Temporal Tables

Temporal Tables represent a concept of a (parameterized) view on a changing history table that returns the content of a table at a specific point in time.

Flink can keep track of the changes applied to an underlying append-only table and allows for accessing the table’s content at a certain point in time within a query.

Motivation

Let’s assume that we have the following table RatesHistory.

SELECT * FROM RatesHistory;

rowtime currency   rate
======= ======== ======
09:00   US Dollar   102
09:00   Euro        114
09:00   Yen           1
10:45   Euro        116
11:15   Euro        119
11:49   Pounds      108

RatesHistory represents an ever growing append-only table of currency exchange rates with respect to Yen (which has a rate of 1). For example, the exchange rate for the period from 09:00 to 10:45 of Euro to Yen was 114. From 10:45 to 11:15 it was 116.

Given that we would like to output all current rates at the time 10:58, we would need the following SQL query to compute a result table:

SELECT *
FROM RatesHistory AS r
WHERE r.rowtime = (
  SELECT MAX(rowtime)
  FROM RatesHistory AS r2
  WHERE r2.currency = r.currency
  AND r2.rowtime <= TIME '10:58');

The correlated subquery determines the maximum time for the corresponding currency that is lower or equal than the desired time. The outer query lists the rates that have a maximum timestamp.

The following table shows the result of such a computation. In our example, the update to Euro at 10:45 is taken into account, however, the update to Euro at 11:15 and the new entry of Pounds are not considered in the table’s version at time 10:58.

rowtime currency   rate
======= ======== ======
09:00   US Dollar   102
09:00   Yen           1
10:45   Euro        116

The concept of Temporal Tables aims to simplify such queries, speed up their execution, and reduce Flink’s state usage. A Temporal Table is a parameterized view on an append-only table that interprets the rows of the append-only table as the changelog of a table and provides the version of that table at a specific point in time. Interpreting the append-only table as a changelog requires the specification of a primary key attribute and a timestamp attribute. The primary key determines which rows are overwritten and the timestamp determines the time during which a row is valid.

In the above example currency would be a primary key for RatesHistory table and rowtime would be the timestamp attribute.

In Flink, a temporal table is represented by a Temporal Table Function.

Temporal Table Functions

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.

Once defined, a Temporal Table Function takes a single time argument timeAttribute and returns a set of rows. This set contains the latest versions of the rows for all of the existing primary keys with respect to the given time attribute.

Assuming that we defined a temporal table function Rates(timeAttribute) based on RatesHistory table, we could query such a function in the following way:

SELECT * FROM Rates('10:15');

rowtime currency   rate
======= ======== ======
09:00   US Dollar   102
09:00   Euro        114
09:00   Yen           1

SELECT * FROM Rates('11:00');

rowtime currency   rate
======= ======== ======
09:00   US Dollar   102
10:45   Euro        116
09:00   Yen           1

Each query to Rates(timeAttribute) would return the state of the Rates for the given timeAttribute.

Note: Currently, Flink doesn’t support directly querying the temporal table functions with a constant time attribute parameter. At the moment, temporal table functions can only be used in joins. The example above was used to provide an intuition about what the function Rates(timeAttribute) returns.

See also the joining page for continuous queries for more information about how to join with a temporal table.

Defining Temporal Table Function

The following code snippet illustrates how to create a temporal table function from an append-only table.

import org.apache.flink.table.functions.TemporalTableFunction;
(...)

// Get the stream and table environments.
StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
StreamTableEnvironment tEnv = TableEnvironment.getTableEnvironment(env);

// Provide a static data set of the rates history table.
List<Tuple2<String, Long>> ratesHistoryData = new ArrayList<>();
ratesHistoryData.add(Tuple2.of("US Dollar", 102L));
ratesHistoryData.add(Tuple2.of("Euro", 114L));
ratesHistoryData.add(Tuple2.of("Yen", 1L));
ratesHistoryData.add(Tuple2.of("Euro", 116L));
ratesHistoryData.add(Tuple2.of("Euro", 119L));

// Create and register an example table using above data set.
// In the real setup, you should replace this with your own table.
DataStream<Tuple2<String, Long>> ratesHistoryStream = env.fromCollection(ratesHistoryData);
Table ratesHistory = tEnv.fromDataStream(ratesHistoryStream, "r_currency, r_rate, r_proctime.proctime");

tEnv.registerTable("RatesHistory", ratesHistory);

// Create and register a temporal table function.
// Define "r_proctime" as the time attribute and "r_currency" as the primary key.
TemporalTableFunction rates = ratesHistory.createTemporalTableFunction("r_proctime", "r_currency"); // <==== (1)
tEnv.registerFunction("Rates", rates);                                                              // <==== (2)
// Get the stream and table environments.
val env = StreamExecutionEnvironment.getExecutionEnvironment
val tEnv = TableEnvironment.getTableEnvironment(env)

// Provide a static data set of the rates history table.
val ratesHistoryData = new mutable.MutableList[(String, Long)]
ratesHistoryData.+=(("US Dollar", 102L))
ratesHistoryData.+=(("Euro", 114L))
ratesHistoryData.+=(("Yen", 1L))
ratesHistoryData.+=(("Euro", 116L))
ratesHistoryData.+=(("Euro", 119L))

// Create and register an example table using above data set.
// In the real setup, you should replace this with your own table.
val ratesHistory = env
  .fromCollection(ratesHistoryData)
  .toTable(tEnv, 'r_currency, 'r_rate, 'r_proctime.proctime)

tEnv.registerTable("Orders", orders)
tEnv.registerTable("RatesHistory", ratesHistory)

// Create and register TemporalTableFunction.
// Define "r_proctime" as the time attribute and "r_currency" as the primary key.
val rates = ratesHistory.createTemporalTableFunction('r_proctime, 'r_currency) // <==== (1)
tEnv.registerFunction("Rates", rates)                                          // <==== (2)

Line (1) creates a rates temporal table function, which allows us to use the function rates in the Table API.

Line (2) registers this function under the name Rates in our table environment, which allows us to use the Rates function in SQL.