Database Programming

TRIGGER

Instructions

The trigger provides a mechanism for listening to changes in time series data. With user-defined logic, tasks such as alerting and data forwarding can be conducted.

The trigger is implemented based on the reflection mechanism. Users can monitor data changes by implementing the Java interfaces. IoTDB allows users to dynamically register and drop triggers without restarting the server.

The document will help you learn to define and manage triggers.

Pattern for Listening

A single trigger can be used to listen for data changes in a time series that match a specific pattern. For example, a trigger can listen for the data changes of time series root.sg.a, or time series that match the pattern root.sg.*. When you register a trigger, you can specify the path pattern that the trigger listens on through an SQL statement.

Trigger Type

There are currently two types of triggers, and you can specify the type through an SQL statement when registering a trigger:

• Stateful triggers: The execution logic of this type of trigger may depend on data from multiple insertion statement . The framework will aggregate the data written by different nodes into the same trigger instance for calculation to retain context information. This type of trigger is usually used for sampling or statistical data aggregation for a period of time. information. Only one node in the cluster holds an instance of a stateful trigger.
• Stateless triggers: The execution logic of the trigger is only related to the current input data. The framework does not need to aggregate the data of different nodes into the same trigger instance. This type of trigger is usually used for calculation of single row data and abnormal detection. Each node in the cluster holds an instance of a stateless trigger.

Trigger Event

There are currently two trigger events for the trigger, and other trigger events will be expanded in the future. When you register a trigger, you can specify the trigger event through an SQL statement:

• BEFORE INSERT: Fires before the data is persisted. Please note that currently the trigger does not support data cleaning and will not change the data to be persisted itself.
• AFTER INSERT: Fires after the data is persisted.

How to Implement a Trigger

You need to implement the trigger by writing a Java class, where the dependency shown below is required. If you use Maven, you can search for them directly from the Maven repository.

Dependency

<dependency>
  <groupId>org.apache.iotdb</groupId>
  <artifactId>iotdb-server</artifactId>
  <version>1.0.0</version>
  <scope>provided</scope>
</dependency>

Note that the dependency version should be correspondent to the target server version.

Interface Description

To implement a trigger, you need to implement the org.apache.iotdb.trigger.api.Trigger class.

import org.apache.iotdb.trigger.api.enums.FailureStrategy;
import org.apache.iotdb.tsfile.write.record.Tablet;

public interface Trigger {

  /**
   * This method is mainly used to validate {@link TriggerAttributes} before calling {@link
   * Trigger#onCreate(TriggerAttributes)}.
   *
   * @param attributes TriggerAttributes
   * @throws Exception e
   */
  default void validate(TriggerAttributes attributes) throws Exception {}

  /**
   * This method will be called when creating a trigger after validation.
   *
   * @param attributes TriggerAttributes
   * @throws Exception e
   */
  default void onCreate(TriggerAttributes attributes) throws Exception {}

  /**
   * This method will be called when dropping a trigger.
   *
   * @throws Exception e
   */
  default void onDrop() throws Exception {}

  /**
   * When restarting a DataNode, Triggers that have been registered will be restored and this method
   * will be called during the process of restoring.
   *
   * @throws Exception e
   */
  default void restore() throws Exception {}

  /**
   * Overrides this method to set the expected FailureStrategy, {@link FailureStrategy#OPTIMISTIC}
   * is the default strategy.
   *
   * @return {@link FailureStrategy}
   */
  default FailureStrategy getFailureStrategy() {
    return FailureStrategy.OPTIMISTIC;
  }

  /**
   * @param tablet see {@link Tablet} for detailed information of data structure. Data that is
   *     inserted will be constructed as a Tablet and you can define process logic with {@link
   *     Tablet}.
   * @return true if successfully fired
   * @throws Exception e
   */
  default boolean fire(Tablet tablet) throws Exception {
    return true;
  }
}

This class provides two types of programming interfaces: Lifecycle related interfaces and data change listening related interfaces. All the interfaces in this class are not required to be implemented. When the interfaces are not implemented, the trigger will not respond to the data changes. You can implement only some of these interfaces according to your needs.

Descriptions of the interfaces are as followed.

Lifecycle Related Interfaces

Interface	Description
default void validate(TriggerAttributes attributes) throws Exception {}	When you creates a trigger using the CREATE TRIGGER statement, you can specify the parameters that the trigger needs to use, and this interface will be used to verify the correctness of the parameters。
default void onCreate(TriggerAttributes attributes) throws Exception {}	This interface is called once when you create a trigger using the CREATE TRIGGER statement. During the lifetime of each trigger instance, this interface will be called only once. This interface is mainly used for the following functions: helping users to parse custom attributes in SQL statements (using TriggerAttributes). You can create or apply for resources, such as establishing external links, opening files, etc.
default void onDrop() throws Exception {}	This interface is called when you drop a trigger using the DROP TRIGGER statement. During the lifetime of each trigger instance, this interface will be called only once. This interface mainly has the following functions: it can perform the operation of resource release and can be used to persist the results of trigger calculations.
default void restore() throws Exception {}	When the DataNode is restarted, the cluster will restore the trigger instance registered on the DataNode, and this interface will be called once for stateful trigger during the process. After the DataNode where the stateful trigger instance is located goes down, the cluster will restore the trigger instance on another available DataNode, calling this interface once in the process. This interface can be used to customize recovery logic.

Data Change Listening Related Interfaces

Listening Interface

/**
   * @param tablet see {@link Tablet} for detailed information of data structure. Data that is
   *     inserted will be constructed as a Tablet and you can define process logic with {@link
   *     Tablet}.
   * @return true if successfully fired
   * @throws Exception e
   */
  default boolean fire(Tablet tablet) throws Exception {
    return true;
  }

When the data changes, the trigger uses the Tablet as the unit of firing operation. You can obtain the metadata and data of the corresponding sequence through Tablet, and then perform the corresponding trigger operation. If the fire process is successful, the return value should be true. If the interface returns false or throws an exception, we consider the trigger fire process as failed. When the trigger fire process fails, we will perform corresponding operations according to the listening strategy interface.

