Documentation

InfluxQL Continuous Queries

This page documents an earlier version of InfluxDB. InfluxDB v2.0 is the latest stable version.

Introduction

Continuous Queries (CQ) are InfluxQL queries that run automatically and periodically on realtime data and store query results in a specified measurement.

Basic SyntaxAdvanced SyntaxCQ Management
Examples of Basic SyntaxExamples of Advanced SyntaxCQ Use Cases
Common Issues with Basic SyntaxCommon Issues with Advanced SyntaxFurther information

Syntax

Basic syntax

CREATE CONTINUOUS QUERY <cq_name> ON <database_name>
BEGIN
  <cq_query>
END

Description of basic syntax

The cq_query

The `cq_query` requires a [function](/influxdb/v1.5/concepts/glossary/#function), an [`INTO` clause](/influxdb/v1.5/query_language/spec/#clauses), and a [`GROUP BY time()` clause](/influxdb/v1.5/query_language/spec/#clauses):
SELECT <function[s]> INTO <destination_measurement> FROM <measurement> [WHERE <stuff>] GROUP BY time(<interval>)[,<tag_key[s]>]

Note: Notice that the cq_query does not require a time range in a WHERE clause. InfluxDB automatically generates a time range for the cq_query when it executes the CQ. Any user-specified time ranges in the cq_query’s WHERE clause will be ignored by the system.

Schedule and Coverage

CQs operate on realtime data. They use the local server’s timestamp, the `GROUP BY time()` interval, and InfluxDB's preset time boundaries to determine when to execute and what time range to cover in the query.

CQs execute at the same interval as the cq_query’s GROUP BY time() interval, and they run at the start of InfluxDB’s preset time boundaries. If the GROUP BY time() interval is one hour, the CQ executes at the start of every hour.

When the CQ executes, it runs a single query for the time range between now() and now() minus the GROUP BY time() interval. If the GROUP BY time() interval is one hour and the current time is 17:00, the query’s time range is between 16:00 and 16:59.999999999.

Examples of basic syntax

The examples below use the following sample data in the transportation database. The measurement bus_data stores 15-minute resolution data on the number of bus passengers and complaints:

name: bus_data
--------------
time                   passengers   complaints
2016-08-28T07:00:00Z   5            9
2016-08-28T07:15:00Z   8            9
2016-08-28T07:30:00Z   8            9
2016-08-28T07:45:00Z   7            9
2016-08-28T08:00:00Z   8            9
2016-08-28T08:15:00Z   15           7
2016-08-28T08:30:00Z   15           7
2016-08-28T08:45:00Z   17           7
2016-08-28T09:00:00Z   20           7
Example 1: Automatically downsampling data

Use a simple CQ to automatically downsample data from a single field and write the results to another measurement in the same database.
CREATE CONTINUOUS QUERY "cq_basic" ON "transportation"
BEGIN
  SELECT mean("passengers") INTO "average_passengers" FROM "bus_data" GROUP BY time(1h)
END

cq_basic calculates the average hourly number of passengers from the bus_data measurement and stores the results in the average_passengers measurement in the transportation database.

cq_basic executes at one-hour intervals, the same interval as the GROUP BY time() interval. Every hour, cq_basic runs a single query that covers the time range between now() and now() minus the GROUP BY time() interval, that is, the time range between now() and one hour prior to now().

Annotated log output on the morning of August 28, 2016:

At 8:00 cq_basic executes a query with the time range time >= '7:00' AND time < '08:00'. cq_basic writes one point to the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T07:00:00Z   7

At 9:00 cq_basic executes a query with the time range time >= '8:00' AND time < '9:00'. cq_basic writes one point to the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T08:00:00Z   13.75

Results:

> SELECT * FROM "average_passengers"
name: average_passengers
------------------------
time                   mean
2016-08-28T07:00:00Z   7
2016-08-28T08:00:00Z   13.75
Example 2: Automatically downsampling data into another retention policy

[Fully qualify](/influxdb/v1.5/query_language/data_exploration/#the-basic-select-statement) the destination measurement to store the downsampled data in a non-`DEFAULT` [retention policy](/influxdb/v1.5/concepts/glossary/#retention-policy-rp) (RP).
CREATE CONTINUOUS QUERY "cq_basic_rp" ON "transportation"
BEGIN
  SELECT mean("passengers") INTO "transportation"."three_weeks"."average_passengers" FROM "bus_data" GROUP BY time(1h)
END

cq_basic_rp calculates the average hourly number of passengers from the bus_data measurement and stores the results in the transportation database, the three_weeks RP, and the average_passengers measurement.

cq_basic_rp executes at one-hour intervals, the same interval as the GROUP BY time() interval. Every hour, cq_basic_rp runs a single query that covers the time range between now() and now() minus the GROUP BY time() interval, that is, the time range between now() and one hour prior to now().

