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diff --git a/wiki/mysql/choose-something-else.md b/wiki/mysql/choose-something-else.md index 96a1d43..24b1fa1 100644 --- a/wiki/mysql/choose-something-else.md +++ b/wiki/mysql/choose-something-else.md @@ -6,6 +6,12 @@ processing; MySQL is differently bad at both poles. (Real apps fall between these poles, and suffer variably from either set of MySQL flaws.) +In the first section, I'll talk about [why MySQL is bad at storing +data](#storage). In the second, I'll talk about [why MySQL is bad at +processing data](#processing). In the third, I'll talk about why these +problems are inherent in the way MySQL was built and are not likely to be +fixed in the foreseeable future. + ## Storage Storage systems have four properties: @@ -51,3 +57,33 @@ familiar with other SQL implementations). ### Preserving Data +... against unexpected changes: like most disk-backed storage systems, MySQL +is as reliable as the disks and filesystems its data lives on. + +... against loss: hoo boy. MySQL's default approach to +[backups](http://dev.mysql.com/doc/refman/5.5/en/backup-methods.html) is to +either dump data to SQL periodically (slow and an inefficient use of space, +with little support for incremental backups) or to use binary logs (plus a +base filesystem backup) for ongoing backups. (Binary logs are also the basis +of MySQL's first-party replication system.) If neither of these are +sufficient, you're left with purchasing [a backup tool from +Oracle](http://dev.mysql.com/doc/refman/5.5/en/glossary.html#glos_mysql_enterprise_backup). + +Like many of MySQL's features, the binary logging feature is +[too](http://dev.mysql.com/doc/refman/5.5/en/binary-log.html) +[configurable](http://dev.mysql.com/doc/refman/5.5/en/replication-options-binary-log.html), +while still, somehow, defaulting to modes that are hazardous or surprising: +the +[default](http://dev.mysql.com/doc/refman/5.5/en/replication-options-binary-log.html#sysvar_binlog_format) +[behaviour](http://dev.mysql.com/doc/refman/5.5/en/replication-formats.html) +is to log SQL statements, rather than logging their side effects. This has +lead to numerous bugs over the years; MySQL now makes an effort to make common +"non-deterministic" cases such as `NOW()` and `RANDOM()` act deterministically +but these have been addressed using ad-hoc solutions. Restoring +binary-log-based backups can easily lead to data that differs from the +original system, and by the time you've noticed the problem, it's too late to +do anything about it. + +(Seriously. The binlog contains the current time on the master and the random +seed for every statement, just in case. If your non-deterministic query uses +any other function, you're still fucked by default.)
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