Single-valued charts

[nblumhardt]

Show one value computed over the whole dashboard range - this might not necessarily be numeric, so that things like circuit breaker status (text) can be shown.

[nblumhardt]

This is coming along; it's a slightly more complex feature than it first appears to be ;-)

Because queries can be expensive, and the dashboard updates frequently, single-valued charts like the one above need to use time-slice caching in order to consume a reasonable amount of resources. Behind the scenes, a new query hint for window() lets Seq know that the query results will hop forward by a certain timespan, i.e. the dashboard interval, so future requests for the same query can re-use partial results.

In 4.1, this will only be used by the dashboard, but a future version might enable this transparently for some queries since the performance difference is substantial.

Remaining work:

• Result modifications - count(*) + 1, Round(min(Elapsed), 2) - etc.
• Get some troublemakers - first(), last(), mean(), percentile() and count(distinct()) - working smoothly
• Orderings

Once this is done, we'll also have a strong foundation to efficiently build rowset-value charts, pie charts and so-on.

[nblumhardt]

Done! The for window() hint makes some pretty aggressive caching possible.

Bare query (debug build):

First run with time slice caching enabled, as used for single-valued charts on the dashboard (slight slow-down):

Subsequent run - 650× faster than the bare query: