对于mysql的query_cache认识的误区

下面我们通过实验及源码具体分析。首先，我们先试验一下：

首先，我们看一下mysql query_cache的状态：

因为这个db是新的db，所以hits，inset都为0，现在我们执行一条select语句：

状态变为：

请注意，这条sql，比刚才那条sql前面多了一个空格。

按照网上的理论，这条sql应该会作为另一个键而插入另一个cache，不会复用先前的cache,但结果呢？

frequently appeared in real life, consequently we can
check all such queries, too.
*/
if ((my_toupper(system_charset_info, sql[i]) != 'S' ||
my_toupper(system_charset_info, sql[i + 1]) != 'E' ||
my_toupper(system_charset_info, sql[i + 2]) != 'L') &&
sql[i] != '/')
{
DBUG_PRINT("qcache", ("The statement is not a SELECT; Not cached"));
goto err;
}

我们看下dispache_command这个方法，在这个方法里有这样一段：

复制代码 代码如下:

if (alloc_query(thd, packet, packet_length))
break; // fatal error is set
char *packet_end= thd->query() + thd->query_length();
/* 'b' stands for 'buffer' parameter', special for 'my_snprintf' */
const char* end_of_stmt= NULL;

复制代码 代码如下:

bool alloc_query(THD *thd, const char *packet, uint packet_length)
{
char *query;
/* Remove garbage at start and end of query */
while (packet_length > 0 && my_isspace(thd->charset(), packet[0]))
{
packet++;
packet_length--;
}
const char *pos= packet + packet_length; // Point at end null
while (packet_length > 0 &&
(pos[-1] == ';' || my_isspace(thd->charset() ,pos[-1])))
{
pos--;
packet_length--;
}
/* We must allocate some extra memory for query cache
The query buffer layout is:
buffer :==
<statement> The input statement(s)
'/0' Terminating null char (1 byte)
<length> Length of following current database name (size_t)
<db_name> Name of current database
<flags> Flags struct
*/
if (! (query= (char*) thd->memdup_w_gap(packet,
packet_length,
1 + sizeof(size_t) + thd->db_length +
QUERY_CACHE_FLAGS_SIZE)))
return TRUE;
query[packet_length]= '/0';
/*
Space to hold the name of the current database is allocated. We
also store this length, in case current database is changed during
execution. We might need to reallocate the 'query' buffer
*/
char *len_pos = (query + packet_length + 1);
memcpy(len_pos, (char *) &thd->db_length, sizeof(size_t));
thd->set_query(query, packet_length);
/* Reclaim some memory */
thd->packet.shrink(thd->variables.net_buffer_length);
thd->convert_buffer.shrink(thd->variables.net_buffer_length);
return FALSE;
}