mvcc相关：innoDB之read view数据格式

末蓝、 2023-09-29 12:09 15阅读 0赞

源码

struct read_view_t{
    	ulint		type;	/*!< VIEW_NORMAL, VIEW_HIGH_GRANULARITY */
    	undo_no_t	undo_no;/*!< 0 or if type is
    				VIEW_HIGH_GRANULARITY
    				transaction undo_no when this high-granularity
    				consistent read view was created */
    	trx_id_t	low_limit_no;
    				/*!< The view does not need to see the undo
    				logs for transactions whose transaction number
    				is strictly smaller (<) than this value: they
    				can be removed in purge if not needed by other
    				views */
    	trx_id_t	low_limit_id;
    				/*!< The read should not see any transaction
    				with trx id >= this value. In other words,
    				this is the "high water mark". */
    	trx_id_t	up_limit_id;
    				/*!< The read should see all trx ids which
    				are strictly smaller (<) than this value.
    				In other words,
    				this is the "low water mark". */
    	ulint		n_trx_ids;
    				/*!< Number of cells in the trx_ids array */
    	trx_id_t*	trx_ids;/*!< Additional trx ids which the read should
    				not see: typically, these are the read-write
    				active transactions at the time when the read
    				is serialized, except the reading transaction
    				itself; the trx ids in this array are in a
    				descending order. These trx_ids should be
    				between the "low" and "high" water marks,
    				that is, up_limit_id and low_limit_id. */
    	trx_id_t	creator_trx_id;
    				/*!< trx id of creating transaction, or
    				0 used in purge */
    	UT_LIST_NODE_T(read_view_t) view_list;
    				/*!< List of read views in trx_sys */
    };

###  ###

### 前置知识 ###

每开启一个事务，数据库会自动为之分配一个tx\_id，它是线性增长的，也就是越后开启的事务，tx\_id越大。

###  ###

### trx\_ids ###

> Additional trx ids which the read should not see: typically, these are the read-write active transactions at the time when the read is serialized, except the reading transaction itself; the trx ids in this array are in a descending order. These trx\_ids should be between the "low" and "high" water marks, that is, up\_limit\_id and low\_limit\_id.

创建该read view 时活跃的事务id数组，**不包含创建该read view的事务id**（except the reading transaction itself）。

### low\_limit\_id ###

> The smallest number not yet assigned as a transaction id or transaction number

活跃id的最大值+1。虽然是low，但数值确实最大的， 其实就是max\_trx\_id

### up\_limit\_id ###

活跃事务id的最小值，最早创建的事务的id，其实就是min\_trx\_id。

### creator\_trx\_id ###

创建该read view的事务id