本節介紹了APP在涉及locks時需要注意的地方.

測試資料:
```
[local]:5432 pg12@testdb=# drop table if exists tbl;
DROP TABLE
Time: 36.136 ms
[local]:5432 pg12@testdb=# create table tbl(id int,c1 varchar(20),c2 varchar(20));
CREATE TABLE
Time: 4.903 ms
[local]:5432 pg12@testdb=# 
[local]:5432 pg12@testdb=# insert into tbl select x,'c1'||x,'c2'||x from generate_series(1,1000000) as x;
INSERT 0 1000000
Time: 3677.812 ms (00:03.678)
[local]:5432 pg12@testdb=# 
```
-- session 1
```
[local]:5432 pg12@testdb=# select pg_backend_pid();
 pg_backend_pid 
----------------
           1541
(1 row)

```
-- session 2
```
[local]:5432 pg12@testdb=# select pg_backend_pid();
 pg_backend_pid 
----------------
           1628
(1 row)

Time: 4.446 ms
```

**1: Never add a column with a default value**
表上新增列時獲取的鎖是AccessExclusiveLock,會阻塞RW(包括SELECT),為了儘快完成列的新增,新增有預設值的列,可拆分為新增列,然後執行UPDATE語句以免出現R阻塞.
```
-- session 1
[local]:5432 pg12@testdb=# begin;
BEGIN
Time: 0.929 ms
[local]:5432 pg12@testdb=#* alter table tbl add column c3 varchar(20) default 'c3';
ALTER TABLE
Time: 32.881 ms
[local]:5432 pg12@testdb=#

-- session 2

[local]:5432 pg12@testdb=# select * from tbl;
-- 阻塞

Time: 29.088 ms
```
使用先新增列,後更新預設值的方法
```
------ session 1
[local]:5432 pg12@testdb=# begin;
BEGIN
Time: 0.330 ms
[local]:5432 pg12@testdb=#* alter table tbl add column c4 varchar(20);
ALTER TABLE
Time: 0.460 ms
[local]:5432 pg12@testdb=#* end;
COMMIT
Time: 0.530 ms
[local]:5432 pg12@testdb=# begin;
BEGIN
Time: 0.199 ms
[local]:5432 pg12@testdb=#* update tbl set c4 = 'c4';
UPDATE 1000000
Time: 5286.769 ms (00:05.287)
[local]:5432 pg12@testdb=#*

------ session 2
[local]:5432 pg12@testdb=# select * from tbl limit 1; 
 id | c1  | c2  | c3 | c4 
----+-----+-----+----+----
  1 | c11 | c21 | c3 | 
(1 row)

Time: 2.793 ms

Time: 1.062 ms
```
雖然更新耗費的時間遠比直接add column設定預設值要大,但鎖等級是RowExclusiveLock,並不會阻塞讀

**2: Beware of lock queues, use lock timeouts**
PG中每一個鎖都有一個佇列,在獲取鎖時如需等待存在衝突的其他鎖,則會阻塞.可透過設定超時時間避免長時間的等待.這樣雖然會失敗,但可透過後臺查詢等方法獲取資料庫活動,保持資料庫可控.
```
------ session 1
[local]:5432 pg12@testdb=# begin;
BEGIN
Time: 1.148 ms
[local]:5432 pg12@testdb=#* alter table tbl add column c5 varchar(20) default 'c3';
ALTER TABLE
Time: 2.726 ms
[local]:5432 pg12@testdb=#*

Time: 2.751 ms

------ session 2
[local]:5432 pg12@testdb=# begin;
BEGIN
Time: 0.861 ms
[local]:5432 pg12@testdb=#* SET lock_timeout TO '1s';
SET
Time: 0.689 ms
[local]:5432 pg12@testdb=#* select * from tbl limit 1;
ERROR:  canceling statement due to lock timeout
LINE 1: select * from tbl limit 1;
                      ^
Time: 1001.031 ms (00:01.001)
[local]:5432 pg12@testdb=#! end;
ROLLBACK
Time: 0.984 ms
[local]:5432 pg12@testdb=#! 
```