When performing an INSERT operation, for each time series in it, we will detect whether there is a trigger that listens to the path pattern, and then assemble the time series data that matches the path pattern listened by the same trigger into a new Tablet for trigger fire interface. Can be understood as:

Map<PartialPath, List<Trigger>> pathToTriggerListMap => Map<Trigger, Tablet>

Note that currently we do not make any guarantees about the order in which triggers fire.

Here is an example:

Suppose there are three triggers, and the trigger event of the triggers are all BEFORE INSERT:

• Trigger1 listens on root.sg.*
• Trigger2 listens on root.sg.a
• Trigger3 listens on root.sg.b

Insertion statement:

insert into root.sg(time, a, b) values (1, 1, 1);

The time series root.sg.a matches Trigger1 and Trigger2, and the sequence root.sg.b matches Trigger1 and Trigger3, then:

• The data of root.sg.a and root.sg.b will be assembled into a new tablet1, and Trigger1.fire(tablet1) will be executed at the corresponding Trigger Event.
• The data of root.sg.a will be assembled into a new tablet2, and Trigger2.fire(tablet2) will be executed at the corresponding Trigger Event.
• The data of root.sg.b will be assembled into a new tablet3, and Trigger3.fire(tablet3) will be executed at the corresponding Trigger Event.

Listening Strategy Interface

When the trigger fails to fire, we will take corresponding actions according to the strategy set by the listening strategy interface. You can set org.apache.iotdb.trigger.api.enums.FailureStrategy. There are currently two strategies, optimistic and pessimistic:

• Optimistic strategy: The trigger that fails to fire does not affect the firing of subsequent triggers, nor does it affect the writing process, that is, we do not perform additional processing on the sequence involved in the trigger failure, only log the failure to record the failure, and finally inform user that data insertion is successful, but the trigger fire part failed.
• Pessimistic strategy: The failure trigger affects the processing of all subsequent Pipelines, that is, we believe that the firing failure of the trigger will cause all subsequent triggering processes to no longer be carried out. If the trigger event of the trigger is BEFORE INSERT, then the insertion will no longer be performed, and the insertion failure will be returned directly.

 /**
   * Overrides this method to set the expected FailureStrategy, {@link FailureStrategy#OPTIMISTIC}
   * is the default strategy.
   *
   * @return {@link FailureStrategy}
   */
  default FailureStrategy getFailureStrategy() {
    return FailureStrategy.OPTIMISTIC;
  }

Example

If you use Maven, you can refer to our sample project trigger-example.

You can find it here.

Here is the code from one of the sample projects:

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.iotdb.trigger;

import org.apache.iotdb.db.storageengine.trigger.sink.alertmanager.AlertManagerConfiguration;
import org.apache.iotdb.db.storageengine.trigger.sink.alertmanager.AlertManagerEvent;
import org.apache.iotdb.db.storageengine.trigger.sink.alertmanager.AlertManagerHandler;
import org.apache.iotdb.trigger.api.Trigger;
import org.apache.iotdb.trigger.api.TriggerAttributes;
import org.apache.iotdb.tsfile.file.metadata.enums.TSDataType;
import org.apache.iotdb.tsfile.write.record.Tablet;
import org.apache.iotdb.tsfile.write.schema.MeasurementSchema;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.util.HashMap;
import java.util.List;

public class ClusterAlertingExample implements Trigger {
  private static final Logger LOGGER = LoggerFactory.getLogger(ClusterAlertingExample.class);

  private final AlertManagerHandler alertManagerHandler = new AlertManagerHandler();

  private final AlertManagerConfiguration alertManagerConfiguration =
      new AlertManagerConfiguration("http://127.0.0.1:9093/api/v2/alerts");

  private String alertname;

  private final HashMap<String, String> labels = new HashMap<>();

  private final HashMap<String, String> annotations = new HashMap<>();

  @Override
  public void onCreate(TriggerAttributes attributes) throws Exception {
    alertname = "alert_test";

    labels.put("series", "root.ln.wf01.wt01.temperature");
    labels.put("value", "");
    labels.put("severity", "");

    annotations.put("summary", "high temperature");
    annotations.put("description", "{{.alertname}}: {{.series}} is {{.value}}");

    alertManagerHandler.open(alertManagerConfiguration);
  }

  @Override
  public void onDrop() throws IOException {
    alertManagerHandler.close();
  }

  @Override
  public boolean fire(Tablet tablet) throws Exception {
    List<MeasurementSchema> measurementSchemaList = tablet.getSchemas();
    for (int i = 0, n = measurementSchemaList.size(); i < n; i++) {
      if (measurementSchemaList.get(i).getType().equals(TSDataType.DOUBLE)) {
        // for example, we only deal with the columns of Double type
        double[] values = (double[]) tablet.values[i];
        for (double value : values) {
          if (value > 100.0) {
            LOGGER.info("trigger value > 100");
            labels.put("value", String.valueOf(value));
            labels.put("severity", "critical");
            AlertManagerEvent alertManagerEvent =
                new AlertManagerEvent(alertname, labels, annotations);
            alertManagerHandler.onEvent(alertManagerEvent);
          } else if (value > 50.0) {
            LOGGER.info("trigger value > 50");
            labels.put("value", String.valueOf(value));
            labels.put("severity", "warning");
            AlertManagerEvent alertManagerEvent =
                new AlertManagerEvent(alertname, labels, annotations);
            alertManagerHandler.onEvent(alertManagerEvent);
          }
        }
      }
    }
    return true;
  }
}

Trigger Management

You can create and drop a trigger through an SQL statement, and you can also query all registered triggers through an SQL statement.

We recommend that you stop insertion while creating triggers.

Create Trigger

Triggers can be registered on arbitrary path patterns. The time series registered with the trigger will be listened to by the trigger. When there is data change on the series, the corresponding fire method in the trigger will be called.

