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PgSQL监控死元组和自动清理状态的SQL语句执行报错ERROR: division by zero原因分析和解决方法

news 2025/9/10 6:44:02

监控死元组和自动清理状态SQL语句，执行后报错，解决方法：

报错SQL：
SELECT schemaname, relname, n_live_tup, n_dead_tup,(n_dead_tup * 100.0 / (n_live_tup + n_dead_tup)) AS dead_tup_ratio,last_autovacuum, last_autoanalyzeFROM pg_stat_user_tablesORDER BY n_dead_tup DESCLIMIT 10;

执行后报错如下：
ERROR: division by zero
怎么修改SQL语句？

这个错误是因为有些表的 `n_live_tup` 和 `n_dead_tup` 都是零。可以使用以下几种方法修复：

方法1：使用 NULLIF() 函数（推荐）

SELECT schemaname, relname,n_live_tup, n_dead_tup,(n_dead_tup * 100.0 / NULLIF((n_live_tup + n_dead_tup), 0)) AS dead_tup_ratio,last_autovacuum, last_autoanalyze
FROM pg_stat_user_tables
ORDER BY n_dead_tup DESC
LIMIT 10;

方法2：使用 CASE 语句

SELECT schemaname, relname,n_live_tup, n_dead_tup,CASE WHEN (n_live_tup + n_dead_tup) = 0 THEN 0 ELSE (n_dead_tup * 100.0 / (n_live_tup + n_dead_tup)) END AS dead_tup_ratio,last_autovacuum, last_autoanalyze
FROM pg_stat_user_tables
ORDER BY n_dead_tup DESC
LIMIT 10;

方法3：使用 COALESCE() 提供默认值

SELECT schemaname, relname,n_live_tup, n_dead_tup,COALESCE((n_dead_tup * 100.0 / NULLIF((n_live_tup + n_dead_tup), 0)), 0) AS dead_tup_ratio,last_autovacuum, last_autoanalyze
FROM pg_stat_user_tables
ORDER BY n_dead_tup DESC
LIMIT 10;

方法4：过滤掉空表（如果业务需要）

SELECT schemaname, relname,n_live_tup, n_dead_tup,(n_dead_tup * 100.0 / (n_live_tup + n_dead_tup)) AS dead_tup_ratio,last_autovacuum, last_autoanalyze
FROM pg_stat_user_tables
WHERE (n_live_tup + n_dead_tup) > 0  -- 只处理有数据的表
ORDER BY n_dead_tup DESC
LIMIT 10;

方法5：更完整的解决方案（包含更多信息）

SELECT schemaname, relname,n_live_tup AS live_tuples,n_dead_tup AS dead_tuples,(n_live_tup + n_dead_tup) AS total_tuples,CASE WHEN (n_live_tup + n_dead_tup) = 0 THEN 0 ELSE ROUND((n_dead_tup * 100.0 / (n_live_tup + n_dead_tup))::numeric, 2) END AS dead_tup_ratio_percent,pg_size_pretty(pg_total_relation_size(schemaname || '.' || relname)) AS total_size,last_autovacuum,last_autoanalyze,age(now(), COALESCE(last_autovacuum, '1970-01-01'::timestamp)) AS since_last_autovacuum
FROM pg_stat_user_tables
ORDER BY n_dead_tup DESC
LIMIT 10;