---
這個問題很簡單，稍微看看mysql客戶端的程式碼上下文即可

---
###一、問題相關

有返回了笛卡爾積資料，導致mysql客戶端吃光了記憶體
![image.png](https://upload-images.jianshu.io/upload_images/7398834-2944ace535f4fd2c.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240)

這種這個mysql客戶端執行緒將會oom導致kill。

###二、問題分析
對於select語句來講，對於mysql客戶端而言是互動式模式而不是batch模式。
```
#9  0x000000000041b2ed in cli_read_rows (mysql=0xaf4760 <mysql>, mysql_fields=0x1e4bf60, fields=26) at ../../mysql-8.0.20/sql-common/client.cc:3013
#10 0x000000000041f615 in mysql_store_result (mysql=mysql@entry=0xaf4760 <mysql>) at ../../mysql-8.0.20/sql-common/client.cc:7333
#11 0x000000000040e1a8 in mysql_store_result_for_lazy (result=0x7ffc05aaf8f8) at ../../mysql-8.0.20/client/mysql.cc:3055
#12 0x0000000000410f3c in com_go (buffer=buffer@entry=0xaf4680 <glob_buffer>, line=line@entry=0x0) at ../../mysql-8.0.20/client/mysql.cc:3299
#13 0x0000000000412cee in add_line (buffer=..., truncated=false, ml_comment=<synthetic pointer="">, in_string=<synthetic pointer="">, line_length=<optimized out="">, line=0x1e41d70 "select * from city,country,countrylanguage,user;") at ../../mysql-8.0.20/client/mysql.cc:2532
#14 read_and_execute(bool) () at ../../mysql-8.0.20/client/mysql.cc:2287
#15 0x0000000000409ccf in main () at ../../mysql-8.0.20/client/mysql.cc:1432
#16 0x00007f70cd61f505 in __libc_start_main () from /lib64/libc.so.6
#17 0x000000000040a730 in _start () at ../../mysql-8.0.20/include/my_alloc.h:86

```

com_go函式會實際的執行sql語句。然後翻開這個函式可以看到如下程式碼：
```
if (quick) {
      if (!(result = mysql_use_result(&mysql)) && mysql_field_count(&mysql)) {
        error = put_error(&mysql);
        goto end;
      }
    } else {
      error = mysql_store_result_for_lazy(&result);
      if (error) goto end;
    }
```

說明如果quick為ture那麼可以不走mysql_store_result_for_lazy分支，那麼看看mysql -q的含義：
```
  -q, --quick         Don't cache result, print it row by row. This may slow
                      down the server if the output is suspended. Doesn't use
                      history file.
```
因此我們在mysql 命令加上-q 就可以解決由於大資料集快取導致的OOM問題。</optimized></synthetic></synthetic></glob_buffer></mysql></mysql>

MySQL:MySQL客戶端快取結果導致OOM

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