如何提前完成 mysql_use_result() / mysql_fetch_row()？
How to prematurely finish mysql_use_result() / mysql_fetch_row()?
我正在为 MySQL 5.5 编写我的第一个 C 客户端，并且在文档中偶然发现了 the following page。几乎在最后，它指出（粗体强调我的，斜体强调不是我的）：
An advantage of mysql_use_result() is [...]. Disadvantages are that
[...]. Furthermore, you must retrieve all the rows even if you
determine in mid-retrieval that you've found the information you were
looking for.
最后一句没看清楚
1) 如果我不遵循那条线会怎样？
2) 我认为如果我确定我有足够的信息，实际上必须有一种方法可以提前结束获取行（否则，这整个事情在我眼中就没有多大意义）。
我知道如果我只是停止获取行然后尝试执行下一条语句，可能会发生一些不好的事情，但是没有像 mysql_finish_fetch() 或类似的函数吗？
如果我调用 mysql_free_result() 会发生什么？即使我还没有获取所有行，这也应该释放结果，所以在检索中调用它并继续我想做的任何事情应该是安全的。我错了吗？
这听起来像是 MySQL 暴露给客户端的内部线程问题。将其归结为各种 MySQL gotchas。简而言之，MySQL 显然内部有有限数量的 "searchers"，而使用 mysql_use_result() 显然将其中一个专用于您的 API 请求。此外，MySQL 显然没有暴露的 API 调用来取消这样的请求。唯一的选择是看到抓取直到结束。
稍长的版本：在内部，MySQL 的游标显然只有一个代码路径——我想是为了在常见情况下提高性能。当光标找不到更多结果时，该代码路径仅 退出 。当您使用更常见的 mysql_store_result() 时，MySQL 已经在 return 将结果发送到应用程序之前完成了此操作。但是，当您使用 mysql_use_result() 时，MySQL 要求您执行 "the dirty work" 迭代结果集的其余部分以清除游标。好玩
mysql_use_result() initiates a result set retrieval but does not actually read the result set into the client like mysql_store_result() does. Instead, each row must be retrieved individually by making calls to mysql_fetch_row(). This reads the result of a query directly from the server without storing it in a temporary table or local buffer, which is somewhat faster and uses much less memory than mysql_store_result(). The client allocates memory only for the current row and a communication buffer that may grow up to max_allowed_packet bytes.
On the other hand, you should not use mysql_use_result() for locking reads if you are doing a lot of processing for each row on the client side, or if the output is sent to a screen on which the user may type a ^S (stop scroll). This ties up the server and prevent other threads from updating any tables from which the data is being fetched.
When using mysql_use_result(), you must execute mysql_fetch_row() until a NULL value is returned, otherwise, the unfetched rows are returned as part of the result set for your next query. The C API gives the error Commands out of sync; you can't run this command now if you forget to do this!
因此，实际回答您的问题：
1) What happens if I don't follow that line?
CAPI会return报错信息：Commands out of sync; you can't run this command now
2) I think that there actually must be a way to prematurely end fetching rows if I decide that I have enough information (otherwise, this whole thing wouldn't make much sense in my eyes).
有人会想，但没有。您必须 完全迭代结果集。
我正在为 MySQL 5.5 编写我的第一个 C 客户端，并且在文档中偶然发现了 the following page。几乎在最后，它指出（粗体强调我的，斜体强调不是我的）：
An advantage of mysql_use_result() is [...]. Disadvantages are that [...]. Furthermore, you must retrieve all the rows even if you determine in mid-retrieval that you've found the information you were looking for.
最后一句没看清楚
1) 如果我不遵循那条线会怎样？
2) 我认为如果我确定我有足够的信息，实际上必须有一种方法可以提前结束获取行（否则，这整个事情在我眼中就没有多大意义）。
我知道如果我只是停止获取行然后尝试执行下一条语句，可能会发生一些不好的事情，但是没有像 mysql_finish_fetch() 或类似的函数吗？
如果我调用 mysql_free_result() 会发生什么？即使我还没有获取所有行，这也应该释放结果，所以在检索中调用它并继续我想做的任何事情应该是安全的。我错了吗？
这听起来像是 MySQL 暴露给客户端的内部线程问题。将其归结为各种 MySQL gotchas。简而言之，MySQL 显然内部有有限数量的 "searchers"，而使用 mysql_use_result() 显然将其中一个专用于您的 API 请求。此外，MySQL 显然没有暴露的 API 调用来取消这样的请求。唯一的选择是看到抓取直到结束。
稍长的版本：在内部，MySQL 的游标显然只有一个代码路径——我想是为了在常见情况下提高性能。当光标找不到更多结果时，该代码路径仅 退出 。当您使用更常见的 mysql_store_result() 时，MySQL 已经在 return 将结果发送到应用程序之前完成了此操作。但是，当您使用 mysql_use_result() 时，MySQL 要求您执行 "the dirty work" 迭代结果集的其余部分以清除游标。好玩
mysql_use_result()initiates a result set retrieval but does not actually read the result set into the client likemysql_store_result()does. Instead, each row must be retrieved individually by making calls tomysql_fetch_row(). This reads the result of a query directly from the server without storing it in a temporary table or local buffer, which is somewhat faster and uses much less memory thanmysql_store_result(). The client allocates memory only for the current row and a communication buffer that may grow up tomax_allowed_packetbytes.On the other hand, you should not use
mysql_use_result()for locking reads if you are doing a lot of processing for each row on the client side, or if the output is sent to a screen on which the user may type a^S(stop scroll). This ties up the server and prevent other threads from updating any tables from which the data is being fetched.When using
mysql_use_result(), you must executemysql_fetch_row()until a NULL value is returned, otherwise, the unfetched rows are returned as part of the result set for your next query. The C API gives the errorCommands out of sync; you can't run this command nowif you forget to do this!
因此，实际回答您的问题：
1) What happens if I don't follow that line?
CAPI会return报错信息：Commands out of sync; you can't run this command now
2) I think that there actually must be a way to prematurely end fetching rows if I decide that I have enough information (otherwise, this whole thing wouldn't make much sense in my eyes).
有人会想，但没有。您必须 完全迭代结果集。