- add mistakes section

- improve error log text

Author: jordansissel

studies/mysql.md

@@ -21,13 +21,25 @@ According to [mysql's docs][mysql-docs], there are about five different kinds of
 Depending on storage engines used, you'll have other log files as well (innodb
 log file, etc).
 
+In most mysql deployments, there are 3 possible actors: the mysql server, mysql
+clients, and replication parters. The logs tend to cover actions between pairs
+of actors, or roughly the purpose of each log:
+
+* internal mysql server: error log
+* replication: binary and relay logs
+* clients: general query and slow query logs
+
 ## Error Log
 
 Relevant my.cnf settings: `log-error`, `log-warnings`.
 
-The error log generally contains information about mysqld's status; generally
-start and stop messages as well as critical errors. Unfortunately, mysql also
-logs warnings to the error log by default.
+According to mysql's docs, the error log generally contains information about
+mysqld's status; generally start and stop messages as well as critical errors.
+Unfortunately, mysql also logs warnings to the error log by default.
+
+What the docs don't say is that pretty much anything not data-related is logged
+here. This log file is a complete mess. Most of what I see in the sample below 
+has nothing to do with errors or warnings.
 
     120707  0:37:09 [Note] Plugin 'FEDERATED' is disabled.
     120707  0:37:09 InnoDB: The InnoDB memory heap is disabled
@@ -51,6 +63,14 @@ The format of this log file is a bit confusing and pretty inconsistent. I'm not
 sure how "highest supported file format is Barracuda." is an error or a
 warning, but it's in the error log anyway. ☹
 
+### Mistakes
+
+This log format is a bit strange. The timestamp seems consistent, even if it is
+odd. The message seems to have no consistency, meaning 'grep' and other
+pattern/text tools may be your chief weapons against this beast.
+
+I wouldn't expect to achieve much in the ways of data mining from this log.
+
 ## General Query Log
 
 This log appears to contain every request sent to the mysql server. Here's what
@@ -92,12 +112,3 @@ And in the general query log:
 
 ## Relay Log
 
-
-# Overview
-
-We can bucket these logs by purpose:
-
-* application internal: error log
-* replication: binary and relay logs
-* user: general query and slow query logs
-