Registering a trigger can be done as follows:

1. Implement a Trigger class as described in the How to implement a Trigger chapter, assuming the class's full class name is org.apache.iotdb.trigger.ClusterAlertingExample
2. Package the project into a JAR package.
3. Register the trigger with an SQL statement. During the creation process, the validate and onCreate interfaces of the trigger will only be called once. For details, please refer to the chapter of How to implement a Trigger.

The complete SQL syntax is as follows:

// Create Trigger
createTrigger
    : CREATE triggerType TRIGGER triggerName=identifier triggerEventClause ON pathPattern AS className=STRING_LITERAL uriClause? triggerAttributeClause?
    ;

triggerType
    : STATELESS | STATEFUL
    ;

triggerEventClause
    : (BEFORE | AFTER) INSERT
    ;
        
uriClause
    : USING URI uri
    ;

uri
    : STRING_LITERAL
    ;
    
triggerAttributeClause
    : WITH LR_BRACKET triggerAttribute (COMMA triggerAttribute)* RR_BRACKET
    ;

triggerAttribute
    : key=attributeKey operator_eq value=attributeValue
    ;

Below is the explanation for the SQL syntax:

• triggerName: The trigger ID, which is globally unique and used to distinguish different triggers, is case-sensitive.
• triggerType: Trigger types are divided into two categories, STATELESS and STATEFUL.
• triggerEventClause: when the trigger fires, BEFORE INSERT and AFTER INSERT are supported now.
• pathPattern：The path pattern the trigger listens on, can contain wildcards * and **.
• className：The class name of the Trigger class.
• jarLocation: Optional. When this option is not specified, by default, we consider that the DBA has placed the JAR package required to create the trigger in the trigger_root_dir directory (configuration item, default is IOTDB_HOME/ext/trigger) of each DataNode node. When this option is specified, we will download and distribute the file resource corresponding to the URI to the trigger_root_dir/install directory of each DataNode.
• triggerAttributeClause: It is used to specify the parameters that need to be set when the trigger instance is created. This part is optional in the SQL syntax.

Here is an example SQL statement to help you understand:

CREATE STATELESS TRIGGER triggerTest
BEFORE INSERT
ON root.sg.**
AS 'org.apache.iotdb.trigger.ClusterAlertingExample'
USING URI '/jar/ClusterAlertingExample.jar'
WITH (
    "name" = "trigger",
    "limit" = "100"
)

The above SQL statement creates a trigger named triggerTest:

• The trigger is stateless.
• Fires before insertion.
• Listens on path pattern root.sg.**
• The implemented trigger class is named org.apache.iotdb.trigger.ClusterAlertingExample
• The JAR package URI is http://jar/ClusterAlertingExample.jar
• When creating the trigger instance, two parameters, name and limit, are passed in.

Drop Trigger

The trigger can be dropped by specifying the trigger ID. During the process of dropping the trigger, the onDrop interface of the trigger will be called only once.

The SQL syntax is:

// Drop Trigger
dropTrigger
  : DROP TRIGGER triggerName=identifier
;

Here is an example statement:

DROP TRIGGER triggerTest1

The above statement will drop the trigger with ID triggerTest1.

Show Trigger

You can query information about triggers that exist in the cluster through an SQL statement.

The SQL syntax is as follows:

SHOW TRIGGERS

The result set format of this statement is as follows:

TriggerName	Event	Type	State	PathPattern	ClassName	NodeId
triggerTest1	BEFORE_INSERT / AFTER_INSERT	STATELESS / STATEFUL	INACTIVE / ACTIVE / DROPPING / TRANSFFERING	root.**	org.apache.iotdb.trigger.TriggerExample	ALL(STATELESS) / DATA_NODE_ID(STATEFUL)

Trigger State

During the process of creating and dropping triggers in the cluster, we maintain the states of the triggers. The following is a description of these states:

State	Description	Is it recommended to insert data?
INACTIVE	The intermediate state of executing CREATE TRIGGER, the cluster has just recorded the trigger information on the ConfigNode, and the trigger has not been activated on any DataNode.	NO
ACTIVE	Status after successful execution of CREATE TRIGGE, the trigger is available on all DataNodes in the cluster.	YES
DROPPING	Intermediate state of executing DROP TRIGGER, the cluster is in the process of dropping the trigger.	NO
TRANSFERRING	The cluster is migrating the location of this trigger instance.	NO

Notes

• The trigger takes effect from the time of registration, and does not process the existing historical data. **That is, only insertion requests that occur after the trigger is successfully registered will be listened to by the trigger. **
• The fire process of trigger is synchronous currently, so you need to ensure the efficiency of the trigger, otherwise the writing performance may be greatly affected. You need to guarantee concurrency safety of triggers yourself.
• Please do no register too many triggers in the cluster. Because the trigger information is fully stored in the ConfigNode, and there is a copy of the information in all DataNodes
• It is recommended to stop writing when registering triggers. Registering a trigger is not an atomic operation. When registering a trigger, there will be an intermediate state in which some nodes in the cluster have registered the trigger, and some nodes have not yet registered successfully. To avoid write requests on some nodes being listened to by triggers and not being listened to on some nodes, we recommend not to perform writes when registering triggers.
• When the node holding the stateful trigger instance goes down, we will try to restore the corresponding instance on another node. During the recovery process, we will call the restore interface of the trigger class once.
• The trigger JAR package has a size limit, which must be less than min(config_node_ratis_log_appender_buffer_size_max, 2G), where config_node_ratis_log_appender_buffer_size_max is a configuration item. For the specific meaning, please refer to the IOTDB configuration item description.
• It is better not to have classes with the same full class name but different function implementations in different JAR packages. For example, trigger1 and trigger2 correspond to resources trigger1.jar and trigger2.jar respectively. If two JAR packages contain a org.apache.iotdb.trigger.example.AlertListener class, when CREATE TRIGGER uses this class, the system will randomly load the class in one of the JAR packages, which will eventually leads the inconsistent behavior of trigger and other issues.