Annotated log output on the morning of August 28, 2016:

At 8:00 cq_basic_rp executes a query with the time range time >= '7:00' AND time < '8:00'. cq_basic_rp writes one point to the three_weeks RP and the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T07:00:00Z   7

At 9:00 cq_basic_rp executes a query with the time range time >= '8:00' AND time < '9:00'. cq_basic_rp writes one point to the three_weeks RP and the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T08:00:00Z   13.75

Results:

> SELECT * FROM "transportation"."three_weeks"."average_passengers"
name: average_passengers
------------------------
time                   mean
2016-08-28T07:00:00Z   7
2016-08-28T08:00:00Z   13.75

cq_basic_rp uses CQs and retention policies to automatically downsample data and keep those downsampled data for an alternative length of time. See the Downsampling and Data Retention guide for an in-depth discussion about this CQ use case.

Example 3: Automatically downsampling a database with backreferencing

Use a function with a wildcard (`*`) and `INTO` query's [backreferencing syntax](/influxdb/v1.5/query_language/data_exploration/#the-into-clause) to automatically downsample data from all measurements and numerical fields in a database.
CREATE CONTINUOUS QUERY "cq_basic_br" ON "transportation"
BEGIN
  SELECT mean(*) INTO "downsampled_transportation"."autogen".:MEASUREMENT FROM /.*/ GROUP BY time(30m),*
END

cq_basic_br calculates the 30-minute average of passengers and complaints from every measurement in the transportation database (in this case, there’s only the bus_data measurement). It stores the results in the downsampled_transportation database.

cq_basic_br executes at 30 minutes intervals, the same interval as the GROUP BY time() interval. Every 30 minutes, cq_basic_br runs a single query that covers the time range between now() and now() minus the GROUP BY time() interval, that is, the time range between now() and 30 minutes prior to now().

Annotated log output on the morning of August 28, 2016:

At 7:30, cq_basic_br executes a query with the time range time >= '7:00' AND time < '7:30'. cq_basic_br writes two points to the bus_data measurement in the downsampled_transportation database:

name: bus_data
--------------
time                   mean_complaints   mean_passengers
2016-08-28T07:00:00Z   9                 6.5

At 8:00, cq_basic_br executes a query with the time range time >= '7:30' AND time < '8:00'. cq_basic_br writes two points to the bus_data measurement in the downsampled_transportation database:

name: bus_data
--------------
time                   mean_complaints   mean_passengers
2016-08-28T07:30:00Z   9                 7.5

[…]

At 9:00, cq_basic_br executes a query with the time range time >= '8:30' AND time < '9:00'. cq_basic_br writes two points to the bus_data measurement in the downsampled_transportation database:

name: bus_data
--------------
time                   mean_complaints   mean_passengers
2016-08-28T08:30:00Z   7                 16

Results:

> SELECT * FROM "downsampled_transportation."autogen"."bus_data"
name: bus_data
--------------
time                   mean_complaints   mean_passengers
2016-08-28T07:00:00Z   9                 6.5
2016-08-28T07:30:00Z   9                 7.5
2016-08-28T08:00:00Z   8                 11.5
2016-08-28T08:30:00Z   7                 16
Example 4: Automatically downsampling data and configuring CQ time boundaries

Use an [offset interval](/influxdb/v1.5/query_language/data_exploration/#advanced-group-by-time-syntax) in the `GROUP BY time()` clause to alter both the CQ's default execution time and preset time boundaries.
CREATE CONTINUOUS QUERY "cq_basic_offset" ON "transportation"
BEGIN
  SELECT mean("passengers") INTO "average_passengers" FROM "bus_data" GROUP BY time(1h,15m)
END

cq_basic_offsetcalculates the average hourly number of passengers from the bus_data measurement and stores the results in the average_passengers measurement.

cq_basic_offset executes at one-hour intervals, the same interval as the GROUP BY time() interval. The 15 minute offset interval forces the CQ to execute 15 minutes after the default execution time; cq_basic_offset executes at 8:15 instead of 8:00.

