PostgreSQL的视图pg

PostgreSQL的视图pg_locks

pg_locks 是 PostgreSQL 提供的系统视图，用于显示当前数据库中的锁信息。通过查询这个视图，数据库管理员可以监控锁的使用情况，识别潜在的锁争用和死锁问题，并优化数据库性能。

`pg_locks` 视图字段说明

以下是 pg_locks 视图中的一些主要字段及其说明：

locktype：锁的类型，如 relation, extend, page, tuple, transaction, etc。
database：对象所属数据库的 OID（对象 ID）。
relation：表或索引的 OID（如果锁对象是一个表或索引）。
page：页号（如果锁对象是一个页）。
tuple：行号（如果锁对象是一个行）。
virtualxid：虚拟事务 ID。
transactionid：事务 ID（如果锁对象是一个事务）。
classid：系统的 OID（如果锁对象是一个泛型的数据库对象）。
objid：对象的 OID（如果锁对象是一个泛型的数据库对象）。
objsubid：对象的子 ID（如果锁对象是一个泛型的数据库对象）。
virtualtransaction：虚拟事务 ID，这是一个唯一标识后台进程的标识符。
pid：持有锁的进程 ID。
mode：锁的模式，如 AccessShareLock, RowExclusiveLock, RowShareLock, etc。
granted：锁是否被授予（true 或 false）。
fastpath：锁是否通过快速路径请求（true 或 false）。

使用示例

查询所有当前锁

通用查询：

SELECT * FROM pg_locks;

根据锁类型查询

例如，查询所有表级锁：

SELECT * FROM pg_locks WHERE locktype = 'relation';

或查询所有行级锁：

SELECT * FROM pg_locks WHERE locktype = 'tuple';

查询特定数据库的锁

可以根据数据库 OID 过滤锁信息：

SELECT * FROM pg_locks WHERE database = (SELECT oid FROM pg_database WHERE datname = 'your_database_name');

查询持有锁的进程

可以根据进程 ID 进行查询：

SELECT * FROM pg_locks WHERE pid = 12345;

查询等待锁的进程

通过过滤 granted 字段为 false 可以找到那些正在等待锁的进程：

SELECT * FROM pg_locks WHERE granted = false;

检测和处理锁争用

在 `pg_stat_activity` 中结合锁信息

可以将 pg_stat_activity 和 pg_locks 视图结合起来，查询所有正在等待锁的会话以及持有这些锁的会话：

SELECTpg_stat_activity.pid,pg_stat_activity.query,pg_locks.locktype,pg_locks.mode,pg_locks.relation::regclass,pg_locks.transactionid,pg_locks.virtualxid,pg_locks.virtualtransaction,pg_locks.granted
FROM pg_stat_activity
JOIN pg_locks ON pg_stat_activity.pid = pg_locks.pid
WHERE pg_locks.granted = false;

解除锁和终止会话

在某些情况下，可能需要手动解锁，例如当某个会话长时间持有锁导致其他事务无法正常进行。可以使用 pg_terminate_backend 函数来终止持有锁的会话：

获取持有锁的进程：

SELECT * FROM pg_locks WHERE mode = 'ExclusiveLock' AND granted = true;

执行终止进程操作：

假设需要终止 PID 为 12345 的会话：

SELECT pg_terminate_backend(12345);

示例脚本：查看锁争用情况并终止占用锁的会话

以下是一个结合 pg_locks 和 pg_stat_activity 的脚本，显示当前锁争用的情况并终止占用锁的会话：

-- 查看当前锁争用情况
SELECTwaiting_locks.pid AS waiting_pid,blocking_locks.pid AS blocking_pid,waiting_activity.query AS waiting_query,blocking_activity.query AS blocking_query
FROM pg_locks AS waiting_locks
JOIN pg_locks AS blocking_locksON waiting_locks.locktype = blocking_locks.locktypeAND waiting_locks.database IS NOT DISTINCT FROM blocking_locks.databaseAND waiting_locks.relation IS NOT DISTINCT FROM blocking_locks.relationAND waiting_locks.page IS NOT DISTINCT FROM blocking_locks.pageAND waiting_locks.tuple IS NOT DISTINCT FROM blocking_locks.tupleAND waiting_locks.virtualxid IS NOT DISTINCT FROM blocking_locks.virtualxidAND waiting_locks.transactionid IS NOT DISTINCT FROM blocking_locks.transactionidAND waiting_locks.classid IS NOT DISTINCT FROM blocking_locks.classidAND waiting_locks.objid IS NOT DISTINCT FROM blocking_locks.objidAND waiting_locks.objsubid IS NOT DISTINCT FROM blocking_locks.objsubidAND waiting_locks.pid <> blocking_locks.pid
JOIN pg_stat_activity AS waiting_activityON waiting_locks.pid = waiting_activity.pid
JOIN pg_stat_activity AS blocking_activityON blocking_locks.pid = blocking_activity.pid
WHERE NOT waiting_locks.granted;-- 终止占用锁的会话（需要确认后再执行）
SELECT pg_terminate_backend(blocking_locks.pid)
FROM pg_locks AS waiting_locks
JOIN pg_locks AS blocking_locksON waiting_locks.locktype = blocking_locks.locktypeAND waiting_locks.database IS NOT DISTINCT FROM blocking_locks.databaseAND waiting_locks.relation IS NOT DISTINCT FROM blocking_locks.relationAND waiting_locks.page IS NOT DISTINCT FROM blocking_locks.pageAND waiting_locks.tuple IS NOT DISTINCT FROM blocking_locks.tupleAND waiting_locks.virtualxid IS NOT DISTINCT FROM blocking_locks.virtualxidAND waiting_locks.transactionid IS NOT DISTINCT FROM blocking_locks.transactionidAND waiting_locks.classid IS NOT DISTINCT FROM blocking_locks.classidAND waiting_locks.objid IS NOT DISTINCT FROM blocking_locks.objidAND waiting_locks.objsubid IS NOT DISTINCT FROM blocking_locks.objsubidAND waiting_locks.pid <> blocking_locks.pid
WHERE NOT waiting_locks.granted;