Question

Hi all wonder if someone could advise a more efficient way to select rows from a table that has roughly 60 millions records in it. Each row has a date stored as a nvarchar, for example ‘20110527030000.106’. I want to select all rows that are 3 months or older based on this date field, so for example i’m only interested in the first part of the date field; ‘20110527’. I have the following code to do that, however its a bit slow and wondering if there was a better way?

DECLARE @tempDate varchar(12)
SET @tempDate = convert(varchar(12),DATEADD(m,-3,GETDATE()),112)

SELECT *
  FROM [TABLE A]
  WHERE SUBSTRING([DATE_FIELD],0,8) < @tempDate

Answer 1

Your query not only it can’t use any index on [DATE_FIELD] and does a full scan but it also applies the SUBSTRING() function to all values of the (date_field column of the) table.

Don’t apply any function on the column so the index of [DATE_FIELD] can be used and the function is only applied once, at the calculation of @tempDate :

SELECT *
  FROM [TABLE A]
  WHERE [DATE_FIELD] < @tempDate

The < comparison works for varchar values. The following will evaluate to True:

'20110526030000.106' < '20110527'

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Is there any reason that the datetime is not stored as datetime type?