[DO NOT MERGE] Rule group select merging

[alanprot]

Rule Group Select Merging — Reduce redundant ingester queries

What

When a rule group has multiple rules querying the same metric with overlapping matchers (e.g., different label filter variants), this optimization pre-fetches the broadest query once and serves individual rules from the cached result with local filtering.

Why

Rule groups often contain multiple rules querying the same metric with slightly different label filters (e.g., per-environment, per-deployment variant). Each rule independently queries the ingester even when one rule's matchers are a superset of the others. This results in redundant postings resolution and series fetching that scales linearly with the number of variants.

How

Uses the existing GroupEvalIterationFunc hook — zero vendored Prometheus changes. The QueryFunc is wrapped at construction time to check context for a prefetch cache, and the iteration func injects the cache via context.

Flow per group evaluation:

1. Plan — Parse all rules' PromQL expressions, group by metric name + expression structure, find superset matchers (e.g., =~".*" covers "blue" and "green")
2. Pre-fetch — Evaluate the broadest rule's full expression once via the existing QueryFunc
3. Evaluate — Subsequent rules are served from the cached Vector, filtered by their specific matchers

Configuration

ruler:
  select_merger_enabled: true
  select_merger_min_rules: 3  # minimum rules on same metric to trigger merging

Flags: --ruler.select-merger-enabled, --ruler.select-merger-min-rules

Safety

• Disabled by default — opt-in via config flag
• Graceful fallback — if pre-fetch errors or no superset exists, rules evaluate normally
• Only merges same expression structure — rate(m{...}[5m]) and sum(m{...}) are never merged
• Only merges when superset rule exists — job="api" + job="web" (no superset) won't merge

Benchmark

BenchmarkSelectMerger/without_merging    114 queries/op    1.60ms/op    11971 allocs/op
BenchmarkSelectMerger/with_merging        38 queries/op    1.30ms/op     8557 allocs/op

66% query reduction, 19% faster, 28% fewer allocations.

Files

File	Change
pkg/ruler/select_merger.go	Core planner: parse rules, group by metric+structure, compute merged matchers
pkg/ruler/prefetch_queryable.go	Cache, QueryFunc wrapper, vector filtering
pkg/ruler/manager.go	Wire iteration func when config enabled
pkg/ruler/ruler.go	Config fields
pkg/ruler/compat.go	Wrap QueryFunc with context-aware select merger