Permanently dropping a field key

I accidentally inserted data for field key “791” and wish to delete it from the measurement.

> SHOW SERIES (just for L3)
name: L3
fieldKey fieldType
-------- ---------
791      float
P789     float
P790     float
P791     float
P792     float
P793     float
P795     float

So I copy the measurement and get what I am looking for.

> SELECT /P/ INTO "L3_copy2" FROM "L3" GROUP BY *
name: result
time written
---- -------
0    12752

> SHOW SERIES (just for L3_copy2)
name: L3_copy2
fieldKey fieldType
-------- ---------
P789     float
P790     float
P791     float
P792     float
P793     float
P795     float

> SELECT * FROM L3_copy2 LIMIT 3
name: L3_copy2
time                P789 P790  P791 P792  P793  P795 day month week year
----                ---- ----  ---- ----  ----  ---- --- ----- ---- ----
1539351900000000000                 208.2            5   10    41   2018
1539352800000000000      489.8      211.1 489.3      5   10    41   2018
1539353700000000000      491              490.8      5   10    41   2018
>

And drop the old measurement (and confirmed with SHOW MEASUREMENTS and SHOW FIELD KEYS that it is no longer present).

DROP MEASUREMENT “L3”

And write the data back to the original measurement name.

> SELECT * INTO "L3" FROM "L3_copy2" GROUP BY *
name: result
time written
---- -------
0    12752

But now…

> SHOW SERIES (just for L3)
name: L3
fieldKey fieldType
-------- ---------
791      float
P789     float
P790     float
P791     float
P792     float
P793     float
P795     float
day      string
month    string
week     string
year     string

> SELECT * FROM L3 LIMIT 3
name: L3
time                791 P789 P790  P791 P792  P793  P795 day day_1 month month_1 week week_1 year year_1
----                --- ---- ----  ---- ----  ----  ---- --- ----- ----- ------- ---- ------ ---- ------
1539351900000000000                     208.2                5           10           41          2018
1539352800000000000          489.8      211.1 489.3          5           10           41          2018
1539353700000000000          491              490.8          5           10           41          2018
> SELECT "791" FROM L3 LIMIT 3
>

As seen , FIELD KEY “791” still exists but has no data. Furthermore, the tag keys have been injected as field keys. I also tried SELECT /P/ INTO "L3" FROM "L3_copy2" GROUP BY * but got the same results. I’ve tried restarting influxd between queries but no change.

To make maters worse, after dropping L3, if I ever write data to L3, the measurement is recreated with the duplicated tag keys as field keys and erroneous “791” field key.

How can a field key be removed? How is a measurement permanently removed and started from scratch?