NOT NULL) / COUNT(*)
    FROM
      a_trigrams
      LEFT JOIN b_trigrams ON tri_a = tri_b
  )
);

这是与 Postgres's pg_trgm 的比较：

select trigram_similarity('saemus', 'seamus');
-- 0.25 vs. pg_trgm 0.272727

select trigram_similarity('shamus', 'seamus');
-- 0.5 vs. pg_trgm 0.4

如何在 Google BigQuery 中执行三元组运算？

How to perform trigram operations in Google BigQuery?

google-bigquery

similarity

trigram