CockroachDB BenchmarkInsights Failure: A Deep Dive

Aug 3, 2025 by Omar Yusuf 51 views

Investigating CockroachDB BenchmarkInsights Failure in pkg/sql/sqlstats

Hey guys,

We've got a bit of a situation here – the BenchmarkInsights test in CockroachDB's pkg/sql/sqlstats package is failing. This article dives deep into the issue, exploring the error, the stack trace, and the potential causes. We'll break it down in a way that's easy to understand, even if you're not a CockroachDB expert.

What's the Issue?

The BenchmarkInsights test within the pkg/sql/sqlstats package of CockroachDB has been failing. This test is crucial for evaluating the performance and stability of insights generated from SQL statistics. The failure was observed in the weekly WIP microbenchmark runs on the master branch, specifically at commit ca0d4076c381609d6487040141c87e36a68b211f. The error manifests as a panic with an "index out of range" message, indicating a problem with array or slice access within the test.

The specific error is a panic: runtime error: index out of range [1] with length 1. This means the code is trying to access an element at index 1 in a slice or array that only has a length of 1. Think of it like trying to grab the second item from a list that only has one thing in it – not gonna work! This type of error usually points to a logic flaw in the code where the index being accessed is not properly validated against the size of the data structure.

Diving into the Details

The error occurred during a microbenchmark run, which are designed to measure the performance of small, isolated pieces of code. This particular benchmark, BenchmarkInsights, is responsible for testing the insights generated from SQL statistics. SQL statistics are vital for the query optimizer, helping CockroachDB make intelligent decisions about how to execute queries efficiently. So, any issues in this area can potentially impact the overall performance of the database.

To make it super clear, let's break down why this is important. CockroachDB uses SQL statistics to figure out the best way to run your queries. It's like having a GPS for your database – it helps find the fastest route. If the stats are off or the insights derived from them are incorrect, the database might take a longer, more inefficient path, slowing things down. That's why these microbenchmarks are so crucial – they help us catch these issues early.

The Stack Trace: A Detective's Clues

Let's examine the stack trace. Stack traces are like a breadcrumb trail that leads us to the source of the error. They show the sequence of function calls that led to the panic. In this case, the stack trace points us to the following:

goroutine 74 gp=0xc0004b6d20 m=9 mp=0xc000108808 [running]:
panic({0x1b9c020?, 0xc000058090?})
	GOROOT/src/runtime/panic.go:811 +0x168 fp=0xc0000dfd50 sp=0xc0000dfca0 pc=0x485948
runtime.goPanicIndex(0x1, 0x1)
	GOROOT/src/runtime/panic.go:115 +0x74 fp=0xc0000dfd90 sp=0xc0000dfd50 pc=0x44b7b4
github.com/cockroachdb/cockroach/pkg/sql/sqlstats/insights_test.BenchmarkInsights.func1(0xc000325088)
	pkg/sql/sqlstats/insights/insights_test.go:68 +0x696 fp=0xc0000dff10 sp=0xc0000dfd90 pc=0x164b996
testing.(*B).runN(0xc000325088, 0x1)
	GOROOT/src/testing/benchmark.go:219 +0x190 fp=0xc0000dffa0 sp=0xc0000dff10 pc=0x6250b0
testing.(*B).run1.func1()
	GOROOT/src/testing/benchmark.go:245 +0x48 fp=0xc0000dffe0 sp=0xc0000dffa0 pc=0x625728
runtime.goexit({})
	src/runtime/asm_amd64.s:1700 +0x1 fp=0xc0000dffe8 sp=0xc0000dffe0 pc=0x48e101
created by testing.(*B).run1 in goroutine 68
	GOROOT/src/testing/benchmark.go:238 +0x90

From the stack trace, the key line is:

github.com/cockroachdb/cockroach/pkg/sql/sqlstats/insights_test.BenchmarkInsights.func1
	pkg/sql/sqlstats/insights/insights_test.go:68

This tells us that the panic occurred within the BenchmarkInsights test, specifically in the func1 function, located in the insights_test.go file at line 68. This is our ground zero – the exact location where the error occurred. Now we know where to focus our attention!

Code Snippet: Line 68 – The Scene of the Crime

Let's take a look at what's happening on line 68 of pkg/sql/sqlstats/insights/insights_test.go:

// Assuming this is a simplified representation of the actual code
func BenchmarkInsights(b *testing.B) {
  // ... some setup ...
  b.Run("numSessions=1", func(b *testing.B) {
    // ... more setup ...
    b.Run("numSessions=1", func(b *testing.B) {
      b.Run("numSessions=1", func(b *testing.B) {
        for i := 0; i < b.N; i++ {
          // line 68
          someSlice[1] = someValue // Potential index out of range
        }
      })
    })
  })
  // ... more tests ...
}

Important: This is a simplified representation. The actual code might be more complex, but the core issue remains the same: an attempt to access an index that might be out of bounds.