Every hour, cq_basic_offset runs a single query that covers the time range between now() and now() minus the GROUP BY time() interval, that is, the time range between now() and one hour prior to now(). The 15 minute offset interval shifts forward the generated preset time boundaries in the CQ’s WHERE clause; cq_basic_offset queries between 7:15 and 8:14.999999999 instead of 7:00 and 7:59.999999999.

Annotated log output on the morning of August 28, 2016:

At 8:15 cq_basic_offset executes a query with the time range time >= '7:15' AND time < '8:15'. cq_basic_offset writes one point to the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T07:15:00Z   7.75

At 9:15 cq_basic_offset executes a query with the time range time >= '8:15' AND time < '9:15'. cq_basic_offset writes one point to the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T08:15:00Z   16.75

Results:

> SELECT * FROM "average_passengers"
name: average_passengers
------------------------
time                   mean
2016-08-28T07:15:00Z   7.75
2016-08-28T08:15:00Z   16.75

Notice that the timestamps are for 7:15 and 8:15 instead of 7:00 and 8:00.

Common issues with basic syntax

Issue 1: Handling time intervals with no data

CQs do not write any results for a time interval if no data fall within that time range.

Note that the basic syntax does not support using fill() to change the value reported for intervals with no data. Basic syntax CQs ignore fill() if it’s included in the CQ query. A possible workaround is to use the advanced CQ syntax.

Issue 2: Resampling previous time intervals

The basic CQ runs a single query that covers the time range between `now()` and `now()` minus the `GROUP BY time()` interval. See the [advanced syntax](#advanced-syntax) for how to configure the query's time range.
Issue 3: Backfilling results for older data

CQs operate on realtime data, that is, data with timestamps that occur relative to [`now()`](/influxdb/v1.5/concepts/glossary/#now). Use a basic [`INTO` query](/influxdb/v1.5/query_language/data_exploration/#the-into-clause) to backfill results for data with older timestamps.
Issue 4: Missing tags in the CQ results

By default, all [`INTO` queries](/influxdb/v1.5/query_language/data_exploration/#the-into-clause) convert any tags in the source measurement to fields in the destination measurement.

Include GROUP BY * in the CQ to preserve tags in the destination measurement.

Advanced syntax

CREATE CONTINUOUS QUERY <cq_name> ON <database_name>
RESAMPLE EVERY <interval> FOR <interval>
BEGIN
  <cq_query>
END

Description of advanced syntax

The cq_query

See [ Description of Basic Syntax](/influxdb/v1.5/query_language/continuous_queries/#description-of-basic-syntax).
Scheduling and coverage

CQs operate on realtime data. With the advanced syntax, CQs use the local server’s timestamp, the information in the `RESAMPLE` clause, and InfluxDB's preset time boundaries to determine when to execute and what time range to cover in the query.

CQs execute at the same interval as the EVERY interval in the RESAMPLE clause, and they run at the start of InfluxDB’s preset time boundaries. If the EVERY interval is two hours, InfluxDB executes the CQ at the top of every other hour.

When the CQ executes, it runs a single query for the time range between now() and now() minus the FOR interval in the RESAMPLE clause. If the FOR interval is two hours and the current time is 17:00, the query’s time range is between 15:00 and 16:59.999999999.

Both the EVERY interval and the FOR interval accept duration literals. The RESAMPLE clause works with either or both of the EVERY and FOR intervals configured. CQs default to the relevant basic syntax behavior if the EVERY interval or FOR interval is not provided (see the first issue in Common Issues with Advanced Syntax for an anomalistic case).

