[barrkel]

I've seen situations where, depending on how MySQL is feeling, it will use an index for an order by, and sometimes not (uses a filesort instead).

For some data sets, it's reasonably quick (100ms), for others it's quite slow (1600ms).

Whereas if I add 'force index for order by' to the query, I get a predictable 150ms across the board.

Predictability is often more important than optimality. Variance in service response time is hard to work around, especially when there's a UI involved. Algorithmically, that often means always using the logarithmic data structure even when the constant overhead is higher for smaller data.

[hinkley]

I have known so many engineers that rely on wishful thinking, it gets pretty frustrating after a while. If they do it with bugs they'll do it with resource planning.

Another place where they do this is oversubscribing a server, by putting 4 services that need 20-30% of the system memory or network to run, and don't understand why throughput drops off by 90% when the system becomes saturated and starts swapping or dropping packets.