Normalize all of the SHOW commands so they allow both using ON to
specify the database and using the default database. Some commands would
require one and some would require the other and it was confusing when
using the query language.
Affected commands:
* SHOW RETENTION POLICIES
* SHOW MEASUREMENTS
* SHOW SERIES
* SHOW TAG KEYS
* SHOW TAG VALUES
* SHOW FIELD KEYS
The `SHOW MEASUREMENTS` and `SHOW TAG VALUES` cannot go through the
query engine to get the speed they need. They also only need access to
the database index and do not need access to specific shards. This
removes the query rewriting that was done to turn these two queries into
a select statement and reimplements them inside of the coordinator as an
interface on the TSDBStore.
This adds support for using regex expressions in SHOW TAG VALUES when
selecting the key. Also supporting the `!=` operation for the
comparison. Now you can do any of the following:
SHOW TAG VALUES WITH KEY != "region"
SHOW TAG VALUES WITH KEY =~ /region/
SHOW TAG VALUES WITH KEY !~ /region/
It also adds a new SetLiteral AST node that will potentially be used in
the future to allow set operations for other comparisons in the future.
Fixes#4532.
`SHOW TAG VALUES` output has been modified to print the measurement name
for every measurement and to return the output in two columns: key and
value. An example output might be:
> SHOW TAG VALUES WITH KEY IN (host, region)
name: cpu
---------
key value
host server01
region useast
name: mem
---------
key value
host server02
region useast
`measurementsByExpr` has been taught how to handle reserved keys (ones
with an underscore at the beginning) to allow reusing that function and
skipping over expressions that don't matter to the call.
Fixes#5593.
Jonathan A. Sternberg
lvheyang