Enable MySQL slow query log for performance diagnostics (#123)

Author: Wiktor Maj

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 * Dependabot updates for `github-actions` and `terraform` ecosystems. (#29)
 * CI workflow to build and push service images to GitHub Container Registry on master merge. (#97)
 * Pull-first with local build fallback for container images during deployment. (#97)
+* MySQL slow query log for queries exceeding 10 seconds or not using indexes. (#76)
 * Strengthen MySQL superset password: secrets module, expanded charset, 24 chars. (#89)
 * SQL Lab query row limits, timeout caps, and validation timeout. (#81)
 * Superset metastore and explore form data caching via Redis. (#79)

docs/PERFORMANCE.md

@@ -39,6 +39,8 @@ The MySQL server is configured for cluster workloads with the following performa
 | `binlog_transaction_dependency_tracking` | `WRITESET` | Enables parallel replication on secondaries |
 | `max_connections` | `50` | Limits concurrent connections per MySQL node |
 | `max_connect_errors` | `50` | Blocks hosts after 50 failed connection attempts |
+| `slow_query_log` | `ON` | Logs queries exceeding `long_query_time` or not using indexes |
+| `long_query_time` | `10` | Threshold in seconds for slow query classification |
 
 ## Nginx Tuning
 
@@ -67,3 +69,5 @@ The following tools and endpoints are available for cluster monitoring and diagn
 - **Celery monitoring**: `celery inspect ping` and `celery inspect stats` provide worker status. See
   [Celery Async Queries](https://superset.apache.org/docs/configuration/async-queries-celery/).
 - **Keepalived logs**: available at `/opt/default/mysql_router/log/keepalived.log` on management nodes.
+- **MySQL slow query log**: available at `/var/log/mysql/slow-queries.log` on each MySQL node. Captures queries
+  exceeding 10 seconds and queries not using indexes.

services/mysql-server/mysql_config.cnf.tpl

@@ -14,6 +14,10 @@ ssl-key = "/etc/mysql/ssl/mysql_server_key.pem"
 require_secure_transport = "ON"
 max_connections = "50"
 max_connect_errors = "50"
+slow_query_log = "ON"
+long_query_time = "10"
+log_queries_not_using_indexes = "ON"
+slow_query_log_file = "/var/log/mysql/slow-queries.log"
 replica_parallel_workers = "4"
 replica_parallel_type = "LOGICAL_CLOCK"
 replica_preserve_commit_order = "ON"