show tag keys returns empty rows

[scotloach]

Bug report

System info: InfluxDB 1.5 built from standard docker container, inmem indexes.

Steps to reproduce:

I updated from 1.4 to 1.5 on a large system with multiple databases.
Since the upgrade, I noticed that on some (but not all) of the databases, a "show tag keys from ." query returns the first row as an empty string, and then the actual tag keys associated with the measurement.

Looks like a regression from 1.4.

Here is actual output, with names redacted.

> show tag keys from "autogen"."meas"
name: meas
tagKey
------

tag1
tag2
tag3
tag4
tag5
tag6
tag7
tag8
tag9
tag10

[e-dard]

@scotloach do you have some test data that can be used to reproduce this issue?

[e-dard]

Reproduced with the following:

CREATE DATABASE ""
USE 
INSERT cpu,tag0=v\, value=1
SHOW TAG KEYS

Results in:

name: cpu
tagKey
------

tag0

Also, rather than this being a formatting issue, it looks like the index is seeing multiple tag keys:

> SHOW TAG KEY CARDINALITY
name: cpu
count
-----
2
>

Yet:

> SHOW SERIES
key
---
cpu,tag0=v\,
>

It's affecting both inmem and tsi1. I have confirmed it's not in 1.4.