Temporal Table Function

Temporal Table Function #

A Temporal table function provides access to the version of a temporal table at a specific point in time. In order to access the data in a temporal table, one must pass a time attribute that determines the version of the table that will be returned. Flink uses the SQL syntax of table functions to provide a way to express it.

Unlike a versioned table, temporal table functions can only be defined on top of append-only streams — it does not support changelog inputs. Additionally, a temporal table function cannot be defined in pure SQL DDL.

Defining a Temporal Table Function #

Temporal table functions can be defined on top of append-only streams using the Table API. The table is registered with one or more key columns, and a time attribute used for versioning.

Suppose we have an append-only table of currency rates that we would like to register as a temporal table function.

SELECT * FROM currency_rates;

update_time   currency   rate
============= =========  ====
09:00:00      Yen        102
09:00:00      Euro       114
09:00:00      USD        1
11:15:00      Euro       119
11:49:00      Pounds     108

Using the Table API, we can register this stream using currency for the key and update_time as the versioning time attribute.

TemporalTableFunction rates = tEnv
    .from("currency_rates").
    .createTemporalTableFunction("update_time", "currency");
 
tEnv.registerFunction("rates", rates);                                                        
rates = tEnv
    .from("currency_rates").
    .createTemporalTableFunction("update_time", "currency")
 
tEnv.registerFunction("rates", rates)

Temporal Table Function Join #

Once defined, a temporal table function is used as a standard table function. Append-only tables (left input/probe side) can join with a temporal table (right input/build side), i.e., a table that changes over time and tracks its changes, to retrieve the value for a key as it was at a particular point in time.

Consider an append-only table orders that tracks customers' orders in different currencies.

SELECT * FROM orders;

order_time amount currency
========== ====== =========
10:15        2    Euro
10:30        1    USD
10:32       50    Yen
10:52        3    Euro
11:04        5    USD

Given these tables, we would like to convert orders to a common currency — USD.

SELECT
  SUM(amount * rate) AS amount
FROM
  orders,
  LATERAL TABLE (rates(order_time))
WHERE
  rates.currency = orders.currency
Table result = orders
    .joinLateral($("rates(order_time)"), $("orders.currency = rates.currency"))
    .select($("(o_amount * r_rate).sum as amount"));
val result = orders
    .joinLateral($"rates(order_time)", $"orders.currency = rates.currency")
    .select($"(o_amount * r_rate).sum as amount"))

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