approx_top_k

Description

Returns the top k most frequently occurring item values in an expr along with their approximate counts.

This function is supported from v3.0.

Syntax

APPROX_TOP_K(<expr> [ , <k> [ , <counter_num> ] ] )

Arguments

• expr: An expression of STRING, BOOLEAN, DATE, DATETIME, or numeric type.

• k: An optional INTEGER literal greater than 0. If k is not specified, it defaults to 5. The maximum value is 100000.

• counter_num: An optional INTEGER literal greater than or equal to k, The larger the counter_num is, the more accurate the result will be. However, this also comes with increased CPU and memory costs.

  • The maximum value is 100000.
  • If counter_num is not specified, it defaults to max(min(2 * k, 100), 100000).

Returns

Results are returned as an ARRAY of type STRUCT, where each STRUCT contains an item field for the value (with its original input type) and a count field (of type BIGINT) with the approximate number of occurrences. The array is sorted by count descending.

The aggregate function returns the top k most frequently occurring item values in an expression expr along with their approximate counts. The error in each count may be up to 2.0 * numRows / counter_num where numRows is the total number of rows. Higher values of counter_num provide better accuracy at the cost of increased memory usage. Expressions that have fewer than counter_num distinct items will yield exact item counts. Results include NULL values as their own item in the results.

Examples

Use data in the scores table as an example.

-- Calculate the score distribution of each subject.
MySQL > SELECT subject, APPROX_TOP_K(score)  AS top_k FROM scores GROUP BY subject;
+---------+--------------------------------------------------------------------------------------------------------------------+
| subject | top_k                                                                                                              |
+---------+--------------------------------------------------------------------------------------------------------------------+
| physics | [{"item":99,"count":2},{"item":null,"count":1},{"item":100,"count":1},{"item":85,"count":1},{"item":60,"count":1}] |
| english | [{"item":null,"count":1},{"item":92,"count":1},{"item":98,"count":1},{"item":100,"count":1},{"item":85,"count":1}] |
| NULL    | [{"item":90,"count":1}]                                                                                            |
| math    | [{"item":80,"count":2},{"item":null,"count":1},{"item":92,"count":1},{"item":95,"count":1},{"item":70,"count":1}]  |
+---------+--------------------------------------------------------------------------------------------------------------------+

-- Calculate the score distribution of the math subject.
MySQL > SELECT subject, APPROX_TOP_K(score)  AS top_k FROM scores WHERE subject IN  ('math') GROUP BY subject;
+---------+-------------------------------------------------------------------------------------------------------------------+
| subject | top_k                                                                                                             |
+---------+-------------------------------------------------------------------------------------------------------------------+
| math    | [{"item":80,"count":2},{"item":null,"count":1},{"item":95,"count":1},{"item":92,"count":1},{"item":70,"count":1}] |
+---------+-------------------------------------------------------------------------------------------------------------------+

keyword

APPROX_TOP_K