Examples of advanced syntax

The examples below use the following sample data in the transportation database. The measurement bus_data stores 15-minute resolution data on the number of bus passengers:

name: bus_data
--------------
time                   passengers
2016-08-28T06:30:00Z   2
2016-08-28T06:45:00Z   4
2016-08-28T07:00:00Z   5
2016-08-28T07:15:00Z   8
2016-08-28T07:30:00Z   8
2016-08-28T07:45:00Z   7
2016-08-28T08:00:00Z   8
2016-08-28T08:15:00Z   15
2016-08-28T08:30:00Z   15
2016-08-28T08:45:00Z   17
2016-08-28T09:00:00Z   20
Example 1: Configuring execution intervals

Use an `EVERY` interval in the `RESAMPLE` clause to specify the CQ's execution interval.
CREATE CONTINUOUS QUERY "cq_advanced_every" ON "transportation"
RESAMPLE EVERY 30m
BEGIN
  SELECT mean("passengers") INTO "average_passengers" FROM "bus_data" GROUP BY time(1h)
END

cq_advanced_every calculates the one-hour average of passengers from the bus_data measurement and stores the results in the average_passengers measurement in the transportation database.

cq_advanced_every executes at 30-minute intervals, the same interval as the EVERY interval. Every 30 minutes, cq_advanced_every runs a single query that covers the time range for the current time bucket, that is, the one-hour time bucket that intersects with now().

Annotated log output on the morning of August 28, 2016:

At 8:00, cq_advanced_every executes a query with the time range WHERE time >= '7:00' AND time < '8:00'. cq_advanced_every writes one point to the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T07:00:00Z   7

At 8:30, cq_advanced_every executes a query with the time range WHERE time >= '8:00' AND time < '9:00'. cq_advanced_every writes one point to the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T08:00:00Z   12.6667

At 9:00, cq_advanced_every executes a query with the time range WHERE time >= '8:00' AND time < '9:00'. cq_advanced_every writes one point to the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T08:00:00Z   13.75

Results:

> SELECT * FROM "average_passengers"
name: average_passengers
------------------------
time                   mean
2016-08-28T07:00:00Z   7
2016-08-28T08:00:00Z   13.75

Notice that cq_advanced_every calculates the result for the 8:00 time interval twice. First, it runs at 8:30 and calculates the average for every available data point between 8:00 and 9:00 (8,15, and 15). Second, it runs at 9:00 and calculates the average for every available data point between 8:00 and 9:00 (8, 15, 15, and 17). Because of the way InfluxDB handles duplicate points , the second result simply overwrites the first result.

Example 2: Configuring time ranges for resampling

Use a `FOR` interval in the `RESAMPLE` clause to specify the length of the CQ's time range.
CREATE CONTINUOUS QUERY "cq_advanced_for" ON "transportation"
RESAMPLE FOR 1h
BEGIN
  SELECT mean("passengers") INTO "average_passengers" FROM "bus_data" GROUP BY time(30m)
END

cq_advanced_for calculates the 30-minute average of passengers from the bus_data measurement and stores the results in the average_passengers measurement in the transportation database.

cq_advanced_for executes at 30-minute intervals, the same interval as the GROUP BY time() interval. Every 30 minutes, cq_advanced_for runs a single query that covers the time range between now() and now() minus the FOR interval, that is, the time range between now() and one hour prior to now().

Annotated log output on the morning of August 28, 2016:

At 8:00 cq_advanced_for executes a query with the time range WHERE time >= '7:00' AND time < '8:00'. cq_advanced_for writes two points to the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T07:00:00Z   6.5
2016-08-28T07:30:00Z   7.5

At 8:30 cq_advanced_for executes a query with the time range WHERE time >= '7:30' AND time < '8:30'. cq_advanced_for writes two points to the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T07:30:00Z   7.5
2016-08-28T08:00:00Z   11.5

At 9:00 cq_advanced_for executes a query with the time range WHERE time >= '8:00' AND time < '9:00'. cq_advanced_for writes two points to the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T08:00:00Z   11.5
2016-08-28T08:30:00Z   16

Notice that cq_advanced_for will calculate the result for every time interval twice. The CQ calculates the average for the 7:30 time interval at 8:00 and at 8:30, and it calculates the average for the 8:00 time interval at 8:30 and 9:00.