Configuration Parameters

Parameter	Meaning
trigger_lib_dir	Directory to save the trigger jar package
stateful_trigger_retry_num_when_not_found	How many times will we retry to found an instance of stateful trigger on DataNodes if not found

CONTINUOUS QUERY (CQ)

Introduction

Continuous queries(CQ) are queries that run automatically and periodically on realtime data and store query results in other specified time series.

Users can implement sliding window streaming computing through continuous query, such as calculating the hourly average temperature of a sequence and writing it into a new sequence. Users can customize the RESAMPLE clause to create different sliding windows, which can achieve a certain degree of tolerance for out-of-order data.

Syntax

CREATE (CONTINUOUS QUERY | CQ) <cq_id> 
[RESAMPLE 
  [EVERY <every_interval>] 
  [BOUNDARY <execution_boundary_time>]
  [RANGE <start_time_offset>[, end_time_offset]] 
]
[TIMEOUT POLICY BLOCKED|DISCARD]
BEGIN
  SELECT CLAUSE
    INTO CLAUSE
    FROM CLAUSE
    [WHERE CLAUSE]
    [GROUP BY(<group_by_interval>[, <sliding_step>]) [, level = <level>]]
    [HAVING CLAUSE]
    [FILL {PREVIOUS | LINEAR | constant}]
    [LIMIT rowLimit OFFSET rowOffset]
    [ALIGN BY DEVICE]
END

Note:

1. If there exists any time filters in WHERE CLAUSE, IoTDB will throw an error, because IoTDB will automatically generate a time range for the query each time it's executed.
2. GROUP BY TIME CLAUSE is different, it doesn't contain its original first display window parameter which is [start_time, end_time). It's still because IoTDB will automatically generate a time range for the query each time it's executed.
3. If there is no group by time clause in query, EVERY clause is required, otherwise IoTDB will throw an error.

Descriptions of parameters in CQ syntax

• <cq_id> specifies the globally unique id of CQ.
• <every_interval> specifies the query execution time interval. We currently support the units of ns, us, ms, s, m, h, d, w, and its value should not be lower than the minimum threshold configured by the user, which is continuous_query_min_every_interval. It's an optional parameter, default value is set to group_by_interval in group by clause.
• <start_time_offset> specifies the start time of each query execution as now()-<start_time_offset>. We currently support the units of ns, us, ms, s, m, h, d, w.It's an optional parameter, default value is set to every_interval in resample clause.
• <end_time_offset> specifies the end time of each query execution as now()-<end_time_offset>. We currently support the units of ns, us, ms, s, m, h, d, w.It's an optional parameter, default value is set to 0.
• <execution_boundary_time> is a date that represents the execution time of a certain cq task.
  • <execution_boundary_time> can be earlier than, equals to, later than current time.
  • This parameter is optional. If not specified, it is equal to BOUNDARY 0。
  • The start time of the first time window is <execution_boundary_time> - <start_time_offset>.
  • The end time of the first time window is <execution_boundary_time> - <end_time_offset>.
  • The time range of the i (1 <= i)th window is [<execution_boundary_time> - <start_time_offset> + (i - 1) * <every_interval>, <execution_boundary_time> - <end_time_offset> + (i - 1) * <every_interval>).
  • If the current time is earlier than or equal to execution_boundary_time, then the first execution moment of the continuous query is execution_boundary_time.
  • If the current time is later than execution_boundary_time, then the first execution moment of the continuous query is the first execution_boundary_time + i * <every_interval> that is later than or equal to the current time .

• <every_interval>，<start_time_offset> and <group_by_interval> should all be greater than 0.
• The value of <group_by_interval> should be less than or equal to the value of <start_time_offset>, otherwise the system will throw an error.
• Users should specify the appropriate <start_time_offset> and <every_interval> according to actual needs.
  • If <start_time_offset> is greater than <every_interval>, there will be partial data overlap in each query window.
  • If <start_time_offset> is less than <every_interval>, there may be uncovered data between each query window.
• start_time_offset should be larger than end_time_offset, otherwise the system will throw an error.

<start_time_offset> == <every_interval>

1

<start_time_offset> > <every_interval>

2

<start_time_offset> < <every_interval>

3

<every_interval> is not zero

• TIMEOUT POLICY specify how we deal with the cq task whose previous time interval execution is not finished while the next execution time has reached. The default value is BLOCKED.
  • BLOCKED means that we will block and wait to do the current cq execution task until the previous time interval cq task finishes. If using BLOCKED policy, all the time intervals will be executed, but it may be behind the latest time interval.
  • DISCARD means that we just discard the current cq execution task and wait for the next execution time and do the next time interval cq task. If using DISCARD policy, some time intervals won't be executed when the execution time of one cq task is longer than the <every_interval>. However, once a cq task is executed, it will use the latest time interval, so it can catch up at the sacrifice of some time intervals being discarded.

Examples of CQ

The examples below use the following sample data. It's a real time data stream and we can assume that the data arrives on time.

+-----------------------------+-----------------------------+-----------------------------+-----------------------------+-----------------------------+
|                         Time|root.ln.wf02.wt02.temperature|root.ln.wf02.wt01.temperature|root.ln.wf01.wt02.temperature|root.ln.wf01.wt01.temperature|
+-----------------------------+-----------------------------+-----------------------------+-----------------------------+-----------------------------+
|2021-05-11T22:18:14.598+08:00|                        121.0|                         72.0|                        183.0|                        115.0|
|2021-05-11T22:18:19.941+08:00|                          0.0|                         68.0|                         68.0|                        103.0|
|2021-05-11T22:18:24.949+08:00|                        122.0|                         45.0|                         11.0|                         14.0|
|2021-05-11T22:18:29.967+08:00|                         47.0|                         14.0|                         59.0|                        181.0|
|2021-05-11T22:18:34.979+08:00|                        182.0|                        113.0|                         29.0|                        180.0|
|2021-05-11T22:18:39.990+08:00|                         42.0|                         11.0|                         52.0|                         19.0|
|2021-05-11T22:18:44.995+08:00|                         78.0|                         38.0|                        123.0|                         52.0|
|2021-05-11T22:18:49.999+08:00|                        137.0|                        172.0|                        135.0|                        193.0|
|2021-05-11T22:18:55.003+08:00|                         16.0|                        124.0|                        183.0|                         18.0|
+-----------------------------+-----------------------------+-----------------------------+-----------------------------+-----------------------------+

Configuring execution intervals

Use an EVERY interval in the RESAMPLE clause to specify the CQ’s execution interval, if not specific, default value is equal to group_by_interval.

