feat(mutations): flip default mutations-pipeline-threshold from 0 to 1

Engages the per-predicate mutation pipeline by default so that systest
runs (and any other test suites that don't override feature-flags)
exercise the pipeline path on every mutation, not the legacy serial
path. Threshold of 1 means "any mutation with ≥1 edge takes the
pipeline" — i.e. always on.

This is a deliberate ramp toward shipping the pipeline. Operators who
want to opt small interactive mutations out of the pipeline (where
benches showed ~2x slowdown for ≤10-edge txns) can set a higher
threshold:

  dgraph alpha --feature-flags="mutations-pipeline-threshold=200"

To turn the pipeline fully off, set 0.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Author: matthewmcneely

worker/server_state.go

@@ -43,7 +43,7 @@ const (
 		`lambda-url=;`
 	CacheDefaults        = `size-mb=4096; percentage=40,40,20; remove-on-update=false`
 	FeatureFlagsDefaults = `normalize-compatibility-mode=; enable-detailed-metrics=false; ` +
-		`log-slow-query-threshold=0; mutations-pipeline-threshold=0`
+		`log-slow-query-threshold=0; mutations-pipeline-threshold=1`
 )
 
 // ServerState holds the state of the Dgraph server.

x/config.go

@@ -141,12 +141,13 @@ type WorkerOptions struct {
 	// MutationsPipelineThreshold gates the per-predicate mutation pipeline
 	// in applyMutations. A mutation runs through the pipeline only when
 	// MutationsPipelineThreshold > 0 and len(m.Edges) >= the threshold;
-	// otherwise it falls back to the legacy serial path. Set to 0 (default)
-	// to disable the pipeline entirely. Set to 1 to always use the pipeline.
-	// The pipeline pays goroutine spin-up cost per predicate, so small
+	// otherwise it falls back to the legacy serial path. Set to 0 to
+	// disable the pipeline entirely. Set to 1 (default) to always use it.
+	// The pipeline pays goroutine spin-up cost per predicate, so tiny
 	// mutations are slower on it; bulk multi-predicate mutations are
-	// faster — pick a value above the per-mutation edge count where the
-	// crossover happens for your workload (~100 in benchmarks here).
+	// faster — set to a value above the per-mutation edge count where the
+	// crossover happens for your workload (~100 in benchmarks here) if
+	// you want only large mutations to take the pipeline path.
 	// Plumbed via the "feature-flags" superflag as
 	// "mutations-pipeline-threshold".
 	MutationsPipelineThreshold int