KWDB Performance Review

Optimize SQL query performance for KaiwuDB time-series and relational engines. Covers: EXPLAIN analysis, time-series optimization, pagination, cross-model queries. Trigger keywords: optimize query, slow query, explain, execution plan, performance, 性能, 查询优化. NOT for: DDL, schema design, deployment, DML writes.

Read the required reference files first.

Tiered Reference Architecture

Tier 1 (Always Read)

• references/key-rules.md - Core engine differences and anti-patterns
• references/query-analysis.md - EXPLAIN output interpretation

Tier 2 (High-Frequency Optimization)

• references/timeseries-optimization.md - Time-series query patterns
• references/pagination-optimization.md - Cursor-based pagination

Tier 3 (Medium-Frequency)

• references/relational-optimization.md - B-tree indexes, join optimization
• references/cross-model-optimization.md - Hybrid query optimization

Tier 4 (Low-Frequency)

• references/schema-tuning.md - Partition interval, TTL, encoding
• references/index-analysis.md - Index review for relational tables
• references/config-optimization.md - Storage configuration parameter optimization

When to Activate

Should trigger:

• "optimize this query" / "优化查询"
• "slow query" / "查询很慢" / "慢查询"
• "explain this query" / "执行计划"
• "query performance" / "查询性能"
• "KWDB query slow"
• "全表扫描" / "查询超时"
• "时序数据查询慢" / "传感器数据"
• "TIME_BUCKET" / "时间聚合"
• "索引优化" (relational tables only)
• "config optimization" / "配置优化"
• "parameter tuning" / "参数调优"
• "存储配置" / "参数调整"

Should NOT trigger:

• Schema design ("create table", "add index") → kwdb-schema-design
• Deployment/configuration questions
• DML write optimization ("fast INSERT")
• Non-KWDB databases

Engine Detection

Before optimizing, determine which engine the query targets:

text

TIME SERIES TABLE:
  - Has ts_column, primary_tags in CREATE TABLE
  - Cannot have secondary indexes
  - Query must include time range filter
  - Primary tag filter uses hash index

RELATIONAL TABLE:
  - Standard SQL table
  - Can have B-tree, inverted indexes
  - Standard SQL optimization applies

Ask user if unclear.

Workflow

Step 1: Parse EXPLAIN Output

Key indicators to look for:

Pattern	Time-Series	Relational	Action
Partition Filter: ts	Good	N/A	Time pruning working
Tag Filter: tag_col	Good	N/A	Hash index hit
Seq Scan in partition	Normal	Check size	Normal for small
Index Scan	N/A	Good	Index being used
Distribute: Shuffle	Warning	Varies	Cross-node traffic

Step 2: Identify Anti-Patterns

Time-Series Critical Issues:

• Missing time range filter -> full partition scan
• Fuzzy match on primary tag (LIKE, SUBSTRING) -> hash index miss
• SELECT * -> unnecessary column IO
• Large OFFSET pagination -> memory pressure
• Manual GROUP BY instead of TIME_BUCKET

Relational Issues:

• Seq Scan on large table -> missing index
• Nested loop on large sets -> bad join order
• Missing index on join column

Step 3: Provide Optimized Query

Always provide:

1. The anti-pattern being fixed
2. The rewritten query
3. Expected improvement in EXPLAIN

Step 4: Validate

Include EXPLAIN (ANALYZE) to verify the optimization works.

Step 5: Configuration Optimization (Conditional)

Only activate when:

1. User explicitly mentions "config optimization" / "parameter tuning" / "配置优化" / "参数调优", OR
2. SQL optimization steps (1-4) are exhausted and performance issues persist

Per-Parameter Trigger (review on demand, not full scan):