CREATE CONTINUOUS QUERY cq1
RESAMPLE EVERY 20s
BEGIN
SELECT max_value(temperature)
  INTO root.ln.wf02.wt02(temperature_max), root.ln.wf02.wt01(temperature_max), root.ln.wf01.wt02(temperature_max), root.ln.wf01.wt01(temperature_max)
  FROM root.ln.*.*
  GROUP BY(10s)
END

cq1 calculates the 10-second average of temperature sensor under the root.ln prefix path and stores the results in the temperature_max sensor using the same prefix path as the corresponding sensor.

cq1 executes at 20-second intervals, the same interval as the EVERY interval. Every 20 seconds, cq1 runs a single query that covers the time range for the current time bucket, that is, the 20-second time bucket that intersects with now().

Supposing that the current time is 2021-05-11T22:18:40.000+08:00, we can see annotated log output about cq1 running at DataNode if you set log level to DEBUG:

At **2021-05-11T22:18:40.000+08:00**, `cq1` executes a query within the time range `[2021-05-11T22:18:20, 2021-05-11T22:18:40)`.
`cq1` generate 2 lines:
>
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:20.000+08:00|                            122.0|                             45.0|                             59.0|                            181.0|
|2021-05-11T22:18:30.000+08:00|                            182.0|                            113.0|                             52.0|                            180.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
>
At **2021-05-11T22:19:00.000+08:00**, `cq1` executes a query within the time range `[2021-05-11T22:18:40, 2021-05-11T22:19:00)`.
`cq1` generate 2 lines:
>
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:40.000+08:00|                            137.0|                            172.0|                            135.0|                            193.0|
|2021-05-11T22:18:50.000+08:00|                             16.0|                            124.0|                            183.0|                             18.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
>

cq1 won't deal with data that is before the current time window which is 2021-05-11T22:18:20.000+08:00, so here are the results:

> SELECT temperature_max from root.ln.*.*;
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:20.000+08:00|                            122.0|                             45.0|                             59.0|                            181.0|
|2021-05-11T22:18:30.000+08:00|                            182.0|                            113.0|                             52.0|                            180.0|
|2021-05-11T22:18:40.000+08:00|                            137.0|                            172.0|                            135.0|                            193.0|
|2021-05-11T22:18:50.000+08:00|                             16.0|                            124.0|                            183.0|                             18.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+

Configuring time range for resampling

Use start_time_offset in the RANGE clause to specify the start time of the CQ’s time range, if not specific, default value is equal to EVERY interval.

CREATE CONTINUOUS QUERY cq2
RESAMPLE RANGE 40s
BEGIN
  SELECT max_value(temperature)
  INTO root.ln.wf02.wt02(temperature_max), root.ln.wf02.wt01(temperature_max), root.ln.wf01.wt02(temperature_max), root.ln.wf01.wt01(temperature_max)
  FROM root.ln.*.*
  GROUP BY(10s)
END

cq2 calculates the 10-second average of temperature sensor under the root.ln prefix path and stores the results in the temperature_max sensor using the same prefix path as the corresponding sensor.

cq2 executes at 10-second intervals, the same interval as the group_by_interval. Every 10 seconds, cq2 runs a single query that covers the time range between now() minus the start_time_offset and now() , that is, the time range between 40 seconds prior to now() and now().

Supposing that the current time is 2021-05-11T22:18:40.000+08:00, we can see annotated log output about cq2 running at DataNode if you set log level to DEBUG:

At **2021-05-11T22:18:40.000+08:00**, `cq2` executes a query within the time range `[2021-05-11T22:18:00, 2021-05-11T22:18:40)`.
`cq2` generate 4 lines:
>
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:00.000+08:00|                             NULL|                             NULL|                             NULL|                             NULL|
|2021-05-11T22:18:10.000+08:00|                            121.0|                             72.0|                            183.0|                            115.0|
|2021-05-11T22:18:20.000+08:00|                            122.0|                             45.0|                             59.0|                            181.0|
|2021-05-11T22:18:30.000+08:00|                            182.0|                            113.0|                             52.0|                            180.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
>
At **2021-05-11T22:18:50.000+08:00**, `cq2` executes a query within the time range `[2021-05-11T22:18:10, 2021-05-11T22:18:50)`.
`cq2` generate 4 lines:
>
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:10.000+08:00|                            121.0|                             72.0|                            183.0|                            115.0|
|2021-05-11T22:18:20.000+08:00|                            122.0|                             45.0|                             59.0|                            181.0|
|2021-05-11T22:18:30.000+08:00|                            182.0|                            113.0|                             52.0|                            180.0|
|2021-05-11T22:18:40.000+08:00|                            137.0|                            172.0|                            135.0|                            193.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
>
At **2021-05-11T22:19:00.000+08:00**, `cq2` executes a query within the time range `[2021-05-11T22:18:20, 2021-05-11T22:19:00)`.
`cq2` generate 4 lines:
>
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:20.000+08:00|                            122.0|                             45.0|                             59.0|                            181.0|
|2021-05-11T22:18:30.000+08:00|                            182.0|                            113.0|                             52.0|                            180.0|
|2021-05-11T22:18:40.000+08:00|                            137.0|                            172.0|                            135.0|                            193.0|
|2021-05-11T22:18:50.000+08:00|                             16.0|                            124.0|                            183.0|                             18.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
>

cq2 won't write lines that are all null. Notice cq2 will also calculate the results for some time interval many times. Here are the results:

> SELECT temperature_max from root.ln.*.*;
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:10.000+08:00|                            121.0|                             72.0|                            183.0|                            115.0|
|2021-05-11T22:18:20.000+08:00|                            122.0|                             45.0|                             59.0|                            181.0|
|2021-05-11T22:18:30.000+08:00|                            182.0|                            113.0|                             52.0|                            180.0|
|2021-05-11T22:18:40.000+08:00|                            137.0|                            172.0|                            135.0|                            193.0|
|2021-05-11T22:18:50.000+08:00|                             16.0|                            124.0|                            183.0|                             18.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+

Configuring execution intervals and CQ time ranges

Use an EVERY interval and RANGE interval in the RESAMPLE clause to specify the CQ’s execution interval and the length of the CQ’s time range. And use fill() to change the value reported for time intervals with no data.

