PostgreSQL的視圖pg_locks
pg_locks 是 PostgreSQL 提供的系統視圖，用于顯示當前數據庫中的鎖信息。通過查詢這個視圖，數據庫管理員可以監控鎖的使用情況，識別潛在的鎖爭用和死鎖問題，并優化數據庫性能。
pg_locks 視圖字段說明
以下是 pg_locks 視圖中的一些主要字段及其說明：
- locktype：鎖的類型，如 relation, extend, page, tuple, transaction, etc。
- database：對象所屬數據庫的 OID（對象 ID）。
- relation：表或索引的 OID（如果鎖對象是一個表或索引）。
- page：頁號（如果鎖對象是一個頁）。
- tuple：行號（如果鎖對象是一個行）。
- virtualxid：虛擬事務 ID。
- transactionid：事務 ID（如果鎖對象是一個事務）。
- classid：系統的 OID（如果鎖對象是一個泛型的數據庫對象）。
- objid：對象的 OID（如果鎖對象是一個泛型的數據庫對象）。
- objsubid：對象的子 ID（如果鎖對象是一個泛型的數據庫對象）。
- virtualtransaction：虛擬事務 ID，這是一個唯一標識后臺進程的標識符。
- pid：持有鎖的進程 ID。
- mode：鎖的模式，如 AccessShareLock, RowExclusiveLock, RowShareLock, etc。
- granted：鎖是否被授予（true 或 false）。
- fastpath：鎖是否通過快速路徑請求（true 或 false）。
使用示例
查詢所有當前鎖
通用查詢：
SELECT * FROM pg_locks;
根據鎖類型查詢
例如，查詢所有表級鎖：
SELECT * FROM pg_locks WHERE locktype = 'relation';
或查詢所有行級鎖：
SELECT * FROM pg_locks WHERE locktype = 'tuple';
查詢特定數據庫的鎖
可以根據數據庫 OID 過濾鎖信息：
SELECT * FROM pg_locks WHERE database = (SELECT oid FROM pg_database WHERE datname = 'your_database_name');
查詢持有鎖的進程
可以根據進程 ID 進行查詢：
SELECT * FROM pg_locks WHERE pid = 12345;
查詢等待鎖的進程
通過過濾 granted 字段為 false 可以找到那些正在等待鎖的進程：
SELECT * FROM pg_locks WHERE granted = false;
檢測和處理鎖爭用
在 pg_stat_activity 中結合鎖信息
可以將 pg_stat_activity 和 pg_locks 視圖結合起來，查詢所有正在等待鎖的會話以及持有這些鎖的會話：
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;
解除鎖和終止會話
在某些情況下，可能需要手動解鎖，例如當某個會話長時間持有鎖導致其他事務無法正常進行。可以使用 pg_terminate_backend 函數來終止持有鎖的會話：
獲取持有鎖的進程：
SELECT * FROM pg_locks WHERE mode = 'ExclusiveLock' AND granted = true;
執行終止進程操作：
假設需要終止 PID 為 12345 的會話：
SELECT pg_terminate_backend(12345);
示例腳本：查看鎖爭用情況并終止占用鎖的會話
以下是一個結合 pg_locks 和 pg_stat_activity 的腳本，顯示當前鎖爭用的情況并終止占用鎖的會話：
-- 查看當前鎖爭用情況
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;-- 終止占用鎖的會話（需要確認后再執行）
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;
小結
pg_locks 視圖提供了監控和管理 PostgreSQL 中鎖的詳細信息。通過合理利用 pg_locks，數據庫管理員可以實時監控鎖的使用情況，及時發現和解決鎖爭用問題，從而提高系統的并發性能和穩定性。
