背景
在資料庫中，通過鎖以及多版本并發控制可以保護資料的一緻性，例如A正在查詢資料，B就無法對A通路的對象執行DDL。A正在更新某條記錄，B就不能删除或更新這條記錄。
鎖是資料庫自動管理的，同時資料庫還提供了ADLOCK或者LOCK文法，允許使用者自己控制鎖。
當然，如果應用程式邏輯設計不慎，就可能導緻嚴重的鎖等待，或者死鎖的産生。
如果你發現SQL請求大多數時候處于等待鎖的狀态，那麼可能出現了業務邏輯的問題。
如何檢查或監控鎖等待呢？
1.pg_locks展示鎖資訊，每一個被鎖或者等待鎖的對象一條記錄。
2.pg_stat_activity，每個會話一條記錄，顯示會話狀态資訊。
我們通過這兩個視圖可以檢視鎖，鎖等待情況。同時可以了解發生鎖沖突的情況。
pg_stat_activity.query反映的是目前正在執行或請求的SQL，而同一個事務中以前已經執行的SQL不能在pg_stat_activity中顯示出來。是以如果你發現兩個會話發生了沖突，但是他們的pg_stat_activity.query沒有沖突的話，那就有可能是他們之間的某個事務之前的SQL擷取的鎖與另一個事務目前請求的QUERY發生了鎖沖突。
追蹤詳細的鎖沖突資訊：
1.可以通過locktrace跟蹤鎖等待的詳細資訊，
2.通過資料庫日志（開啟lock_timeout,log_lockwait參數）(csvlog)跟蹤鎖等待資訊，
3.或者通過資料庫日志（開啟log_statements='all'，SQL審計）追蹤事務中所有的SQL(csvlog)，分析事務之間的鎖沖突。
4.通過SQL檢視持鎖，等鎖的事務狀态。
鎖的釋放時機：
大多數鎖要等待事務結束後釋放，某些輕量級鎖（資料庫自動控制）是随用随釋放的。
檢視目前事務鎖等待、持鎖資訊的SQL
這條SQL非常有用，建議DBA珍藏。
with
t_wait as
(
select a.mode,a.locktype,a.database,a.relation,a.page,a.tuple,a.classid,a.granted,
a.objid,a.objsubid,a.pid,a.virtualtransaction,a.virtualxid,a.transactionid,a.fastpath,
b.state,b.query,b.xact_start,b.query_start,b.usename,b.datname,b.client_addr,b.client_port,b.application_name
from pg_locks a,pg_stat_activity b where a.pid=b.pid and not a.granted
),
t_run as
(
a.objid,a.objsubid,a.pid,a.virtualtransaction,a.virtualxid,a.transactionid,a.fastpath,
from pg_locks a,pg_stat_activity b where a.pid=b.pid and a.granted
t_overlap as
select r.* from t_wait w join t_run r on
(
r.locktype is not distinct from w.locktype and
r.database is not distinct from w.database and
r.relation is not distinct from w.relation and
r.page is not distinct from w.page and
r.tuple is not distinct from w.tuple and
r.virtualxid is not distinct from w.virtualxid and
r.transactionid is not distinct from w.transactionid and
r.classid is not distinct from w.classid and
r.objid is not distinct from w.objid and
r.objsubid is not distinct from w.objsubid and
r.pid <> w.pid
)
),
t_unionall as
select r.* from t_overlap r
union all
select w.* from t_wait w
)
select locktype,datname,relation::regclass,page,tuple,virtualxid,transactionid::text,classid::regclass,objid,objsubid,
string_agg(
'Pid: '||case when pid is null then 'NULL' else pid::text end||chr(10)||
'Lock_Granted: '||case when granted is null then 'NULL' else granted::text end||' , Mode: '||case when mode is null then 'NULL' else mode::text end||' , FastPath: '||case when fastpath is null then 'NULL' else fastpath::text end||' , VirtualTransaction: '||case when virtualtransaction is null then 'NULL' else virtualtransaction::text end||' , Session_State: '||case when state is null then 'NULL' else state::text end||chr(10)||
'Username: '||case when usename is null then 'NULL' else usename::text end||' , Database: '||case when datname is null then 'NULL' else datname::text end||' , Client_Addr: '||case when client_addr is null then 'NULL' else client_addr::text end||' , Client_Port: '||case when client_port is null then 'NULL' else client_port::text end||' , Application_Name: '||case when application_name is null then 'NULL' else application_name::text end||chr(10)||
'Xact_Start: '||case when xact_start is null then 'NULL' else xact_start::text end||' , Query_Start: '||case when query_start is null then 'NULL' else query_start::text end||' , Xact_Elapse: '||case when (now()-xact_start) is null then 'NULL' else (now()-xact_start)::text end||' , Query_Elapse: '||case when (now()-query_start) is null then 'NULL' else (now()-query_start)::text end||chr(10)||
'SQL (Current SQL in Transaction): '||chr(10)||
case when query is null then 'NULL' else query::text end,
chr(10)||'--------'||chr(10)
order by
( case mode
when 'INVALID' then 0
when 'AccessShareLock' then 1
when 'RowShareLock' then 2
when 'RowExclusiveLock' then 3
when 'ShareUpdateExclusiveLock' then 4
when 'ShareLock' then 5
when 'ShareRowExclusiveLock' then 6
when 'ExclusiveLock' then 7
when 'AccessExclusiveLock' then 8
else 0
end ) desc,
(case when granted then 0 else 1 end)
) as lock_conflict
from t_unionall
group by
locktype,datname,relation,page,tuple,virtualxid,transactionid::text,classid,objid,objsubid ;
如果覺得寫SQL麻煩，可以将它建立為視圖
create view v_locks_monitor as
locktype,datname,relation,page,tuple,virtualxid,transactionid::text,classid,objid,objsubid ;
ksl=> create table locktest(id int primary key, info text);
CREATE TABLE
ksl=> insert into locktest values (1,'a');
INSERT 0 1
會話A
ksl=> begin;
BEGIN
ksl=> update locktest set info='a' where id=1;
UPDATE 1
ksl=> select * from locktest ;
id | info
----+------
1 | a
(1 row)
ksl=>
會話B
會話C
ksl=> insert into locktest values (2,'test');
會話D
一直處于等待狀态
會話E
也一直處于等待狀态
處理方法
前面的鎖查詢SQL，已經清晰的顯示了每一個發生了鎖等待的對象，按鎖的大小排序，要快速解出這種狀态，terminate最大的鎖對應的PID即可。
ksl=> select pg_terminate_backend(43600);
-[ RECORD 1 ]--------+--
pg_terminate_backend | t
會話D
sl=> begin;
ksl=> truncate locktest ;
FATAL: terminating connection due to administrator command
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Succeeded.
幹掉43600後，大家都清淨了
再查詢該表資料
ksl=> select * from v_locks_monitor ;
(0 rows)
END