spanconfig/rangefeed: investigate microbenchmark regression

[irfansharif]

Describe the problem

One of our microbenchmarks show additional memory usage and was bisected to #74555. See #74555 (comment).

To Reproduce

I ran the following and used go tool pprof -http localhost:8082 -diff_base=mem-old.out -normalize mem-new.out to see where the differences are coming from.

dev bench pkg/bench -f='BenchmarkSQL/MultinodeCockroach/InsertSecondaryIndex/count=1000' --count=10 --bench-mem --timeout=24h -v --test-args '-test.memprofile=mem.out'

Looks like it's additional rangefeed event processing. I'm not sure off-hand why; I suspect it's got something to do with ranges having to enable rangefeeds by default when no targeted config is available.

Expected resolution

Pin down exactly why we're doing more processing, and whether it's expected.

Jira issue: CRDB-14939

[nvanbenschoten]

We return true from Replica.isRangefeedEnabled when !r.mu.spanConfigExplicitlySet:

cockroach/pkg/kv/kvserver/replica.go

Lines 1043 to 1045 in 81d3dc1

	if !r.mu.spanConfigExplicitlySet {
	return true
	}

Is this unintentionally enabling rangefeed and MVCCLogicalOp publication on most ranges?

[irfansharif]

Discussed internally: spanConfigExplicitlySet is not whether a span config exists for the range or not, it’s whether the current replica’s span config has been applied from a rangefeed on the spanconfig table. So we're not enabling rangefeed + MVCCLogicalOp publication on most ranges. That said, this logic does have weird artifacts in test code that use scratch ranges for ex. where unless the ConfigureScratchRange testing knob is set to true, we don’t use span configs in that portion of the keyspan. So automatically have rangefeed enabled.

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