I always knew proper indexes were important, but hadn't realized just HOW important until recently. We purchased an off-the-shelf eCommerce solution and implemented it. The SQL server CPU utilization would skyrocket to 99% when we hit about 600 concurrent users. After some investigation, I found that the Users table was NOT indexed properly. I added a couple indexes, built them, and as soon as I did that I watched the CPU utilization DROP IMMEDIATELY to 10%.
I couldn't believe my eyes. At THAT second I realized just how important proper indexes truly are.
Anyone else experience an eye-opener like this?