Benchmarking Transaction Patterns

Benchmarking Transaction Patterns

Guides users through benchmarking, explaining, and comparing two formulations of the same transactional business workflow in CockroachDB: explicit multi-statement transactions versus single-statement CTE transactions. Focuses on performance under contention, fair test methodology, and result interpretation.

Complement to design skills: For general transaction design principles, see designing-application-transactions. For SQL syntax and query patterns, see cockroachdb-sql.

Core Concept

Under contention, the transaction formulation itself is a primary performance lever. The explicit model (multi-statement BEGIN/COMMIT) keeps the transaction open across round trips, widening the contention window. The CTE model (single-statement) collapses the same logic into one atomic statement, reducing transaction duration and retries.

Explicit Transaction Model

BEGIN;
SELECT balance FROM accounts WHERE id = $1;
-- Application decides whether transfer is allowed
UPDATE accounts SET balance = balance - $2 WHERE id = $1;
UPDATE accounts SET balance = balance + $2 WHERE id = $3;
INSERT INTO transfers (from_acct, to_acct, amount, created_at)
VALUES ($1, $3, $2, now());
COMMIT;

CTE Transaction Model

CockroachDB rejects multiple mutations of the same table in a single statement by default (sql.multiple_modifications_of_table.enabled), so the debit and credit are folded into one UPDATE using CASE.

WITH funded AS (
  SELECT 1 FROM accounts WHERE id = $1 AND balance >= $2
), upd AS (
  UPDATE accounts
  SET balance = CASE WHEN id = $1 THEN balance - $2 ELSE balance + $2 END
  WHERE id IN ($1, $3) AND EXISTS (SELECT 1 FROM funded)
  RETURNING id
), ins AS (
  INSERT INTO transfers (from_acct, to_acct, amount, created_at)
  SELECT $1, $3, $2, now() WHERE (SELECT count(*) FROM upd) = 2
  RETURNING id
)
SELECT id FROM ins;

Steps

1. Prepare the Benchmark Environment

Set up a dedicated test database and schema. Do not mix benchmark workloads with other traffic.

CREATE DATABASE IF NOT EXISTS bankbench;
USE bankbench;

CREATE TABLE accounts (
  id INT PRIMARY KEY,
  balance DECIMAL(18,2) NOT NULL DEFAULT 0
);

CREATE TABLE transfers (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  from_acct INT NOT NULL,
  to_acct INT NOT NULL,
  amount DECIMAL(18,2) NOT NULL,
  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
);

2. Seed the Test Data

Use multi-row UPSERT for efficient seeding. Single-row inserts distort setup cost.

INSERT INTO accounts (id, balance)
SELECT generate_series(1, 10000), 1000.00
ON CONFLICT (id) DO UPDATE SET balance = 1000.00;

3. Run the Explicit Transaction Benchmark

Execute with realistic concurrency. Example using pgbench (PostgreSQL-compatible):

# Create a pgbench script file: explicit_transfer.sql
# \set from_id random(1, 10000)
# \set to_id random(1, 10000)
# \set amount 10.00
# BEGIN;
# SELECT balance FROM accounts WHERE id = :from_id;
# UPDATE accounts SET balance = balance - :amount WHERE id = :from_id;
# UPDATE accounts SET balance = balance + :amount WHERE id = :to_id;
# INSERT INTO transfers (from_acct, to_acct, amount, created_at) VALUES (:from_id, :to_id, :amount, now());
# COMMIT;

pgbench -n -c 64 -j 8 -T 120 -f explicit_transfer.sql \
  "postgresql://root@localhost:26257/bankbench?sslmode=disable"

Record throughput (tps), retries, p50/p95/p99 latency, max latency, and failures.

4. Reset Between Runs for Fair Comparison

For a fair benchmark, reset account balances between explicit and CTE runs so table size, index size, and account state remain comparable.

UPDATE accounts SET balance = 1000.00;

5. Run the CTE Transaction Benchmark

Execute with the same concurrency, duration, and parameters as the explicit run:

# Create a pgbench script file: cte_transfer.sql containing the CTE query above
pgbench -n -c 64 -j 8 -T 120 -f cte_transfer.sql \
  "postgresql://root@localhost:26257/bankbench?sslmode=disable"

6. Compare Results

Always compare these metrics side by side:

7. Validate Benchmark Integrity

Before interpreting results, verify the benchmark ran cleanly:

-- Confirm expected transfer volume
SELECT COUNT(*) AS total_transfers FROM transfers;

# Check node liveness and start times (no node restarts mid-benchmark)
cockroach node status --certs-dir=<certs-dir>     # or --insecure for an insecure cluster

Benchmark Reference Results

In a reported high-contention run comparing the two models:

Extended runs preserved the same directional result at higher total volume, with the explicit model continuing to accumulate retries and occasional failures while the CTE model stayed at zero retries and zero failures.

Impact Summary

Decision Guidance

Prefer the Explicit Pattern When

• The business workflow truly cannot be expressed cleanly in one SQL statement
• Readability or staged business logic matters more than peak throughput
• The contention level is low enough that retry amplification is not the dominant cost

Prefer the CTE Pattern When

• The workflow is contention-heavy
• The operation is naturally atomic
• The application currently performs read-decide-write across multiple statements
• The main goal is higher throughput, lower retries, and more stable p95/p99 latency

Fair Benchmark Rules

1. Reset between runs for fair comparison so balances, table size, and index size stay consistent
2. Treat no-reset runs as a demo, not an apples-to-apples benchmark
3. Use --batch-size=1 when you want one business unit of work at a time for clean comparison
4. Compare the right metrics — always include throughput, retries, p50, p95, p99, max latency, and failures
5. Use multi-row UPSERT for seeding — single-row seeding distorts setup cost

Common Misconceptions

• "CTE always wins" — Only when the workflow is contention-sensitive and expressible as a single atomic statement.
• "SQL Activity showing waiting means CTE failed" — CTE reduces but does not eliminate contention. Compare throughput, tail latency, and retry profile.
• "Single-statement means no contention" — CTE narrows the contention window but does not eliminate it.

Safety Considerations

• Run benchmarks on dedicated test clusters, not production
• Reset data between runs for fair comparison
• Monitor cluster health during benchmark execution
• Use realistic but not destructive concurrency levels
• Validate that benchmark results transfer to your specific workload before making production changes

References

• CockroachDB Transactions Documentation
• Advanced Client-Side Transaction Retries
• Performance Best Practices
• Comparing Multi-Statement vs Single-Statement Transactions
• Set-Based Operations with CockroachDB
• Deep Dive into Transaction Retry Failures
• Troubleshooting CockroachDB Performance
• CockroachDB Transaction Demo
• CockroachDB Best Practices & Anti-Patterns Demo -- Demos 1-2 show retry patterns and contention scaling under concurrency