Add Postgres support for like-for-like benchmark comparisons

Gemfile

@@ -4,6 +4,7 @@ gem "railties", "~> 8.0.0"
 gem "activerecord", "~> 8.0.0"
 gem "activejob", "~> 8.0.0"
 gem "trilogy"
+gem "pg"
 
 case (sq_source = ENV.fetch("SOLID_QUEUE_SOURCE", "upstream"))
 when "upstream"

README.md

@@ -7,9 +7,11 @@ reports.
 ## Prerequisites
 
 - [mise](https://mise.jdx.dev) for the pinned Ruby version
-- Docker for MySQL 8.0
+- Docker for MySQL 8.0 and Postgres 16
+- `libpq` dev headers on the host, to compile the `pg` gem's native extension
+  (`libpq-dev` on Debian/Ubuntu, `postgresql` via Homebrew on macOS)
 
-Workers run on the host. Only MySQL is containerized.
+Workers run on the host. Only the database (MySQL or Postgres) is containerized.
 
 ## Setup
 
@@ -20,25 +22,28 @@ mise run setup
 
 If mise reports that `mise.toml` is not trusted, run `mise trust` once in the checkout, then retry.
 
-`setup` bundles the default upstream source and pulls the MySQL image.
+`setup` bundles the default upstream source and pulls both the MySQL and Postgres images.
 
 ## Run a Benchmark
 
 ```sh
-# latest official gem
+# latest official gem, against MySQL (default)
 bin/bench run baseline --source upstream
 
+# same run against Postgres, for a like-for-like comparison
+bin/bench run baseline --source upstream --database postgres
+
 # pinned RubyGems release
 bin/bench run baseline --source upstream@1.2.4
 
 # local fork; must be clean unless --allow-dirty is passed
 bin/bench run sprawl --source path:~/Projects/solid_queue
 ```
 
-Each run starts a fresh MySQL volume, loads schema from the selected gem source, starts the
-solid_queue supervisor, runs the scenario, waits for drain, and writes
-`results/<run-id>/result.json` plus logs. Results include the resolved gem version and, for
-`path:` sources, the git SHA.
+Each run starts a fresh database volume (MySQL or Postgres, per `--database`; default `mysql`),
+loads schema from the selected gem source, starts Solid Queue workers, runs the scenario, waits
+for drain, and writes `results/<run-id>/result.json` plus logs. Results include the resolved gem
+version, the database engine used, the process launcher, and, for `path:` sources, the git SHA.
 
 ## Scenarios
 
@@ -59,17 +64,28 @@ bin/bench run sprawl --source path:~/Projects/solid_queue --set seeds=5 --set fa
 
 ## Profiles and Topology
 
-Profiles live in `profiles/*.yml` and bundle MySQL resources with worker topology so comparisons
-stay comparable. The default profile uses MySQL pinned to 1 CPU / 1 GB, 10 worker processes x 2
-threads, and 1 dispatcher.
+Profiles live in `profiles/*.yml` and bundle database resource limits with worker topology so
+comparisons stay comparable, independent of which engine a run targets. The default profile
+pins the database container to 1 CPU / 1 GB, with 10 worker processes x 2 threads and 1
+dispatcher.
 
 ```sh
 bin/bench run baseline --source upstream --profile default
 bin/bench run baseline --source upstream --profile smoke
-bin/bench run baseline --source upstream --workers 50 --mysql-cpus 2 --mysql-memory 2g
+bin/bench run baseline --source upstream --workers 50 --db-cpus 2 --db-memory 2g
+bin/bench run baseline --source upstream --database postgres --profile default
 ```
 
-`bin/bench compare` refuses profile mismatches unless `--force` is passed.
+`bin/bench compare` refuses profile mismatches unless `--force` is passed. Comparing a MySQL run
+against a Postgres run under the same profile is not treated as a mismatch — the report simply
+shows which engine each side used.
+
+By default, the harness uses Solid Queue's fork supervisor. On macOS with Postgres, the harness
+uses `direct` launching instead: each worker and dispatcher is started as a fresh Ruby process to
+avoid the `pg`/libpq crashes seen when forked children connect after Rails has booted. The chosen
+launcher is recorded in `result.json` under `profile.process_launcher`, so mixed launcher
+comparisons are guarded like other topology changes. Override with
+`BENCH_SOLID_QUEUE_LAUNCHER=supervisor` or `BENCH_SOLID_QUEUE_LAUNCHER=direct` when needed.
 
 ## Compare Runs
 
@@ -78,8 +94,9 @@ bin/bench list
 bin/bench compare results/<A>/result.json results/<B>/result.json
 ```
 
-Reports are written to `reports/<A>__vs__<B>/report.md` and `report.html`. They include metric
-deltas, MySQL CPU and ready-depth overlay charts, and top SQL statements by total database time.
+Reports are written to `reports/<A>__vs__<B>/report.md` and `report.html`. They include which
+database engine each run used, metric deltas, DB CPU and ready-depth overlay charts, and top SQL
+statements by total database time.
 
 Use `--force` only when you intentionally want to compare different profiles:
 
@@ -99,9 +116,9 @@ Repeats keep every raw result and print median throughput across completed runs.
 
 - Throughput, including a per-second series
 - Latency percentiles for enqueue-to-start and enqueue-to-finish
-- MySQL container CPU samples from `docker stats`
+- Database container CPU samples from `docker stats`
 - Queue depth samples for ready, scheduled, claimed, blocked, failed, and completed jobs
-- Top statement digests from MySQL `performance_schema`
+- Top statement digests from MySQL `performance_schema` or Postgres `pg_stat_statements`
 
 ## Adding a Scenario
 

docker-compose.yml

@@ -8,12 +8,30 @@ services:
       MYSQL_ROOT_PASSWORD: bench
       MYSQL_DATABASE: bench
     command: --performance-schema=ON --max-connections=500
-    cpus: "${BENCH_MYSQL_CPUS:-1.0}"
-    mem_limit: "${BENCH_MYSQL_MEMORY:-1g}"
+    cpus: "${BENCH_DB_CPUS:-1.0}"
+    mem_limit: "${BENCH_DB_MEMORY:-1g}"
     ports:
       - "127.0.0.1:${BENCH_MYSQL_PORT:-13306}:3306"
     healthcheck:
       test: ["CMD", "mysqladmin", "ping", "-h", "127.0.0.1", "-pbench"]
       interval: 2s
       timeout: 2s
       retries: 60
+
+  postgres:
+    image: postgres:16
+    container_name: sq-bench-postgres
+    environment:
+      POSTGRES_USER: bench
+      POSTGRES_PASSWORD: bench
+      POSTGRES_DB: bench
+    command: postgres -c shared_preload_libraries=pg_stat_statements -c max_connections=500
+    cpus: "${BENCH_DB_CPUS:-1.0}"
+    mem_limit: "${BENCH_DB_MEMORY:-1g}"
+    ports:
+      - "127.0.0.1:${BENCH_POSTGRES_PORT:-15432}:5432"
+    healthcheck:
+      test: ["CMD", "pg_isready", "-U", "bench"]
+      interval: 2s
+      timeout: 2s
+      retries: 60