Results:

> SELECT * FROM "average_passengers"
name: average_passengers
------------------------
time                   mean
2016-08-28T07:00:00Z   6.5
2016-08-28T07:30:00Z   7.5
2016-08-28T08:00:00Z   11.5
2016-08-28T08:30:00Z   16
Example 3: Configuring execution intervals and CQ time ranges

Use an `EVERY` interval and `FOR` interval in the `RESAMPLE` clause to specify the CQ's execution interval and the length of the CQ's time range.
CREATE CONTINUOUS QUERY "cq_advanced_every_for" ON "transportation"
RESAMPLE EVERY 1h FOR 90m
BEGIN
  SELECT mean("passengers") INTO "average_passengers" FROM "bus_data" GROUP BY time(30m)
END

cq_advanced_every_for calculates the 30-minute average of passengers from the bus_data measurement and stores the results in the average_passengers measurement in the transportation database.

cq_advanced_every_for executes at one-hour intervals, the same interval as the EVERY interval. Every hour, cq_advanced_every_for runs a single query that covers the time range between now() and now() minus the FOR interval, that is, the time range between now() and 90 minutes prior to now().

Annotated log output on the morning of August 28, 2016:

At 8:00 cq_advanced_every_for executes a query with the time range WHERE time >= '6:30' AND time < '8:00'. cq_advanced_every_for writes three points to the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T06:30:00Z   3
2016-08-28T07:00:00Z   6.5
2016-08-28T07:30:00Z   7.5

At 9:00 cq_advanced_every_for executes a query with the time range WHERE time >= '7:30' AND time < '9:00'. cq_advanced_every_for writes three points to the average_passengers measurement:

name: average_passengers
------------------------
time                   mean
2016-08-28T07:30:00Z   7.5
2016-08-28T08:00:00Z   11.5
2016-08-28T08:30:00Z   16

Notice that cq_advanced_every_for will calculate the result for every time interval twice. The CQ calculates the average for the 7:30 interval at 8:00 and 9:00.

Results:

> SELECT * FROM "average_passengers"
name: average_passengers
------------------------
time                   mean
2016-08-28T06:30:00Z   3
2016-08-28T07:00:00Z   6.5
2016-08-28T07:30:00Z   7.5
2016-08-28T08:00:00Z   11.5
2016-08-28T08:30:00Z   16
Example 4: Configuring CQ time ranges and filling empty results

Use a `FOR` interval and `fill()` to change the value reported for time intervals with no data. Note that at least one data point must fall within the `FOR` interval for `fill()` to operate. If no data fall within the `FOR` interval the CQ writes no points to the destination measurement.
CREATE CONTINUOUS QUERY "cq_advanced_for_fill" ON "transportation"
RESAMPLE FOR 2h
BEGIN
  SELECT mean("passengers") INTO "average_passengers" FROM "bus_data" GROUP BY time(1h) fill(1000)
END

cq_advanced_for_fill calculates the one-hour average of passengers from the bus_data measurement and stores the results in the average_passengers measurement in the transportation database. Where possible, it writes the value 1000 for time intervals with no results.

cq_advanced_for_fill executes at one-hour intervals, the same interval as the GROUP BY time() interval. Every hour, cq_advanced_for_fill runs a single query that covers the time range between now() and now() minus the FOR interval, that is, the time range between now() and two hours prior to now().

Annotated log output on the morning of August 28, 2016:

At 6:00, cq_advanced_for_fill executes a query with the time range WHERE time >= '4:00' AND time < '6:00'. cq_advanced_for_fill writes nothing to average_passengers; bus_data has no data that fall within that time range.

At 7:00, cq_advanced_for_fill executes a query with the time range WHERE time >= '5:00' AND time < '7:00'. cq_advanced_for_fill writes two points to average_passengers:

name: average_passengers
------------------------
time                   mean
2016-08-28T05:00:00Z   1000          <------ fill(1000)
2016-08-28T06:00:00Z   3             <------ average of 2 and 4

[…]

At 11:00, cq_advanced_for_fill executes a query with the time range WHERE time >= '9:00' AND time < '11:00'. cq_advanced_for_fill writes two points to average_passengers:

name: average_passengers
------------------------
2016-08-28T09:00:00Z   20            <------ average of 20
2016-08-28T10:00:00Z   1000          <------ fill(1000)

At 12:00, cq_advanced_for_fill executes a query with the time range WHERE time >= '10:00' AND time < '12:00'. cq_advanced_for_fill writes nothing to average_passengers; bus_data has no data that fall within that time range.