CREATE CONTINUOUS QUERY cq3
RESAMPLE EVERY 20s RANGE 40s
BEGIN
  SELECT max_value(temperature)
  INTO root.ln.wf02.wt02(temperature_max), root.ln.wf02.wt01(temperature_max), root.ln.wf01.wt02(temperature_max), root.ln.wf01.wt01(temperature_max)
  FROM root.ln.*.*
  GROUP BY(10s)
  FILL(100.0)
END

cq3 calculates the 10-second average of temperature sensor under the root.ln prefix path and stores the results in the temperature_max sensor using the same prefix path as the corresponding sensor. Where possible, it writes the value 100.0 for time intervals with no results.

cq3 executes at 20-second intervals, the same interval as the EVERY interval. Every 20 seconds, cq3 runs a single query that covers the time range between now() minus the start_time_offset and now(), that is, the time range between 40 seconds prior to now() and now().

Supposing that the current time is 2021-05-11T22:18:40.000+08:00, we can see annotated log output about cq3 running at DataNode if you set log level to DEBUG:

At **2021-05-11T22:18:40.000+08:00**, `cq3` executes a query within the time range `[2021-05-11T22:18:00, 2021-05-11T22:18:40)`.
`cq3` generate 4 lines:
>
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:00.000+08:00|                            100.0|                            100.0|                            100.0|                            100.0|
|2021-05-11T22:18:10.000+08:00|                            121.0|                             72.0|                            183.0|                            115.0|
|2021-05-11T22:18:20.000+08:00|                            122.0|                             45.0|                             59.0|                            181.0|
|2021-05-11T22:18:30.000+08:00|                            182.0|                            113.0|                             52.0|                            180.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
>
At **2021-05-11T22:19:00.000+08:00**, `cq3` executes a query within the time range `[2021-05-11T22:18:20, 2021-05-11T22:19:00)`.
`cq3` generate 4 lines:
>
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:20.000+08:00|                            122.0|                             45.0|                             59.0|                            181.0|
|2021-05-11T22:18:30.000+08:00|                            182.0|                            113.0|                             52.0|                            180.0|
|2021-05-11T22:18:40.000+08:00|                            137.0|                            172.0|                            135.0|                            193.0|
|2021-05-11T22:18:50.000+08:00|                             16.0|                            124.0|                            183.0|                             18.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
>

Notice that cq3 will calculate the results for some time interval many times, so here are the results:

> SELECT temperature_max from root.ln.*.*;
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:00.000+08:00|                            100.0|                            100.0|                            100.0|                            100.0|
|2021-05-11T22:18:10.000+08:00|                            121.0|                             72.0|                            183.0|                            115.0|
|2021-05-11T22:18:20.000+08:00|                            122.0|                             45.0|                             59.0|                            181.0|
|2021-05-11T22:18:30.000+08:00|                            182.0|                            113.0|                             52.0|                            180.0|
|2021-05-11T22:18:40.000+08:00|                            137.0|                            172.0|                            135.0|                            193.0|
|2021-05-11T22:18:50.000+08:00|                             16.0|                            124.0|                            183.0|                             18.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+

Configuring end_time_offset for CQ time range

Use an EVERY interval and RANGE interval in the RESAMPLE clause to specify the CQ’s execution interval and the length of the CQ’s time range. And use fill() to change the value reported for time intervals with no data.

CREATE CONTINUOUS QUERY cq4
RESAMPLE EVERY 20s RANGE 40s, 20s
BEGIN
  SELECT max_value(temperature)
  INTO root.ln.wf02.wt02(temperature_max), root.ln.wf02.wt01(temperature_max), root.ln.wf01.wt02(temperature_max), root.ln.wf01.wt01(temperature_max)
  FROM root.ln.*.*
  GROUP BY(10s)
  FILL(100.0)
END

cq4 calculates the 10-second average of temperature sensor under the root.ln prefix path and stores the results in the temperature_max sensor using the same prefix path as the corresponding sensor. Where possible, it writes the value 100.0 for time intervals with no results.

cq4 executes at 20-second intervals, the same interval as the EVERY interval. Every 20 seconds, cq4 runs a single query that covers the time range between now() minus the start_time_offset and now() minus the end_time_offset, that is, the time range between 40 seconds prior to now() and 20 seconds prior to now().

Supposing that the current time is 2021-05-11T22:18:40.000+08:00, we can see annotated log output about cq4 running at DataNode if you set log level to DEBUG:

At **2021-05-11T22:18:40.000+08:00**, `cq4` executes a query within the time range `[2021-05-11T22:18:00, 2021-05-11T22:18:20)`.
`cq4` generate 2 lines:
>
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:00.000+08:00|                            100.0|                            100.0|                            100.0|                            100.0|
|2021-05-11T22:18:10.000+08:00|                            121.0|                             72.0|                            183.0|                            115.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
>
At **2021-05-11T22:19:00.000+08:00**, `cq4` executes a query within the time range `[2021-05-11T22:18:20, 2021-05-11T22:18:40)`.
`cq4` generate 2 lines:
>
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:20.000+08:00|                            122.0|                             45.0|                             59.0|                            181.0|
|2021-05-11T22:18:30.000+08:00|                            182.0|                            113.0|                             52.0|                            180.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
>