This line, someSlice[1] = someValue, is the likely culprit. If someSlice has a length of 1 or less, accessing index 1 will cause the "index out of range" panic. The benchmark test might be setting up the slice incorrectly or not handling cases where the slice is smaller than expected.

Examining the Logs: More Clues Uncovered

The logs preceding the fatal error provide additional context:

goos: linux
goarch: amd64
cpu: Intel(R) Xeon(R) CPU @ 2.60GHz
BenchmarkInsights
BenchmarkInsights/numSessions=1
BenchmarkInsights/numSessions=1-32         	33837946	        35.41 ns/op	       0 B/op	       0 allocs/op
BenchmarkInsights/numSessions=10

These logs indicate that the benchmark was running with different numbers of sessions (numSessions). The error seems to have occurred specifically when numSessions=1. This suggests that the issue might be related to how the code handles scenarios with a single session, potentially leading to a slice or array with an unexpected size.

Potential Causes: Putting the Pieces Together

Based on the error message, stack trace, and logs, here are some potential causes for the failure:

Incorrect Slice Initialization: The slice someSlice might not be initialized correctly, resulting in a length less than 2 when numSessions=1. This is a classic off-by-one error.
Logic Error in Index Calculation: There might be a flaw in the logic that calculates the index being accessed. If the calculation doesn't account for the size of the slice, it could lead to out-of-bounds access.
Race Condition: Although less likely in a benchmark test, a race condition could theoretically lead to inconsistent slice sizes. However, this is less probable given the nature of the test.
Edge Case Handling: The code might not be properly handling an edge case where numSessions=1, leading to an unexpected state for the slice.

How to Investigate Further: Time to Dig Deeper

To pinpoint the exact cause and fix the issue, here are the steps we can take:

Inspect the Code: Carefully examine the BenchmarkInsights function and the surrounding code, paying close attention to how someSlice is initialized and how indices are calculated. Look for any potential off-by-one errors or logic flaws.
Add Logging: Insert logging statements to print the size and contents of someSlice before the line that panics. This will help us understand the state of the slice at the time of the error.
Run the Benchmark Locally: Re-run the benchmark locally with the -count=1 flag to reproduce the error in a controlled environment. This allows for easier debugging.
Use a Debugger: Employ a debugger to step through the code line by line, inspecting variables and data structures as the benchmark runs. This is a powerful way to identify the root cause.
Write a Unit Test: Create a unit test that specifically targets the scenario where numSessions=1. This will help ensure that the issue is fixed and prevent regressions in the future.

Repair Input Keywords

Okay, let's make sure we're all on the same page with some key terms and questions:

BenchmarkInsights Failure: What caused the BenchmarkInsights test to fail?
Index out of Range: What does the "index out of range" error mean?
Stack Trace Analysis: How can the stack trace help in debugging the error?
pkg/sql/sqlstats: What is the role of the pkg/sql/sqlstats package?
ca0d4076c381609d6487040141c87e36a68b211f: What is the significance of this commit hash?
Insights Discussion: What are the insights being discussed in this context?
numSessions=1: Why does the error occur specifically with numSessions=1?
Line 68 insights_test.go: What is the code at line 68 in insights_test.go doing?
goroutine 74: What does the goroutine ID 74 signify in the stack trace?
*Testing.(B).runN: What is the function Testing.(*B).runN responsible for?

Conclusion: Solving the Puzzle

The BenchmarkInsights failure in CockroachDB's pkg/sql/sqlstats package is a critical issue that needs to be addressed. By carefully analyzing the error message, stack trace, and logs, we can narrow down the potential causes. Guys, it seems like the error likely stems from an index out-of-range panic due to incorrect slice access within the benchmark test, particularly when dealing with a single session (numSessions=1). Further investigation is needed to pinpoint the exact line of code and implement a fix.

By following the investigation steps outlined above, we can identify the root cause, fix the bug, and ensure the stability and performance of CockroachDB's SQL statistics insights. This, in turn, helps maintain the overall efficiency and reliability of the database.

This detailed investigation is crucial for maintaining the quality and robustness of CockroachDB. These benchmark failures, while initially alarming, provide valuable opportunities to strengthen the database and ensure its continued performance and reliability. So, let's roll up our sleeves and get to the bottom of this!