If you've ever used MySQL, you've probably used either the SHOW STATUS; query or the equivalent mysqladmin status command at least once.

$ mysqladmin status
Uptime: 569459  Threads: 2  Questions: 17666  Slow queries: 3  Opens: 118
  Flush tables: 1  Open tables: 111  Queries per second avg: 0.31

Notice anything funny there? You should. Pulling out the trusty calculator tells me 17666 / 569459 is in fact closer to .0310, not 0.31. This a a classic case of floating point string printing gone wrong. Why does this happen? Turns out the internal printf() function used by MySQL doesn't support %f, the normal format specifier for floats. So instead, the float is printed using two integers separated by a period. Of course, this is total failsauce if you don't zero-pad your values.

The fix is easy; simply make sure you zero-pad the printed value right of the decimal point if doing division and modulo tricks. Here is the patch with a couple of extra lines of context to see what is going on:

diff -r -U8 mysql-5.5.12-original/sql/sql_parse.cc mysql-5.5.12/sql/sql_parse.cc
--- mysql-5.5.12-original/sql/sql_parse.cc      2011-04-11 05:44:03.000000000 -0500
+++ mysql-5.5.12/sql/sql_parse.cc       2011-05-17 09:07:28.000000000 -0500
@@ -1303,17 +1303,17 @@
     if (!(uptime= (ulong) (thd->start_time - server_start_time)))
       queries_per_second1000= 0;
       queries_per_second1000= thd->query_id * LL(1000) / uptime;

     length= my_snprintf(buff, buff_len - 1,
                         "Uptime: %lu  Threads: %d  Questions: %lu  "
                         "Slow queries: %lu  Opens: %lu  Flush tables: %lu  "
-                        "Open tables: %u  Queries per second avg: %u.%u",
+                        "Open tables: %u  Queries per second avg: %u.%03u",
                         (int) thread_count, (ulong) thd->query_id,
                         (uint) (queries_per_second1000 / 1000),
                         (uint) (queries_per_second1000 % 1000));

Now the values print as expected when the decimal portion is less than 100:

$ mysqladmin status
Uptime: 3  Threads: 1  Questions: 2  Slow queries: 0  Opens: 33
  Flush tables: 1  Open tables: 26  Queries per second avg: 0.666

$ mysqladmin status
Uptime: 50  Threads: 1  Questions: 3  Slow queries: 0  Opens: 33
  Flush tables: 1  Open tables: 26  Queries per second avg: 0.060

I filed a bug report with the above patch, and got at least one piece of quick feedback, so we'll see if it gets fixed anytime soon. I also looked quickly for any other suspect format specifiers (the regex was '[ud]\.%[ud]') and didn't see any.

Shockingly this bug has been around since some changes in April 2007. I can't believe I was the only one noticing this and expecting it to get fixed at any time. I guess it just finally pissed me off enough today to do something about it.