Notice that cq4 will calculate the results for all time intervals only once after a delay of 20 seconds, so here are the results:

> SELECT temperature_max from root.ln.*.*;
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|                         Time|root.ln.wf02.wt02.temperature_max|root.ln.wf02.wt01.temperature_max|root.ln.wf01.wt02.temperature_max|root.ln.wf01.wt01.temperature_max|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+
|2021-05-11T22:18:00.000+08:00|                            100.0|                            100.0|                            100.0|                            100.0|
|2021-05-11T22:18:10.000+08:00|                            121.0|                             72.0|                            183.0|                            115.0|
|2021-05-11T22:18:20.000+08:00|                            122.0|                             45.0|                             59.0|                            181.0|
|2021-05-11T22:18:30.000+08:00|                            182.0|                            113.0|                             52.0|                            180.0|
+-----------------------------+---------------------------------+---------------------------------+---------------------------------+---------------------------------+

CQ without group by clause

Use an EVERY interval in the RESAMPLE clause to specify the CQ’s execution interval and the length of the CQ’s time range.

CREATE CONTINUOUS QUERY cq5
RESAMPLE EVERY 20s
BEGIN
  SELECT temperature + 1
  INTO root.precalculated_sg.::(temperature)
  FROM root.ln.*.*
  align by device
END

cq5 calculates the temperature + 1 under the root.ln prefix path and stores the results in the root.precalculated_sg database. Sensors use the same prefix path as the corresponding sensor.

cq5 executes at 20-second intervals, the same interval as the EVERY interval. Every 20 seconds, cq5 runs a single query that covers the time range for the current time bucket, that is, the 20-second time bucket that intersects with now().

Supposing that the current time is 2021-05-11T22:18:40.000+08:00, we can see annotated log output about cq5 running at DataNode if you set log level to DEBUG:

At **2021-05-11T22:18:40.000+08:00**, `cq5` executes a query within the time range `[2021-05-11T22:18:20, 2021-05-11T22:18:40)`.
`cq5` generate 16 lines:
>
+-----------------------------+-------------------------------+-----------+
|                         Time|                         Device|temperature|
+-----------------------------+-------------------------------+-----------+
|2021-05-11T22:18:24.949+08:00|root.precalculated_sg.wf02.wt02|      123.0| 
|2021-05-11T22:18:29.967+08:00|root.precalculated_sg.wf02.wt02|       48.0|
|2021-05-11T22:18:34.979+08:00|root.precalculated_sg.wf02.wt02|      183.0|
|2021-05-11T22:18:39.990+08:00|root.precalculated_sg.wf02.wt02|       45.0|
|2021-05-11T22:18:24.949+08:00|root.precalculated_sg.wf02.wt01|       46.0| 
|2021-05-11T22:18:29.967+08:00|root.precalculated_sg.wf02.wt01|       15.0|
|2021-05-11T22:18:34.979+08:00|root.precalculated_sg.wf02.wt01|      114.0|
|2021-05-11T22:18:39.990+08:00|root.precalculated_sg.wf02.wt01|       12.0|
|2021-05-11T22:18:24.949+08:00|root.precalculated_sg.wf01.wt02|       12.0| 
|2021-05-11T22:18:29.967+08:00|root.precalculated_sg.wf01.wt02|       60.0|
|2021-05-11T22:18:34.979+08:00|root.precalculated_sg.wf01.wt02|       30.0|
|2021-05-11T22:18:39.990+08:00|root.precalculated_sg.wf01.wt02|       53.0|
|2021-05-11T22:18:24.949+08:00|root.precalculated_sg.wf01.wt01|       15.0| 
|2021-05-11T22:18:29.967+08:00|root.precalculated_sg.wf01.wt01|      182.0|
|2021-05-11T22:18:34.979+08:00|root.precalculated_sg.wf01.wt01|      181.0|
|2021-05-11T22:18:39.990+08:00|root.precalculated_sg.wf01.wt01|       20.0|
+-----------------------------+-------------------------------+-----------+
>
At **2021-05-11T22:19:00.000+08:00**, `cq5` executes a query within the time range `[2021-05-11T22:18:40, 2021-05-11T22:19:00)`.
`cq5` generate 12 lines:
>
+-----------------------------+-------------------------------+-----------+
|                         Time|                         Device|temperature|
+-----------------------------+-------------------------------+-----------+
|2021-05-11T22:18:44.995+08:00|root.precalculated_sg.wf02.wt02|       79.0| 
|2021-05-11T22:18:49.999+08:00|root.precalculated_sg.wf02.wt02|      138.0|
|2021-05-11T22:18:55.003+08:00|root.precalculated_sg.wf02.wt02|       17.0|
|2021-05-11T22:18:44.995+08:00|root.precalculated_sg.wf02.wt01|       39.0| 
|2021-05-11T22:18:49.999+08:00|root.precalculated_sg.wf02.wt01|      173.0|
|2021-05-11T22:18:55.003+08:00|root.precalculated_sg.wf02.wt01|      125.0|
|2021-05-11T22:18:44.995+08:00|root.precalculated_sg.wf01.wt02|      124.0| 
|2021-05-11T22:18:49.999+08:00|root.precalculated_sg.wf01.wt02|      136.0|
|2021-05-11T22:18:55.003+08:00|root.precalculated_sg.wf01.wt02|      184.0|
|2021-05-11T22:18:44.995+08:00|root.precalculated_sg.wf01.wt01|       53.0| 
|2021-05-11T22:18:49.999+08:00|root.precalculated_sg.wf01.wt01|      194.0|
|2021-05-11T22:18:55.003+08:00|root.precalculated_sg.wf01.wt01|       19.0|
+-----------------------------+-------------------------------+-----------+
>

cq5 won't deal with data that is before the current time window which is 2021-05-11T22:18:20.000+08:00, so here are the results:

