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
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
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
