MVCC原理探究及MySQL源码实现分析

dict_table_add_system_columns(
/*==========================*/
dict_table_t*   table, /*!< in/out: table */
mem_heap_t* heap) /*!< in: temporary heap */
{
ut_ad(table);
ut_ad(table->n_def == (table->n_cols - table->get_n_sys_cols()));
ut_ad(table->magic_n == DICT_TABLE_MAGIC_N);
ut_ad(!table->cached);
/* NOTE: the system columns MUST be added in the following order
(so that they can be indexed by the numerical value of DATA_ROW_ID,
etc.) and as the last columns of the table memory object.
The clustered index will not always physically contain all system
columns.
Intrinsic table don't need DB_ROLL_PTR as UNDO logging is turned off
for these tables. */
dict_mem_table_add_col(table, heap, "DB_ROW_ID", DATA_SYS,
     DATA_ROW_ID | DATA_NOT_NULL,
     DATA_ROW_ID_LEN);
#if (DATA_ITT_N_SYS_COLS != 2)
#error "DATA_ITT_N_SYS_COLS != 2"
#endif
#if DATA_ROW_ID != 0
#error "DATA_ROW_ID != 0"
#endif
dict_mem_table_add_col(table, heap, "DB_TRX_ID", DATA_SYS,
     DATA_TRX_ID | DATA_NOT_NULL,
     DATA_TRX_ID_LEN);
#if DATA_TRX_ID != 1
#error "DATA_TRX_ID != 1"
#endif
if (!table->is_intrinsic()) {
dict_mem_table_add_col(table, heap, "DB_ROLL_PTR", DATA_SYS,
     DATA_ROLL_PTR | DATA_NOT_NULL,
     DATA_ROLL_PTR_LEN);
#if DATA_ROLL_PTR != 2
#error "DATA_ROLL_PTR != 2"
#endif
/* This check reminds that if a new system column is added to
the program, it should be dealt with here */
#if DATA_N_SYS_COLS != 3
#error "DATA_N_SYS_COLS != 3"
#endif
}
}

dhy@10.16.70.190:3306 12:25:47 [dhy]>select * from dhytest;
+------+
| id   |
+------+
| 10 |
+------+
1 row in set (7.99 sec)

dhy@10.10.80.199:3306 15:28:24 [dhy]>update dhytest set id = 20;
Query OK, 3 rows affected (40.71 sec)
Rows matched: 3 Changed: 3 Warnings: 0

/** The read should not see any transaction with trx id >= this
value. In other words, this is the "high water mark". */
trx_id_t    m_low_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". */
trx_id_t    m_up_limit_id;
/** trx id of creating transaction, set to TRX_ID_MAX for free
views. */
trx_id_t    m_creator_trx_id;

bool
lock_clust_rec_cons_read_sees(
/*==========================*/
const rec_t*    rec, /*!< in: user record which should be read or
passed over by a read cursor */
dict_index_t*   index, /*!< in: clustered index */
const ulint*    offsets,/*!< in: rec_get_offsets(rec, index) */
ReadView*   view) /*!< in: consistent read view */
{
ut_ad(index->is_clustered());
ut_ad(page_rec_is_user_rec(rec));
ut_ad(rec_offs_validate(rec, index, offsets));
/* Temp-tables are not shared across connections and multiple
transactions from different connections cannot simultaneously
operate on same temp-table and so read of temp-table is
always consistent read. */
//只读事务或者临时表是不需要一致性读的判断
if (srv_read_only_mode || index->table->is_temporary()) {
ut_ad(view == 0 || index->table->is_temporary());
return(true);
}
/* NOTE that we call this function while holding the search
system latch. */
trx_id_t    trx_id = row_get_rec_trx_id(rec, index, offsets); //获取记录上的TRX_ID这里需要解释下，我们一个查询可能满足的记录数有多个。那我们每读取一条记录的时候就要根据这条记录上的TRX_ID判断这条记录是否可见
return(view->changes_visible(trx_id, index->table->name)); //判断记录可见性
}

bool changes_visible(
trx_id_t    id,
const table_name_t& name) const
MY_ATTRIBUTE((warn_unused_result))
{
ut_ad(id > 0);
//如果ID小于Read View中最小的, 则这条记录是可以看到。说明这条记录是在select这个事务开始之前就结束的
if (id < m_up_limit_id || id == m_creator_trx_id) {
return(true);
}
check_trx_id_sanity(id, name);
//如果比Read View中最大的还要大，则说明这条记录是在事务开始之后进行修改的，所以此条记录不应查看到
if (id >= m_low_limit_id) {
return(false);
} else if (m_ids.empty()) {
return(true);
}
const ids_t::value_type*    p = m_ids.data();
return(!std::binary_search(p, p + m_ids.size(), id)); //判断是否在Read View中， 如果在说明在创建Read View时 此条记录还处于活跃状态则不应该查询到，否则说明创建Read View是此条记录已经是不活跃状态则可以查询到
}