TL;DR InfluxDB Tech Tips - InfluxDB Field Keys Issues And Solutions

By Regan Kuchan / Developer
Dec 22, 2016

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In this weekly post we recap the most interesting InfluxDB field keys and TICK-stack related issues, workarounds, how-tos and Q&A from GitHub, IRC and the InfluxDB Google Group that you might have missed in the last week or so.

A Case of Missing Data

Q: My writes to InfluxDB are succeeding but my queries don’t return any results. I can see the database I created and the measurements that I’ve written to, but the actual data points seem to be in hiding. Do you have any advice?

A: Without knowing a bit more about your data setup and queries it’ll be hard to identify exactly what’s happening on your end. Here are a couple things that might explain why your queries aren’t returning anything:

The first and most common explanation involves retention policies (RP). InfluxDB automatically queries data in a database’s DEFAULT RP. If your data are stored in an RP other than the DEFAULT RP, InfluxDB won’t return any results unless you specify the alternative RP.

Another possible explanation has to do with your query’s time range. By default, most SELECT queries cover the time range between 1677-09-21 00:12:43.145224194 and 2262-04-11T23:47:16.854775806Z UTC. SELECT queries that also include a GROUP BY time() clause, however, cover the time range between 1677-09-21 00:12:43.145224194 and now(). If any of your data occur after now() a GROUP BY time() query will not cover those data points. Your query will need to provide an alternative upper bound for the time range if the query includes a GROUP BY time() clause and if any of your data occur after now().

The final and more obscure explanation involves schemas with identical field keys and tag keys. If a field key matches a tag key and your query only specifies the key, InfluxDB will assume that you are querying the field key. In some cases, this can make it seem as though your data are missing. You’ll need to use the :: syntax to differentiate between the field key and tag key.

A Study in *

Q: I’m including GROUP BY * in my query. Could you help me understand the output from my query? How does InfluxDB determine what to GROUP BY?

> SELECT MEAN("rache") FROM "scarlet" GROUP BY *
name: scarlet
tags: level=1, location=
time                  mean
----                  ----
1970-01-01T00:00:00Z  13

name: scarlet
tags: level=3, location=2
time                  mean
----                  ----
1970-01-01T00:00:00Z  2

name: scarlet
tags: level=4, location=5
time                  mean
----                  ----
1970-01-01T00:00:00Z  14

A: When you GROUP BY *, InfluxDB returns results for every possible tag set in the measurement. In your case, the scarlet measurement has three unique tag combinations:

  • level = 1 and location = ''
  • level = 3 and location = 2
  • level = 4 and location = 5

InfluxDB groups the data points in the scarlet measurement by the unique tag set combinations and calculates the average rache for each of those groups.

The Sign of the FOR

Q: I’m running a Continuous Query (CQ) and I’ve noticed that it misses some of my data points. My main problem seems to be that my data are arriving late; so data arrive for a specific time interval only after the CQ runs for that time interval. Is there a way to get around this? Here’s my CQ:

CREATE CONTINUOUS QUERY in_1888 ON treasure BEGIN SELECT MAX("darts") INTO "items" FROM "pearls" GROUP BY time(15m) END

A: Yes! There is a way to get around that behavior. You can use the advanced CQ syntax to configure the CQ’s time range. Your current CQ runs every 15 minutes and queries data that fall within the past 15 minutes. The CQ below still runs every 15 minutes, but it queries data that fall within the past 30 minutes:

CREATE CONTINUOUS QUERY in_1888 ON treasure ?RESAMPLE FOR 30m? BEGIN SELECT MAX("darts") INTO "items" FROM "pearls" GROUP BY time(15m) END

Check out the CQ documentation for examples of the advanced syntax and for additional information.

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