Looks like MySQL Camp 2006 was really interesting and useful for its attendees and for entire MySQL community. As the result of this meeting of many MySQL-related professionals we’ve got lots of interesting publications and I want to refer one of them in this post. Very interesting list of 84 MySQL performance tips has beed created on first day of this year MySQL Camp at Google headquarters:

1. Index stuff.
2. Don’t Index Everything
3. Use benchmarking
4. Minimize traffic by fetching only what you need.
   • Paging/chunked data retrieval to limit
   • Don’t use SELECT *
   • Be wary of lots of small quick queries if a longer query can be more efficient
5. Use EXPLAIN to profile the query …

Maybe not breaking news, but I think it’s interesting enough of a point, and I didn’t really find anything about the topic when I googled it. If you do any addition, subtraction, multiplication, or division (and probably a lot more mathematical functions for that matter) and NULL is one of your values, the entire expression will evaluate to NULL.

For example, this statement returns NULL:

select 4 + NULL;

+———-+
| 4 + NULL |
+———-+
| NULL |
+———-+

Normally you wouldn’t do the above in such a simple way, for instance, you might do some addition in a subquery. For example,

select 4 + (select val from table1 WHERE id < 3 LIMIT 1);

+---------------------------------------------------+
| 4 + (select val from table1 WHERE id < 3 LIMIT 1) |
+---------------------------------------------------+
| NULL |
…

Finding your Independent Data Path:

dbmcli ?d -u , dbm_getpath IndepDataPath

Finding your Run Directory
dbmcli ?d -u , param_directget RUNDIRECTORY