Parameter	Config Group	Trigger Condition
ts.compress.stage	Compression Group	User wants compression optimization or smaller disk space usage
ts.compress.algorithm	Compression Group	User wants compression optimization or smaller disk space usage
ts.compress.level	Compression Group	User wants compression optimization or smaller disk space usage
ts.rows_per_block.min_limit	Rows Per Block Group	User reports excessive small blocks from flushing, long write visibility delay, or high per-device data volume with low compression ratio
ts.rows_per_block.max_limit	Rows Per Block Group	User reports excessive small blocks from flushing, long write visibility delay, or high per-device data volume with low compression ratio
ts.compress.last_segment.enabled	Independent	User wants compression optimization or smaller disk space usage, or needs to optimize write performance
ts.block.lru_cache.max_limit	Independent	User wants to optimize overall query performance, or memory usage is too high
ts.last_cache_size.max_limit	Independent	User wants to optimize last-related SQL query performance, or memory usage is too high
ts.mem_segment_size.max_limit	Independent	Write performance optimization (after ts.compress.last_segment.enabled reviewed), or memory usage is too high (after ts.block.lru_cache.max_limit and ts.last_cache_size.max_limit reviewed)
ts.reserved_last_segment.max_limit	Independent	Frequent compaction triggers or disk space is tight
ts.compact.max_limit	Independent	User reports compaction backlog with significant CPU idle, or CPU usage is too high
ts.auto_vacuum.enabled	Independent	User wants to clean up data
ts.block_filter.sampling_ratio	Independent	User reports poor query performance with range conditions or null checks, suspects inefficient filter pushdown

Decision Tree:

1. Compression optimization / disk space reduction → Compression Group
   • Performance priority, disk sufficient → snappy/lz4, level=any, stage=1; extreme: stage=0
   • Disk space priority, CPU sufficient → zstd, level=high, stage=3
   • CPU usage too high → lz4, level=any, stage=1; if still high → stage=0
2. Excessive small blocks / write visibility delay / low compression ratio → Rows Per Block Group
   • High-throughput write → increase max (8192-16384)
   • Memory constrained → decrease max (2048)
   • Point queries → decrease max
   • Sequential scan → increase max
   • Low-latency small batch → increase min (1024+)
3. Write performance → ts.compress.last_segment.enabled (SSD: false, HDD: true)
4. Query performance / high memory → ts.block.lru_cache.max_limit
5. Last query performance / high memory → ts.last_cache_size.max_limit
6. Write performance (after #3) / high memory (after #4,#5) → ts.mem_segment_size.max_limit
7. Frequent compaction / disk tight → ts.reserved_last_segment.max_limit
8. Compaction backlog / high CPU → ts.compact.max_limit
9. Data cleanup → ts.auto_vacuum.enabled
10. Poor filter pushdown → ts.block_filter.sampling_ratio

Pre-conditions (confirm relevant resources before suggesting):

• Memory-related params: confirm available free memory with user
• Disk-related params: confirm available disk space with user
• CPU-related params: confirm CPU availability with user

Read references/config-optimization.md for detailed parameter guidance. See assets/example-configs.md for configuration tuning examples.

Config query approach:

• Use MCP tool if available: mcp__kwdb__read-query("SHOW CLUSTER SETTING ts.xxx")
• Otherwise, ask user to run: SHOW CLUSTER SETTING ts.xxx;
• NEVER use SHOW CLUSTER SETTINGS

Important:

• NEVER execute SET CLUSTER SETTING automatically
• Only provide SQL statements for user to review and execute

NOT for

• DDL operations: Creating/dropping tables, indexes (→ kwdb-schema-design)
• Deployment/Config: Memory settings, installation, replication
• Write optimization: Bulk INSERT, import performance
• Data migration: Moving data between databases
• Hardware sizing: Server specs, disk I/O recommendations
• Application tuning: Connection pooling, caching (beyond SQL)

Guardrails

1. Never suggest CREATE INDEX on time-series tables - they don't support secondary indexes
2. Time-series queries MUST have time range filters - warn if missing
3. Never recommend OFFSET for deep pagination - use time-based cursor
4. Always specify SELECT columns for time-series - no SELECT *
5. Verify table type before index recommendations
6. Explain WHY the optimization works - not just what to change
7. Validate with EXPLAIN before finishing
8. Confirm memory and disk before config changes - never suggest cache/memory size increases without confirmed free resources
9. Never auto-execute SET CLUSTER SETTING - only provide SQL for user to review
10. Never use SHOW CLUSTER SETTINGS - always query specific settings individually
11. Resource overload reduce warning - when recommending reduced values for memory/CPU-impact parameters, always include the risk warning about potential performance degradation

Output Format

markdown

## Intent
[Brief description of the optimization goal]

## Engine Type
[time-series / relational / mixed]

## Anti-Pattern Detected
[What was causing the slowness]

## Original Query
```sql
[query before optimization]

Optimized Query

sql

[rewritten query]

Expected Improvement

[What should change in EXPLAIN]

Validation

sql

EXPLAIN (ANALYZE) [optimized query];

text

For configuration optimization output format, see `assets/config-output-template.md`.