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?
Eldon Ziegler
ProAtion Systems, Inc.
www.proation.com
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