PostgreSQL 性能调优实战：索引与缓冲池优化

flowchart TD
    A[INSERT 一行数据] --> B[Heap Tuple v1: xmin=100, xmax=0]
    B --> C[UPDATE 该行]
    C --> D[Heap Tuple v1: xmin=100, xmax=200 标记过期]
    C --> E[Heap Tuple v2: xmin=200, xmax=0 新版本]
    E --> F[DELETE 该行]
    F --> G[Heap Tuple v2: xmin=200, xmax=300 标记删除]
    D --> H[Dead Tuple: 等待 VACUUM 回收]
    G --> H
    H --> I[VACUUM 扫描]
    I --> J[标记空间为可复用: FSM 更新]
    I --> K[更新统计信息: pg_statistic]
    style H fill:#ffebee
    style J fill:#e8f5e9
    style K fill:#e8f5e9

flowchart TD
    A[查询: WHERE user_id = 12345] --> B[B-Tree Root 节点]
    B --> C[Branch 节点: 二分查找确定子节点]
    C --> D[Leaf 节点: 找到 CTID 指针]
    D --> E[Heap Fetch: 根据 CTID 读取行数据]
    E --> F{版本检查}
    F -->|xmin 可见, xmax=0| G[返回数据]
    F -->|xmax 已提交| H[沿更新链查找新版本]
    H --> G
    style B fill:#e3f2fd
    style D fill:#e3f2fd
    style G fill:#e8f5e9

-- 启用慢查询日志（会话级别）
SET log_min_duration_statement = 100;
-- 查看当前活跃慢查询
SELECT
    pid,
    now() - pg_stat_activity.query_start AS duration,
    query,
    state
FROM pg_stat_activity
WHERE state = 'active'
  AND now() - pg_stat_activity.query_start > interval '100 milliseconds'
ORDER BY duration DESC;
-- EXPLAIN ANALYZE 分析执行计划
EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)
SELECT user_id, feature_vector, last_updated
FROM user_features
WHERE tenant_id = 42
  AND last_updated > NOW() - INTERVAL '7 days'
ORDER BY last_updated DESC
LIMIT 100;
-- 创建复合索引（等值列在前，范围列在后）
CREATE INDEX CONCURRENTLY idx_user_features_tenant_updated
ON user_features (tenant_id, last_updated DESC);
-- 覆盖索引消除回表
CREATE INDEX CONCURRENTLY idx_user_features_covering
ON user_features (tenant_id, last_updated DESC)
INCLUDE (feature_vector);

-- 内存配置
ALTER SYSTEM SET shared_buffers = '4GB'; -- 系统内存25%
ALTER SYSTEM SET effective_cache_size = '12GB'; -- 系统内存75%
ALTER SYSTEM SET work_mem = '64MB'; -- 按并发量计算
ALTER SYSTEM SET maintenance_work_mem = '1GB';
-- WAL与检查点配置
ALTER SYSTEM SET checkpoint_completion_target = 0.9;
ALTER SYSTEM SET max_wal_size = '4GB';
ALTER SYSTEM SET min_wal_size = '1GB';
ALTER SYSTEM SET wal_buffers = '64MB';
-- 自动清理配置
ALTER SYSTEM SET autovacuum_vacuum_scale_factor = 0.05;
ALTER SYSTEM SET autovacuum_analyze_scale_factor = 0.02;
-- 特定大表设置更激进策略
ALTER TABLE large_feature_table SET (
    autovacuum_vacuum_scale_factor = 0.01,
    autovacuum_analyze_scale_factor = 0.005
);
-- 应用配置变更
SELECT pg_reload_conf();

-- 缓冲池命中率监控
SELECT
    'index hit ratio' AS metric,
    ROUND(
        (sum(idx_blks_hit)::float / NULLIF(sum(idx_blks_hit + idx_blks_read), 0)) * 100,
        2
    ) AS ratio_pct
FROM pg_statio_user_indexes
UNION ALL
SELECT
    'table hit ratio' AS metric,
    ROUND(
        (sum(heap_blks_hit)::float / NULLIF(sum(heap_blks_hit + heap_blks_read), 0)) * 100,
        2
    ) AS ratio_pct
FROM pg_statio_user_tables;
-- 表膨胀检测
SELECT
    schemaname,
    tablename,
    pg_size_pretty(pg_total_relation_size(schemaname || '.' || tablename)) AS total_size,
    ROUND(
        100.0 * pg_total_relation_size(schemaname || '.' || tablename)
        / NULLIF(pg_relation_size(schemaname || '.' || tablename), 0),
        1
    ) AS bloat_ratio_pct
FROM pg_tables
WHERE schemaname NOT IN ('pg_catalog', 'information_schema')
ORDER BY pg_total_relation_size(schemaname || '.' || tablename) DESC
LIMIT 20;