Reduce an unoptimized-query-oracle test failure log to the simplest possible reproduction case. Use when you have unoptimized-query-oracle*.log files from a failed roachtest and need to find the minimal SQL to reproduce the bug.
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Reduce an unoptimized-query-oracle test failure log to the simplest possible reproduction case. Use when you have unoptimized-query-oracle*.log files from a failed roachtest and need to find the minimal SQL to reproduce the bug.
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Reduce Unoptimized Query Oracle Test Failure
Reduce an unoptimized-query-oracle test failure log to the simplest possible
reproduction case.
The unoptimized-query-oracle roachtest runs a series of random SQL statements to
create a random dataset, and then executes a random "Query of Interest" twice,
with different optimization settings. If the two executions return different
results, it indicates a bug in CockroachDB.
When to Use
Use this skill when:
You have a test failure from the unoptimized-query-oracle roachtest.
You need to find the minimal SQL to reproduce the test failure.
Step 1: Locate artifacts
Ask the user where the artifacts directory is.
Find the relevant files in the artifacts directory:
Test parameters: params.log (the parameters from the roachtest)
Test log: test.log (the log from the roachtest)
Failure log: failure*.log (the failure log from the roachtest)
Full SQL log: unoptimized-query-oracle*.log (the SQL statements that led to failure)
Query of interest log: unoptimized-query-oracle*.failure.log (containing
the query of interest and possibly more information about the failure)
Cockroach log: logs/1.unredacted/cockroach.log or
logs/unredacted/cockroach.log (contains the git commit)
Step 2: Determine test configuration
Determine the git commit from cockroach.log:
grep "binary: CockroachDB" cockroach.log
Look for the commit hash in the version string (e.g., cb94db961b8f55e3473f279d98ae90f0eeb0adcb).
Determine if runtime assertions are enabled by checking for:
"runtimeAssertionsBuild": "true" in params.log
or Runtime assertions enabled in test.log
Determine if metamorphic settings apply by looking for:
Check that the output reproduces the test failure described in the failure
log. There are many possible failure modes. Look for one of the following,
which should match the failure log:
Different results between the two executions of the "Query of Interest"
(which is the randomly generated SELECT statement repeated twice near the end
of the log, wrapped in various SET and RESET staements). These different
results could take the form of different result sets, or could also be an
error in one case and no error in the other case. This is an "oracle"
failure.
Or, internal error or assertion failure. Note the error message for the
reduce step.
Or, a panic. Note the error message for the reduce step.
Or, a timeout. Note the statement that timed out.
Troubleshooting
IMPORTANT: Many failures are nondeterministic, especially for multi-region
tests. If no failure happens on the first run, try up to 10 times before
concluding it doesn't reproduce.
It can be helpful at this point to compare the output with the failure*.log
which should show the failure from the original test run.
If the initial run fails to reproduce after 10 times, pause here and report to
the user that the failure cannot be reproduced, and show the command that was
tried. The user might have additional instructions.
If it looks like it reproduces, it's time to move on to the next step.
Step 6: Use the Reduce Tool
Build the reduce tool:
./dev build reduce
Prepare the Full SQL Log File again
For multi-region tests, remove \connect lines (they cause syntax errors in the
reduce tool):
IMPORTANT: Immediately save a backup of the reduce output before manual simplification:
cp reduced.sql reduced_original.sql
This provides a recovery point if the working file gets corrupted during simplification.
If the reduce tool fails to reproduce, pause here and report this to the
user. They might have additional instructions. Occasionally we have to modify
the reduce tool itself, if the test failure is not reproducing.
Step 7: Create Test Script and Determine Reproduction Rate
IMPORTANT: Many bugs are nondeterministic. Before manual simplification,
create a reusable test script and determine the reproduction rate.
Create a small test script (adjust as needed):
cat > test_repro.sh << 'EOF'#!/bin/bash# Test if reduced_v2.sql reproduces the error (exits on first success, up to 10 attempts)for i in {1..10}; doif ./cockroach demo --multitenant=false --nodes=9 --insecure \
--set=errexit=false --no-example-database --format=tsv \
-f reduced_v2.sql 2>&1 | grep -q "<error-pattern>"; thenecho"Run $i: REPRODUCED"exit 0
elseecho"Run $i: no error"fidoneecho"FAILED"
EOF
chmod +x test_repro.sh
For "oracle" failures, instead of checking for an error pattern, the test script
probably needs to isolate and diff the results of the two executions of the
"Query of Interest".
Run the test script to determine the reproduction rate. It's not always 100%.
This rate determines how many attempts you need when testing simplifications:
100% rate: Single attempt sufficient
50% rate: 2-3 attempts usually sufficient
10% rate: Need ~10 attempts to be confident
<5% rate: May need 20+ attempts
Note that in some cases, the following settings might need to be added back to
the reduced file to get a repro:
SET statement_timeout='1m0s';
SET sql_safe_updates =false;
If the reduced SQL fails to reproduce after 10 attempts, pause here and report
this to the user. They might have additional instructions.
Step 8: Manual Simplification
Now iteratively simplify the SQL while maintaining reproduction.
CRITICAL: For nondeterministic failures, you MUST test each simplification
with enough attempts based on the repro rate. A single failed attempt does NOT
mean the simplification broke the repro - it may just be nondeterminism.
Workflow for Each Simplification
Copy reduced.sql to reduced_v2.sql
Make ONE small change to reduced_v2.sql
Run ./test_repro.sh (which tests reduced_v2.sql)
If it reproduces: Copy reduced_v2.sql to reduced.sql, continue simplifying
If it doesn't reproduce after enough attempts: Discard reduced_v2.sql, try
a different change (i.e. backtrack).
This workflow avoids needing to restore files - you always keep the last working
version in reduced.sql.
