Table Functions

ksqlDB Table Functions

A table function is a function that returns a set of zero or more rows. Contrast this to a scalar function, which returns a single value.

Table functions are analogous to the FlatMap operation commonly found in functional programming or stream processing frameworks such as Kafka Streams.

Table functions are used in the SELECT clause of a query. They cause the query to output potentially more than one row for each input value.

The current implementation of table functions only allows a single column to be returned. This column can be any valid SQL type.

Here's an example of the EXPLODE built-in table function, which takes an ARRAY and outputs one value for each element of the array:

  {sensor_id:12345 readings: [23, 56, 3, 76, 75]}
  {sensor_id:54321 readings: [12, 65, 38]}

The following stream:

  CREATE STREAM exploded_stream AS
    SELECT sensor_id, EXPLODE(readings) AS reading FROM batched_readings;

Would emit:

  {sensor_id:12345 reading: 23}
  {sensor_id:12345 reading: 56}
  {sensor_id:12345 reading: 3}
  {sensor_id:12345 reading: 76}
  {sensor_id:12345 reading: 75}
  {sensor_id:54321 reading: 12}
  {sensor_id:54321 reading: 65}
  {sensor_id:54321 reading: 38}

When scalar values are mixed with table function return values in a SELECT clause, the scalar values, like sensor_id in the previous example, are copied for each value returned from the table function.

You can also use multiple table functions in a SELECT clause. In this situation, the results of the table functions are "zipped" together. The total number of rows returned is equal to the greatest number of values returned from any of the table functions. If some of the functions return fewer rows than others, the missing values are replaced with null.

Here's an example that illustrates using multiple table functions in a SELECT clause.

With the following input data:

  {country:'UK', customer_names: ['john', 'paul', 'george', 'ringo'], customer_ages: [23, 56, 3]}
  {country:'USA', customer_names: ['chad', 'chip', 'brad'], customer_ages: [56, 34, 76, 84, 56]}

And the following stream:

  CREATE STREAM country_customers AS
    SELECT country, EXPLODE(customer_names) AS name, EXPLODE(customer_ages) AS age FROM country_batches;

Would give:

  {country: 'UK', name: 'john', age: 23}
  {country: 'UK', name: 'paul', age: 56}
  {country: 'UK', name: 'george', age: 3}
  {country: 'UK', name: 'ringo', age: null}
  {country: 'USA', name: 'chad', age: 56}
  {country: 'USA', name: 'chip', age: 34}
  {country: 'USA', name: 'brad', age: 76}
  {country: 'USA', name: null, age: 84}
  {country: 'USA', name: null, age: 56}

CUBE

cube_explode(array[col1, ..., colN])

Array

Takes as argument an array of columns and outputs all possible combinations of them. It produces 2^d new rows where d is the number of columns given as parameter. Duplicate entries for columns with null value are skipped.

EXPLODE

EXPLODE(col1)

Array

This function takes an Array and outputs one value for each of the elements of the array. The output values have the same type as the array elements.


Last update: 2020-02-12