≈472 tokens on demand. Your agent reads this file only when SKILL.md points to it.
RDS MySQL/MariaDB — Query Load Analysis & Explain Plan Review
Step 1: Get Top 5 Queries by Load
SELECT DIGEST_TEXT, COUNT_STAR, SUM_TIMER_WAIT/1000000000000 AS total_time_sec,
AVG_TIMER_WAIT/1000000000 AS avg_time_ms, SUM_ROWS_EXAMINED, SUM_ROWS_SENT,
FIRST_SEEN, LAST_SEEN
FROM performance_schema.events_statements_summary_by_digest
WHERE SCHEMA_NAME NOT IN ('mysql','information_schema','performance_schema','sys')
ORDER BY SUM_TIMER_WAIT DESC LIMIT 5;
Step 2: Run EXPLAIN on Each Query
EXPLAIN FORMAT=JSON <query>;
Step 3: Flag Upgrade-Impacting Patterns
🔴 Critical
| Pattern |
Why It Matters in 8.0 |
Action |
using_temporary_table: true |
TempTable engine replaces MEMORY. Overflow goes to mmap, not MyISAM. |
Tune temptable_max_ram. Monitor Created_tmp_disk_tables. |
using_filesort: true + large rows |
Sort algorithm changed. |
Benchmark on test instance. |
| GROUP BY implicit sort relied upon |
8.0 no longer implicitly sorts GROUP BY. |
Add explicit ORDER BY. |
🟡 Warning
| Pattern |
Why It Matters |
Action |
Block Nested Loop joins |
8.0 may replace with hash join. Usually faster. |
Test. Use optimizer_switch to disable hash_join if regression. |
| Derived table materialization |
8.0 improved derived table merging. |
Usually beneficial. Monitor. |
index_merge usage |
Behavior refined in 8.0. |
Verify same indexes used post-upgrade. |
🟢 Clean
| Pattern |
Notes |
Simple index lookups (ref, eq_ref, const) |
No change. |
Covering indexes (Using index) |
No change. |
Key MySQL 8.0 Optimizer Changes
- Hash joins for equi-joins without indexes
- TempTable engine replaces MEMORY for internal temp tables
- GROUP BY no longer implicitly sorts
- Descending indexes supported natively
- Updated cost model for I/O and memory
