After earlier work with DATETIME and indexes, I am still seeing some performance issues with storing logs with a full timestamp. Since a lot of the reporting is based on day-on-day and week-on-week comparisons, it could be more efficient to store the date and time components separately.
For my purposes, I’m using time to a resolution of about 15 minutes so there’s no real need to suffer the extra storage and performance overhead of working with a TIME field.
Instead, I’ve opted for a TINYINT with a value between 0-95 inclusive – one for every fifteen minutes of the day. Naturally, this is indexed.
The issue comes with how to convert time into these segments, and back again. Logically, 00:00 to 00:15 should be the first segment, but I have to be careful when reporting this back.
Technically if I’m quoting the range in a graph, there’s no problem. It’s between 00:00 and 00:15. If I’m forced to plot a point – say in a graph – the easiest solution would be to drop the dot at 00:00:00. The technically most accurate would be at 00:07:30.
That could be awfully messy, but it’s the midpoint of each range and better than quoting a measurement (say, originally at 00:14:59.999999….) as 00:00:00 when it would clearly be better as 00:15:00.
In this case, I suspect simplicity prevails. Most normal people (i.e. not me) would be more comfortable seeing nice round numbers: 00:00, 00:15 and so on. Seeing 00:07:30, 00:22:30 and so on is likely to be unnecessarily confusing. Thankfully the incoming quality of data is not that precise, so nobody will scrutinise this.
Quoting the full range (00:00 – 00:15) would be more appropriate anyway, in tables and text reports.