Results:

> SELECT * FROM "average_passengers"
name: average_passengers
------------------------
time                   mean
2016-08-28T05:00:00Z   1000
2016-08-28T06:00:00Z   3
2016-08-28T07:00:00Z   7
2016-08-28T08:00:00Z   13.75
2016-08-28T09:00:00Z   20
2016-08-28T10:00:00Z   1000

Note: fill(previous) doesn’t fill the result for a time interval if the previous value is outside the query’s time range. See Frequently Asked Questions for more information.

Common issues with advanced syntax

Issue 1: If the EVERY interval is greater than the GROUP BY time() interval

If the `EVERY` interval is greater than the `GROUP BY time()` interval, the CQ executes at the same interval as the `EVERY` interval and runs a single query that covers the time range between `now()` and `now()` minus the `EVERY` interval (not between `now()` and `now()` minus the `GROUP BY time()` interval).

For example, if the GROUP BY time() interval is 5m and the EVERY interval is 10m, the CQ executes every ten minutes. Every ten minutes, the CQ runs a single query that covers the time range between now() and now() minus the EVERY interval, that is, the time range between now() and ten minutes prior to now().

This behavior is intentional and prevents the CQ from missing data between execution times.

Issue 2: If the FOR interval is less than the execution interval

If the `FOR` interval is less than the `GROUP BY time()` interval or, if specified, the `EVERY` interval, InfluxDB returns the following error:
error parsing query: FOR duration must be >= GROUP BY time duration: must be a minimum of <minimum-allowable-interval> got <user-specified-interval>

To avoid missing data between execution times, the FOR interval must be equal to or greater than the GROUP BY time() interval or, if specified, the EVERY interval.

Currently, this is the intended behavior. GitHub Issue #6963 outlines a feature request for CQs to support gaps in data coverage.

Continuous query management

Only admin users are allowed to work with CQs. For more on user privileges, see Authentication and Authorization.

Listing Continuous Queries

List every CQ on an InfluxDB instance with:

SHOW CONTINUOUS QUERIES

SHOW CONTINUOUS QUERIES groups results by database.

Example

The output shows that the `telegraf` and `mydb` databases have CQs: ``` > SHOW CONTINUOUS QUERIES name: _internal --------------- name query

name: telegraf

name query idle_hands CREATE CONTINUOUS QUERY idle_hands ON telegraf BEGIN SELECT min(usage_idle) INTO telegraf.autogen.min_hourly_cpu FROM telegraf.autogen.cpu GROUP BY time(1h) END feeling_used CREATE CONTINUOUS QUERY feeling_used ON telegraf BEGIN SELECT mean(used) INTO downsampled_telegraf.autogen.:MEASUREMENT FROM telegraf.autogen./.*/ GROUP BY time(1h) END

name: downsampled_telegraf

name query

name: mydb

name query vampire CREATE CONTINUOUS QUERY vampire ON mydb BEGIN SELECT count(dracula) INTO mydb.autogen.all_of_them FROM mydb.autogen.one GROUP BY time(5m) END


### Deleting Continuous Queries

Delete a CQ from a specific database with:

DROP CONTINUOUS QUERY <cq_name> ON <database_name>

`DROP CONTINUOUS QUERY` returns an empty result.
##### Example
<br>
Drop the `idle_hands` CQ from the `telegraf` database:

DROP CONTINUOUS QUERY “idle_hands” ON “telegraf”`


### Altering Continuous Queries

CQs cannot be altered once they're created.
To change a CQ, you must `DROP` and re`CREATE` it with the updated settings.

### Continuous Query Statistics

If `query-stats-enabled` is set to `true` in your `influxdb.conf` or using the `INFLUXDB_CONTINUOUS_QUERIES_QUERY_STATS_ENABLED` environment variable, data will be written to `_internal` with information about when continuous queries ran and their duration.
Information about CQ configuration settings is available in the [Configuration](/influxdb/v1.5/administration/config/#continuous-queries-settings-continuous-queries) documentation.

> **Note:** `_internal` houses internal system data and is meant for internal use.
The structure of and data stored in `_internal` can change at any time.
Use of this data falls outside the scope of official InfluxData support.

## Continuous Query Use Cases

### Downsampling and Data Retention

Use CQs with InfluxDB's
[retention policies](/influxdb/v1.5/concepts/glossary/#retention-policy-rp)
(RPs) to mitigate storage concerns.
Combine CQs and RPs to automatically downsample high precision data to a lower
precision and remove the dispensable, high precision data from the database.