IMPORTANT: Run copy, edit, and test as separate bash commands (not chained with &&).
This reduces the number of permission checks.
What to Try Removing (in rough order)
Query projections and aggregations - Simplify SELECT list to just essential columns
Query predicates - Simplify WHERE clause
Indexes - Try removing secondary indexes
Query joins - Simplify WHERE clause
Columns from CREATE TABLE - Remove columns not referenced in the failing query
Weird characters - Remove or replace non-ASCII characters from names and data
other SQL simplifications
For "oracle" failures, when editing the Query of Interest, be sure to edit
BOTH copies of the Query of Interest so that they are identical. Otherwise
it won't be an apples-to-apples comparison when diffing the result sets.
Common Required Elements
These often cannot be removed:
Optimizer random seed: SET testing_optimizer_random_seed = <value> - this
specific value often cannot be changed, as it determines which optimizer rules
are disabled
Optimizer rule probability: SET testing_optimizer_disable_rule_probability
affects query plan selection
Specific RESET/SET sequences for optimizer settings, such as distsql and vectorize
Certain indexes (affect query plans)
Multi-node setup (--nodes=9) for distributed query bugs (though try
single-node first - it may work and is simpler)
Discard reduced_v2.sql (don't copy it to reduced.sql)
Verify reduced.sql still reproduces. If it doesn't, this means the repro is
nondeterministic. (It might have started out nondeterministic, or might have
become nondeterministic over the course of simplification.) Try reproducing
it 10 times and note the new repro rate. Use the new repro rate to adjust the
number of repro attempts during each simplification step going forward.
Try a DIFFERENT simplification
Never continue simplifying from a broken state.
If you get stuck (i.e. cannot reproduce again after backtracking), stop and
report to the user with the exact command you were trying.
Step 9: Final Verification and Output
After about 20 minutes of simplification, or if there are no more
simplifications after backtracking a few times, it's time to stop.
Run reproduction 10+ times to confirm stability and determine final repro rate
Document the minimal reproduction steps
Note which elements were required vs optional
Output
The final output should include two files that can be shown to the user:
reduced.sql - The minimal SQL script that reproduces the bug
bisect_run.sh - A script for use with git bisect run
Write the output in such a way that it could be copied and pasted into a
terminal.
Example Output Format
(The commands in this output should be edited to match what was necessary to
reproduce.)
# Minimal Reproduction# reduced.sqlcat > reduced.sql << 'EOF'
CREATE TABLE t ();
SET testing_optimizer_random_seed = 1234567890;
SET testing_optimizer_disable_rule_probability = 0.5;
SELECT ...;
EOF
# bisect_run.shcat > bisect_run.sh << 'EOF'#!/bin/bash# Git bisect run script# Exit codes: 0=good (bug not present), 1=bad (bug present), 125=skip (build failed)
REPO_DIR="/path/to/cockroach"
REPRO_SQL="/path/to/reduced.sql"cd"$REPO_DIR" || exit 125
echo"=== Testing commit $(git rev-parse --short HEAD) ==="# Build (use --crdb_test if runtime assertions were enabled in the original test)if ! ./dev build short -- --crdb_test 2>&1 | grep -q "Successfully built"; thenecho"BUILD FAILED - skipping"exit 125
fi# Test for bug (try 3 times for flaky bugs)for i in {1..3}; doif ./cockroach demo --multitenant=false --insecure \
--set=errexit=false --no-example-database --format=tsv \
-f "$REPRO_SQL" 2>&1 | grep -q "<error-pattern>"; thenecho"BUG PRESENT - marking as BAD"exit 1
fidoneecho"Bug not present - marking as GOOD"exit 0
EOF
chmod +x bisect_run.sh
# Command to reproduce
git checkout <commit-hash>
./bisect_run.sh
# Command to bisect
git bisect start ...
git bisect run bisect_run.sh
# Failure# <paste stacktrace or relevant failure details here># Repro rate: ~X% (may need multiple attempts)
After showing this output, ask the user if they want to try reproducing the
bug on master branch.
Optional Step 10: Check if Bug is Fixed on Master
Before bisecting, check whether the bug has already been fixed on master.
Run this a few times to account for flakiness. Note whether the bug reproduces
on master or not.
Optional Step 11: Bisect
If the user wants to find the commit that introduced or fixed the bug, use
git bisect.
If the Bug is Already Fixed on Master
Bisect to find the fix commit (the first commit where the bug no longer
reproduces). Use custom terms since the "good" commit (master) is newer than the
"bad" commit:
git bisect start --first-parent --term-old=broken --term-new=fixed
git bisect broken <commit-where-bug-exists> # e.g., the original failing commit
git bisect fixed master # master is fixed
git bisect run ./bisect_run.sh
# When done
git bisect reset
Note: The --first-parent option follows only merge commits on the main
branch, avoiding detours into feature branches. The bisect script must return 0
when the bug is NOT present (fixed) and 1 when the bug IS present (broken).
If the Bug Still Exists on Master
Bisect to find the regression commit (the first commit where the bug was
introduced):
git bisect start --first-parent
git bisect good <known-good-commit> # e.g., a previous release tag
git bisect bad master # master has the bug
git bisect run ./bisect_run.sh
# When done
git bisect reset
The bisect will identify the commit that introduced or fixed the bug.
Finding a Good Commit
If you don't know a good commit (where the bug doesn't exist), you can jump back
in time to find one.
# Find a commit from ~6 months ago on the main branch
git rev-list --first-parent -1 --before="6 months ago" HEAD
Test whether the bug exists at that commit. If not, use it as the good commit
for bisect. If the bug still exists, try going back further in time, but don't
go back further than 1 year.
If a known good commit can't be found within 1 year, stop and report this to
the user.