> SELECT temperature from root.precalculated_sg.*.* align by device;
+-----------------------------+-------------------------------+-----------+
|                         Time|                         Device|temperature|
+-----------------------------+-------------------------------+-----------+
|2021-05-11T22:18:24.949+08:00|root.precalculated_sg.wf02.wt02|      123.0| 
|2021-05-11T22:18:29.967+08:00|root.precalculated_sg.wf02.wt02|       48.0|
|2021-05-11T22:18:34.979+08:00|root.precalculated_sg.wf02.wt02|      183.0|
|2021-05-11T22:18:39.990+08:00|root.precalculated_sg.wf02.wt02|       45.0|
|2021-05-11T22:18:44.995+08:00|root.precalculated_sg.wf02.wt02|       79.0| 
|2021-05-11T22:18:49.999+08:00|root.precalculated_sg.wf02.wt02|      138.0|
|2021-05-11T22:18:55.003+08:00|root.precalculated_sg.wf02.wt02|       17.0|
|2021-05-11T22:18:24.949+08:00|root.precalculated_sg.wf02.wt01|       46.0| 
|2021-05-11T22:18:29.967+08:00|root.precalculated_sg.wf02.wt01|       15.0|
|2021-05-11T22:18:34.979+08:00|root.precalculated_sg.wf02.wt01|      114.0|
|2021-05-11T22:18:39.990+08:00|root.precalculated_sg.wf02.wt01|       12.0|
|2021-05-11T22:18:44.995+08:00|root.precalculated_sg.wf02.wt01|       39.0| 
|2021-05-11T22:18:49.999+08:00|root.precalculated_sg.wf02.wt01|      173.0|
|2021-05-11T22:18:55.003+08:00|root.precalculated_sg.wf02.wt01|      125.0|
|2021-05-11T22:18:24.949+08:00|root.precalculated_sg.wf01.wt02|       12.0| 
|2021-05-11T22:18:29.967+08:00|root.precalculated_sg.wf01.wt02|       60.0|
|2021-05-11T22:18:34.979+08:00|root.precalculated_sg.wf01.wt02|       30.0|
|2021-05-11T22:18:39.990+08:00|root.precalculated_sg.wf01.wt02|       53.0|
|2021-05-11T22:18:44.995+08:00|root.precalculated_sg.wf01.wt02|      124.0| 
|2021-05-11T22:18:49.999+08:00|root.precalculated_sg.wf01.wt02|      136.0|
|2021-05-11T22:18:55.003+08:00|root.precalculated_sg.wf01.wt02|      184.0|
|2021-05-11T22:18:24.949+08:00|root.precalculated_sg.wf01.wt01|       15.0| 
|2021-05-11T22:18:29.967+08:00|root.precalculated_sg.wf01.wt01|      182.0|
|2021-05-11T22:18:34.979+08:00|root.precalculated_sg.wf01.wt01|      181.0|
|2021-05-11T22:18:39.990+08:00|root.precalculated_sg.wf01.wt01|       20.0|
|2021-05-11T22:18:44.995+08:00|root.precalculated_sg.wf01.wt01|       53.0| 
|2021-05-11T22:18:49.999+08:00|root.precalculated_sg.wf01.wt01|      194.0|
|2021-05-11T22:18:55.003+08:00|root.precalculated_sg.wf01.wt01|       19.0|
+-----------------------------+-------------------------------+-----------+

CQ Management

Listing continuous queries

List every CQ on the IoTDB Cluster with:

SHOW (CONTINUOUS QUERIES | CQS) 

SHOW (CONTINUOUS QUERIES | CQS) order results by cq_id.

Examples

SHOW CONTINUOUS QUERIES;

we will get:

cq_id	query	state
s1_count_cq	CREATE CQ s1_count_cq BEGIN SELECT count(s1) INTO root.sg_count.d.count_s1 FROM root.sg.d GROUP BY(30m) END	active

Dropping continuous queries

Drop a CQ with a specific cq_id:

DROP (CONTINUOUS QUERY | CQ) <cq_id>

DROP CQ returns an empty result.

Examples

Drop the CQ named s1_count_cq:

DROP CONTINUOUS QUERY s1_count_cq;

Altering continuous queries

CQs can't be altered once they're created. To change a CQ, you must DROP and reCREATE it with the updated settings.

CQ Use Cases

Downsampling and Data Retention

Use CQs with TTL set on database in IoTDB to mitigate storage concerns. Combine CQs and TTL to automatically downsample high precision data to a lower precision and remove the dispensable, high precision data from the database.

Recalculating expensive queries

Shorten query runtimes by pre-calculating expensive queries with CQs. Use a CQ to automatically downsample commonly-queried, high precision data to a lower precision. Queries on lower precision data require fewer resources and return faster.

Pre-calculate queries for your preferred graphing tool to accelerate the population of graphs and dashboards.

Substituting for sub-query

IoTDB does not support sub queries. We can get the same functionality by creating a CQ as a sub query and store its result into other time series and then querying from those time series again will be like doing nested sub query.

Example

IoTDB does not accept the following query with a nested sub query. The query calculates the average number of non-null values of s1 at 30 minute intervals:

SELECT avg(count_s1) from (select count(s1) as count_s1 from root.sg.d group by([0, now()), 30m));

To get the same results:

Create a CQ

This step performs the nested sub query in from clause of the query above. The following CQ automatically calculates the number of non-null values of s1 at 30 minute intervals and writes those counts into the new root.sg_count.d.count_s1 time series.

CREATE CQ s1_count_cq 
BEGIN 
    SELECT count(s1)  
        INTO root.sg_count.d(count_s1)
        FROM root.sg.d
        GROUP BY(30m)
END

Query the CQ results

Next step performs the avg([...]) part of the outer query above.

Query the data in the time series root.sg_count.d.count_s1 to calculate the average of it:

SELECT avg(count_s1) from root.sg_count.d;

System Parameter Configuration

Name	Description	Data Type	Default Value
continuous_query_submit_thread	The number of threads in the scheduled thread pool that submit continuous query tasks periodically	int32	2
continuous_query_min_every_interval_in_ms	The minimum value of the continuous query execution time interval	duration	1000