I’ve poked around the source before to try and figure things out, but code for handling explain seems to be a bit distributed on the code base, so it takes more than just a few minutes reading to understand it.

So that leaves me with my second best guess: strace.

Take a look at this:

(I’m using lookup from udf_example.so)

mysql> select lookup(url) from urls;
+—————+
| lookup(url) |
+—————+
| 69.80.205.238 |
| 69.89.31.108 |
| 69.80.205.238 |
+—————+
3 rows in set (0.09 sec)

And here’s the strace relevant for the lookup:

[pid 8205] 0.000120 write(3, “100127 16:40:23\t 9 Query select lookup(url) from urls\n”…, 65) = 65
…
[pid 8205] 0.000878 open(“/etc/resolv.conf”, O_RDONLY) = 37
[pid 8205] 0.000179 fstat(37, {st_mode=S_IFREG|0644, st_size=240, …}) = 0

Now with explain:

mysql> explain select lookup(url) from urls;
+—-+————-+——-+——+—————+——+———+——+——+——-+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+—-+————-+——-+——+—————+——+———+——+——+——-+
| 1 | SIMPLE | urls | ALL | NULL | NULL | NULL | NULL | 3 | |
+—-+————-+——-+——+—————+——+———+——+——+——-+
1 row in set (0.00 sec)

[pid 8205] 0.000137 write(3, “100127 16:40:35\t 9 Query explain select lookup(url) from urls\n”…, 73) = 73
[pid 8205] 0.000185 sched_setscheduler(8205, SCHED_OTHER, { 6 }) = -1 EINVAL (Invalid argument)
[pid 8205] 0.000280 gettimeofday({1264617635, 884763}, NULL) = 0
[pid 8205] 0.000091 gettimeofday({1264617635, 884852}, NULL) = 0
[pid 8205] 0.000266 write(29, “\x01\x00\x00\x01\x0a\x18\x00\x00\x02\x03\x64\x65\x66\x00\x00\x00\x02\x69\x64\x00\x0c\x3f\x00\x03\x00\x00\x00\x08\xa1\x00
\x00\x00\x00\x21\x00\x00\x03\x03\x64\x65\x66\x00\x00\x00\x0b\x73\x65\x6c\x65\x63\x74\x5f\x74\x79\x70\x65\x00\x0c\x08\x00\x13\x00\x00\x00\xfd\x01\x00\x1f\x00\
x00\x1b\x00\x00\x04\x03\x64\x65\x66\x00\x00\x00\x05\x74\x61\x62\x6c\x65\x00\x0c\x08\x00\x40\x00\x00\x00\xfd\x00\x00\x1f\x00\x00\x1a\x00\x00\x05\x03\x64\x65\x
66\x00\x00\x00\x04\x74\x79\x70\x65\x00\x0c\x08\x00\x0a\x00\x00\x00\xfd\x00\x00\x1f\x00\x00\x23\x00\x00\x06\x03\x64\x65\x66\x00\x00\x00\x0d\x70\x6f\x73\x73\x6
9\x62\x6c\x65\x5f\x6b\x65\x79\x73\x00\x0c\x08\x00\x00\x10\x00\x00\xfd\x00\x00\x1f\x00\x00\x19\x00\x00\x07\x03\x64\x65\x66\x00\x00\x00\x03\x6b\x65\x79\x00\x0c
\x08\x00\x40\x00\x00\x00\xfd\x00\x00\x1f\x00\x00\x1d\x00\x00\x08\x03\x64\x65\x66\x00\x00\x00\x07\x6b\x65\x79\x5f\x6c\x65\x6e\x00\x0c\x08\x00\x00\x10\x00\x00\
xfd\x00\x00\x1f\x00\x00\x19\x00\x00\x09\x03\x64\x65\x66\x00\x00\x00\x03\x72\x65\x66\x00\x0c\x08\x00\x00\x04\x00\x00\xfd\x00\x00\x1f\x00\x00\x1a\x00\x00\x0a\x
03\x64\x65\x66\x00\x00\x00\x04\x72\x6f\x77\x73\x00\x0c\x3f\x00\x0a\x00\x00\x00\x08\xa0\x00\x00\x00\x00\x1b\x00\x00\x0b\x03\x64\x65\x66\x00\x00\x00\x05\x45\x7
8\x74\x72\x61\x00\x0c\x08\x00\xff\x00\x00\x00\xfd\x01\x00\x1f\x00\x00\x05\x00\x00\x0c\xfe\x00\x00\x02\x00\x19\x00\x00\x0d\x01\x31\x06\x53\x49\x4d\x50\x4c\x45
\x04\x75\x72\x6c\x73\x03\x41\x4c\x4c\xfb\xfb\xfb\xfb\x01\x33\x00\x05\x00\x00\x0e\xfe\x00\x00\x02\x00″…, 369) = 369
[pid 8205] 0.000457 gettimeofday({1264617635, 885575}, NULL) = 0
[pid 8205] 0.000092 sched_setscheduler(8205, SCHED_OTHER, { 8 }) = -1 EINVAL (Invalid argument)
[pid 8205] 0.000103 gettimeofday({1264617635, 885770}, NULL) = 0
[pid 8205] 0.000084 gettimeofday({1264617635, 885853}, NULL) = 0
[pid 8205] 0.000086 gettimeofday({1264617635, 885939}, NULL) = 0
[pid 8205] 0.000116 gettimeofday({1264617635, 886058}, NULL) = 0
[pid 8205] 0.000130 write(4, “# Time: 100127 16:40:35\n# User@Host: root[root] @ localhost []\n# Thread_id: 9 Schema: test\n# Query_time: 0.001985 Lo
ck_time: 0.000690 Rows_sent: 1 Rows_examined: 0 Rows_affected: 0 Rows_read: 1\nexplain select lookup(url) from urls;\n”…, 238) = 238
…

To discard a possible dns caching effect, I ran the original query twice on the same connection, and got the same strace output, so it’s always going to /etc/resolv.conf

Still, without looking at the source code, this is just anecdotical evidence from one example, I’ll try to look more into this and perhaps make a new post on the subject

explain select id from projects where id = (select my_udf(max(id)) from projects where name like ‘en%’);

Would this EXPLAIN then actually call my_udf()? Seems a bit scary (but maybe one should expect that?)

mysql -e ‘create database rtest’
[... create some inodb tables on rtest, with foreign key constraints ... ]

java pl.kernelpanic.dbmonster.Launcher -c rtest.properties –grab > rtest.xml

After properly editing the begining of the rtest.xml file (since it will included some non-xml output from dbmonster):

java pl.kernelpanic.dbmonster.Launcher -c rtest.properties -s rtest.xml

This successfully generates rows for all tables, satisfying referential integrity

This might be of interest to you too: http://www.cohprog.com/mod_bandwidth.html, though I don’t know if it’s still active, or if it works across apache v 1.x and 2.x.

The good side of the iptables approach is that it can be used to limit concurrent connections to any network based service

Good luck,

I have been trying to find a way to prevent Apache from spawning too many child processes whenever some bad robot or hacker throws a bunch of concurrent requests at our website.

This may just do the trick…