* Eldon Ziegler
>
> mysql is slow when finding the last value before a period of time (I'm
> plotting the temperature values during a period of time and want the last
> value before the period begins).
>
> The query is:
> select DateTime, Target, Value from log
> where DateTime < "2001-12-21 00:00:00"
> and Source = "StatIO#1"
> and Target = "SN1"
> and (Attribute = NULL or Attribute = "" or Attribute = "T")
> order by DateTime desc limit 1;
>
> The table has two indexes,
> 1. DateTime only
> 2. dtSrcTarAttr on log (DateTime, Source, Target, Attribute)
>
> "Explain" shows the query using the DateTime index instead of the
> dtSrcTarAttr that contains all items in the WHERE clause.
>
> How can I speed up this query?
Change (DateTime, Source, Target, Attribute) to (Source, Target, DateTime,
Attribute)
--
Roger