See the
[Downsampling and Data Retention](/influxdb/v1.5/guides/downsampling_and_retention/)
guide for a detailed walkthrough of this common use case.

### Precalculating 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.

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

### Substituting for a `HAVING` Clause

InfluxQL does not support [`HAVING` clauses](https://en.wikipedia.org/wiki/Having_%28SQL%29).
Get the same functionality by creating a CQ to aggregate the data and querying
the CQ results to apply the `HAVING` clause.

> **Note:** InfluxQL supports [subqueries](/influxdb/v1.5/query_language/data_exploration/#subqueries) which also offer similar functionality to `HAVING` clauses.
See [Data Exploration](/influxdb/v1.5/query_language/data_exploration/#subqueries) for more information.

##### Example
<br>
InfluxDB does not accept the following query with a `HAVING` clause.
The query calculates the average number of `bees` at `30` minute intervals and
requests averages that are greater than `20`.

SELECT mean(“bees”) FROM “farm” GROUP BY time(30m) HAVING mean(“bees”) > 20


To get the same results:

**1. Create a CQ**
<br>
This step performs the `mean("bees")` part of the query above.
Because this step creates CQ you only need to execute it once.

The following CQ automatically calculates the average number of `bees` at
`30` minutes intervals and writes those averages to the `mean_bees` field in the
`aggregate_bees` measurement.

CREATE CONTINUOUS QUERY “bee_cq” ON “mydb” BEGIN SELECT mean(“bees”) AS “mean_bees” INTO “aggregate_bees” FROM “farm” GROUP BY time(30m) END


**2. Query the CQ results**
<br>
This step performs the `HAVING mean("bees") > 20` part of the query above.

Query the data in the measurement `aggregate_bees` and request values of the `mean_bees` field that are greater than `20` in the `WHERE` clause:

SELECT “mean_bees” FROM “aggregate_bees” WHERE “mean_bees” > 20


### Substituting for nested functions

Some InfluxQL functions
[support nesting](/influxdb/v1.5/troubleshooting/frequently-asked-questions/#which-influxql-functions-support-nesting)
of other functions.
Most do not.
If your function does not support nesting, you can get the same functionality using a CQ to calculate
the inner-most function.
Then simply query the CQ results to calculate the outer-most function.

> **Note:** InfluxQL supports [subqueries](/influxdb/v1.5/query_language/data_exploration/#subqueries) which also offer the same functionality as nested functions.
See [Data Exploration](/influxdb/v1.5/query_language/data_exploration/#subqueries) for more information.

##### Example
<br>
InfluxDB does not accept the following query with a nested function.
The query calculates the number of non-null values
of `bees` at `30` minute intervals and the average of those counts:

SELECT mean(count(“bees”)) FROM “farm” GROUP BY time(30m)


To get the same results:

**1. Create a CQ**
<br>
This step performs the `count("bees")` part of the nested function above.
Because this step creates a CQ you only need to execute it once.

The following CQ automatically calculates the number of non-null values of `bees` at `30` minute intervals
and writes those counts to the `count_bees` field in the `aggregate_bees` measurement.

CREATE CONTINUOUS QUERY “bee_cq” ON “mydb” BEGIN SELECT count(“bees”) AS “count_bees” INTO “aggregate_bees” FROM “farm” GROUP BY time(30m) END


**2. Query the CQ results**
<br>
This step performs the `mean([...])` part of the nested function above.

Query the data in the measurement `aggregate_bees` to calculate the average of the
`count_bees` field:

SELECT mean(“count_bees”) FROM “aggregate_bees” WHERE time >= <start_time> AND time <= <end_time>


## Further information

See the
[Downsampling and data retention](/influxdb/v1.5/guides/downsampling_and_retention/)
guide to see how to combine two InfluxDB features, CQs, and retention policies,
to periodically downsample data and automatically expire the dispensable high
precision data.

Kapacitor, InfluxData's data processing engine, can do the same work as
InfluxDB's CQs.
Check out [examples of continuous queries in Kapacitor](/kapacitor/v1.6/guides/continuous_queries/) to learn when
to use Kapacitor instead of InfluxDB and how to perform the same CQ
functionality with a TICKscript.

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