本節簡單介紹了tuple locks.
關於tuple locks，原始碼中的README檔案給出以下解釋：
>Locking tuples is not as easy as locking tables or other database objects.
The problem is that transactions might want to lock large numbers of tuples at
any one time, so it's not possible to keep the locks objects in shared memory.
To work around this limitation, we use a two-level mechanism.  The first level
is implemented by storing locking information in the tuple header: a tuple is
marked as locked by setting the current transaction's XID as its XMAX, and
setting additional infomask bits to distinguish this case from the more normal
case of having deleted the tuple.  When multiple transactions concurrently
lock a tuple, a MultiXact is used; see below.  This mechanism can accommodate
arbitrarily large numbers of tuples being locked simultaneously.

相對於tables或者其他資料庫物件，元組的locks並不太容易處理，問題是事務可能在某個時間點
鎖定大批次的tuples，因此不可能在共享記憶體中儲存這些鎖資訊。為了突破這個限制，PG使用了兩級機制。
第一級：把鎖資訊儲存在tuple header實現：把當前事務的XID儲存在XMAX中，同時設定特別的
infomask bits來區分已刪除的tuples和這些被鎖定的tuples。
第二級：如有併發操作，則使用MultiXact。
這種機制可以容納同時被鎖定的任意數量的元組。

**示例**
插入資料，並執行update，update操作會lock tuples
```
[local:/data/run/pg12]:5120 pg12@testdb=# begin;
BEGIN
[local:/data/run/pg12]:5120 pg12@testdb=#* insert into t values(1),(2);
INSERT 0 2
[local:/data/run/pg12]:5120 pg12@testdb=#* commit;
COMMIT
[local:/data/run/pg12]:5120 pg12@testdb=# begin;
BEGIN
[local:/data/run/pg12]:5120 pg12@testdb=#* update t set id = 2;
UPDATE 2
[local:/data/run/pg12]:5120 pg12@testdb=#* 
[local:/data/run/pg12]:5120 pg12@testdb=#* select txid_current();
   1512070049

--

[local:/data/run/pg12]:5120 pg12@testdb=# select xmin,xmax from t;
    xmin    |    xmax    
------------+------------
 1512070048 | 1512070049
 1512070048 | 1512070049
(2 rows)

[local:/data/run/pg12]:5120 pg12@testdb=#

```

**參考資料**
[Locking tuples](https://github.com/postgres/postgres/blob/master/src/backend/access/heap/README.tuplock#L8)

PostgreSQL DBA(158) - Locks(tuple locks)

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