Summary from

• Understanding Prometheus Range Vectors
• The Anatomy of a PromQL Query

• What’s a Vector?
  • Since Prometheus is a timeseries database, all data is in the context of some timestamp. The series that maps a timestamp to recorded data is called a timeseries
  • a set of related timeseries is called a vector
  • Ex.
    • http_requests_total is a vector representing the total number of http requests received by a service
    • http_requests_total{code="200"} http_requests_total refers to the entire set of timeseries that are named that. And by appending a {code="200"}, we’re selecting a subset.
• Types of Vectors
  • Instant vector - a set of timeseries where every timestamp maps to a single data point at that “instant”
    • Imagine evaluating the expression http_requests_total at a given timestamp. http_requests_total is an instant vector selector that selects the latest sample for any time series with the metric name http_requests_total. More specifically, "latest" means "at most 5 minutes old and not stale", relative to the evaluation timestamp. So this selector will only yield a result for series that have a sample at most 5 minutes prior to the evaluation timestamp, and where the last sample before the evaluation timestamp is not a stale marker (an explicit way of marking a series as terminating at a certain time in the Prometheus TSDB).
  • Range vector - a set of timeseries where every timestamp maps to a “range” of data points, recorded some duration into the past.
    • Range vector is mostly used for graphs, where you want to show a PromQL expression over a given time range. A range query works exactly like many completely independent instant queries that are evaluated at subsequent time steps over a given range of time. Of course, this is highly optimized under the hood and Prometheus doesn't actually run many independent instant queries in this case.
• Differences
  • Instant vectors can be charted; Range vectors cannot. This is because charting something involves displaying a data point on the y-axis for every timestamp on the x-axis. Instant vectors have a single value for every timestamp, while range vectors have many of them. For the purpose of charting a metric, it is undefined1 how to show multiple data points for a single timestamp in a timeseries.
  • Instant vectors can be compared and have arithmetic performed on them; Range vectors cannot. This is also due to the way comparison and arithmetic operators are defined. For every timestamp, if we have multiple values, we don’t know how to add1 or compare them to another timeseries of a similar nature.
  • Range Vectors for counters. We take the instant vector and append our duration [15m]. This part is called the range selector and it transforms the instant vector into a range vector. We then use a function like increase which effectively subtracts the data point at the start of the range from the one at the end. increase(http_requests_total{code="200",handler="/api/v1/query"}[15m]) represent it is the increase in the total number of requests over the past fifteen minutes

PromQL doesn't allow specifying time range for the query. Instead, the time range is specified via HTTP API args passed to the corresponding endpoints:

• time query arg at /api/v1/query
• start and end query args at /api/v1/query_range

Such a design decision allows executing the same PromQL query at any timestamp or on any time range without the need to change the query itself.