This adds query syntax support for subqueries and adds support to the
query engine to execute queries on subqueries.
Subqueries act as a source for another query. It is the equivalent of
writing the results of a query to a temporary database, executing
a query on that temporary database, and then deleting the database
(except this is all performed in-memory).
The syntax is like this:
SELECT sum(derivative) FROM (SELECT derivative(mean(value)) FROM cpu GROUP BY *)
This will execute derivative and then sum the result of those derivatives.
Another example:
SELECT max(min) FROM (SELECT min(value) FROM cpu GROUP BY host)
This would let you find the maximum minimum value of each host.
There is complete freedom to mix subqueries with auxiliary fields. The only
caveat is that the following two queries:
SELECT mean(value) FROM cpu
SELECT mean(value) FROM (SELECT value FROM cpu)
Have different performance characteristics. The first will calculate
`mean(value)` at the shard level and will be faster, especially when it comes to
clustered setups. The second will process the mean at the top level and will not
include that optimization.
Instead of having the parser set the defaults, the command will set the
defaults so that the constants for that are actually used. This way we
can also identify which things the user provided and which ones we are
filling with default values.
This allows the meta client to be able to make smarter decisions when
determining if the user requested a conflict or if the requested
capabilities match with what is currently available. If you just say
`CREATE DATABASE WITH NAME myrp`, the user doesn't really care what the
duration of the retention policy is and just wants to use the default.
Now, we can use that information to determine if an existing retention
policy would conflict with what the user requested rather than returning
an error if a default value ever gets changed since the meta client
command can communicate intent more easily.
Stuart Carnie
Joe LeGasse
Jonathan A. Sternberg
Cory LaNou
David Norton
Jason Wilder