**3: Create indexes CONCURRENTLY**
使用CONCURRENTLY模式建立Index.
新插入1000w資料
```
[local]:5432 pg12@testdb=# insert into tbl select x,'c1'||x,'c2'||x,'c3'||x from generate_series(1,10000000) as x;
INSERT 0 10000000
Time: 32784.183 ms (00:32.784)
```
普通模式建立索引
```
------ session 1
[local]:5432 pg12@testdb=# begin;
BEGIN
Time: 29.276 ms
[local]:5432 pg12@testdb=#* create index idx_tbl_id on tbl(id);
CREATE INDEX
Time: 7261.828 ms (00:07.262)
[local]:5432 pg12@testdb=#*

------ session 2
[local]:5432 pg12@testdb=# begin;
BEGIN
Time: 0.358 ms
[local]:5432 pg12@testdb=#* insert into tbl(id) values(0);
-- 阻塞

Time: 0.795 ms
```
回滾事務後,使用CONCURRENTLY模式建立索引,要注意的是CONCURRENTLY模式不能用在事務中
```
[local]:5432 pg12@testdb=#* -- only blocks other DDL
[local]:5432 pg12@testdb=#* create index CONCURRENTLY idx_tbl_id on tbl(id);
ERROR:  CREATE INDEX CONCURRENTLY cannot run inside a transaction block
Time: 0.491 ms
[local]:5432 pg12@testdb=#! 
```
不啟動事務,直接執行
```
------ session 1
[local]:5432 pg12@testdb=# -- only blocks other DDL
create index CONCURRENTLY idx_tbl_id on tbl(id);
CREATE INDEX
Time: 9718.400 ms (00:09.718)

------ session 2
[local]:5432 pg12@testdb=# begin;
BEGIN
Time: 0.373 ms
[local]:5432 pg12@testdb=#* insert into tbl(id) values(0);
INSERT 0 1
Time: 0.686 ms
[local]:5432 pg12@testdb=#*

Time: 0.754 ms
```
使用CONCURRENTLY模式建立索引,獲取的lock是ShareUpdateExclusiveLock,不會阻塞INSERT/UPDATE/DELETE操作(請求的鎖是RowExclusiveLock)

**4: Take aggressive locks as late as possible**
這個跟程式設計中定義變數類似 : 離需要用到的地方越近的地方才定義.文中的例子見仁見智,選擇使用.

**5: Adding a primary key with minimal locking**
重新構建測試資料
```
[local]:5432 pg12@testdb=# truncate table tbl;
TRUNCATE TABLE
Time: 91.815 ms
[local]:5432 pg12@testdb=# insert into tbl select x,'c1'||x,'c2'||x,'c3'||x from generate_series(1,12000000) as x;
INSERT 0 12000000
Time: 59285.694 ms (00:59.286)
```
把add primary key這一個動作拆解為先新增唯一索引,再新增primary key constraint這兩個動作.
```
------ session 1
[local]:5432 pg12@testdb=# begin;
BEGIN
Time: 1.155 ms
[local]:5432 pg12@testdb=#* alter table tbl add primary key(id);
ALTER TABLE
Time: 10572.201 ms (00:10.572)
[local]:5432 pg12@testdb=#*

------ session 2
[local]:5432 pg12@testdb=# begin;
BEGIN
Time: 0.703 ms
[local]:5432 pg12@testdb=#* insert into tbl(id) values(0);
-- 阻塞

Time: 1.397 ms
```
拆解後,使用CONCURRENTLY模式建立索引,與第3點類似
```
------ session 1
[local]:5432 pg12@testdb=# -- takes a long time, but doesn’t block queries
[local]:5432 pg12@testdb=# CREATE UNIQUE INDEX CONCURRENTLY idx_tbl_id ON tbl (id); 
CREATE INDEX
Time: 9908.405 ms (00:09.908)
[local]:5432 pg12@testdb=# -- blocks queries, but only very briefly
[local]:5432 pg12@testdb=# ALTER TABLE tbl ADD CONSTRAINT pk_tbl PRIMARY KEY USING INDEX idx_tbl_id;  
NOTICE:  ALTER TABLE / ADD CONSTRAINT USING INDEX will rename index "idx_tbl_id" to "pk_tbl"
ALTER TABLE
Time: 4582.013 ms (00:04.582)

```

**6: Never VACUUM FULL**
```
------ session 1
[local]:5432 pg12@testdb=# vacuum full;

------ session 2

Time: 0.803 ms
```
vacuum full請求的鎖是AccessExclusiveLock,會阻塞讀寫,在目前vacuum full並不智慧的情況下,手工發起對單個表的vacuum full會保險許多.

**7: Avoid deadlocks by ordering commands**
注意命令的順序,避免死鎖
```
------ session 1
[local]:5432 pg12@testdb=# begin;
BEGIN
Time: 0.440 ms
[local]:5432 pg12@testdb=#* delete from tbl where id = 1;
DELETE 1
Time: 0.567 ms
[local]:5432 pg12@testdb=#*

------ session 2
[local]:5432 pg12@testdb=# begin;
BEGIN
Time: 0.960 ms
[local]:5432 pg12@testdb=#* delete from tbl where id = 2;
DELETE 1
Time: 1.783 ms
[local]:5432 pg12@testdb=#*

------ session 3

```
產生死鎖
```
------ session 1
[local]:5432 pg12@testdb=#* delete from tbl where id = 2;

------ session 2
[local]:5432 pg12@testdb=#* delete from tbl where id = 1;
ERROR:  deadlock detected
DETAIL:  Process 1628 waits for ShareLock on transaction 623; blocked by process 1541.
Process 1541 waits for ShareLock on transaction 624; blocked by process 1628.
HINT:  See server log for query details.
CONTEXT:  while deleting tuple (0,1) in relation "tbl"
Time: 1004.485 ms (00:01.004)
[local]:5432 pg12@testdb=#!

------ session 3

```

**參考資料**
[When Postgres blocks: 7 tips for dealing with locks](https://www.citusdata.com/blog/2018/02/22/seven-tips-for-dealing-with-postgres-locks/)

PostgreSQL DBA(78) - Locks(When Postgres blocks 7 tips for dealing with locks)

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