How to improve performance for datetime filtering in SQL Server?

Question

I have a problem with filtering by datetime columns.

I tried these two methods:

datefield < '2013-03-15 17:17:55.179'
datefield < CAST('2013-03-15 17:17:55.179' AS datetime)

I have a large database with over 3.000.000 main objects.

So I need to improve performance for my datetime filtering. I was reading about UNIX timestamp (convert all datetime to UNIX timestamp and then filter by this UNIX field).

I think it's a better way than filtering by datetime. But if anyone knows some other way, I would appreciate it.

My query is:

SELECT TOP (100)  ev.Title as Event_name, po.Name as POI_name, 
po.Address, po.City, po.Region, po.Country, po.Latitude, po.Longitude, ev.Start_time, 
(Select ID_Category FROM SubCategory s where ev.ID_SubCategory = s.ID_SubCategory) as ID_Category, 
ev.ID_SubCategory, ev.ID_Event, ev.ID_Channel, IDChanelEvent, 
ev.FavoriteCount, po.gmtOffset, v.IsFavorite, v1.IsFavorite  
FROM Events ev 
JOIN POI po ON ev.ID_POI = po.ID_POI 
JOIN (SELECT et.id_event as joinIdEv FROM EventTagLink et, tags t 
 WHERE t.id_tag = et.id_tag 
 AND ( t.Title = N'music' ) 
 ) as joinEvents 
 ON joinEvents.joinIdEv = ev.ID_Event 
LEFT JOIN Viewed v ON v.ID_Event = ev.ID_Event AND v.ID_User = 1 AND v.IsFavorite = 1 LEFT join Viewed v1 ON v1.ID_Event = ev.ID_Event AND v1.ID_User = 1 AND v1.IsFavorite = 0
WHERE 
--ev.GmtStop_time > '2013-03-15 14:17:55.188' AND 
po.Latitude > 41.31423 AND po.Latitude < 61.60511 
AND  po.Longitude > -6.676602 AND po.Longitude < 17.04498  
AND ev.ID_SubCategory in (3, 12, 21, 4, 30, 13, 22, 6, 14, 40, 23, 7, 32, 15, 41, 8, 50, 33, 16, 42, 25, 9, 34, 17, 35, 18, 44, 27, 36, 19, 45, 28, 37, 46, 29, 38, 47, 39, 48, 49, 10, 1, 11, 2, 20) 
--AND ev.GmtStart_time< '2013-03-15 17:17:55.179'
AND v1.IsFavorite is null

filtering by the time I commented.

If I turn off these filters, request duration is several seconds. If I turn them on then request duration is over 25 seconds.

• Execution plan with filtering datetime
• Execution plan without datetime filter

So there is a lot of discussion about execute plans, indexes and so on. But what about UNIX timestamp, which is the main reason why I've put the question there. Would it improve performance for datetime filtering?

Thanks in advance.

Answer 1

Just a suggestion when it comes to indexes on datetime in msql is the index footprint impacts search times (Yes this seems obvious...but please read onward).

The importances to this when indexing on the datetime say for instance '2015-06-05 22:47:20.102' the index has to account for every place within the datetime. This becomes very large spatially and bulky. A successful approach that I've leveraged is create a new datetime column and populate the data by rounding the time to the hour and then building the index upon this new column. Example '2015-06-05 22:47:20.102' translates to '2015-06-05 22:00:00.000'. By taking this approach we leave the detailed data alone and can display it or use it by search on this new column which gives us approximately a 10x (at minimum) return on how fast results are returned. This is due to the fact that the index doesn't have to account for the minutes, seconds and millisecond fields.

Answer 2

You need to look at your execution plan first to see what SQL Server is doing. More than likely, you just need add an index. Little conversions like this are almost never the reason why your query is slow. Indices are a good first stop for fixing queries.

You don't need to make this the clustered index. Making it the clustered index means that you don't need to do a lookup, but for only 100 rows, lookup is very fast. I would put datetime and subcategory into a nonclustered index, in that order.

If you are ordering, you should also make sure that's in an index. Since it only makes sense to use one index per table, you'll need to make sure all the relevant columns are in the same index, in the right order.

But first, get your actual execution plan!

Answer 3

For better performance I suggest you create new indexes:

CREATE INDEX x1 ON LiveCity.dbo.Tags(Title) INCLUDE(ID_Tag)
CREATE INDEX x2 ON LiveCity.dbo.Tags(ID_Event, GmtStart_time, GmtStop_time) 
  INCLUDE(
          FavoriteCount, 
          ID_Channel, 
          ID_POI, 
          ID_SubCategory, 
          IDChanelEvent, 
          Start_time, 
          Title
          )
CREATE INDEX x ON LiveCity.dbo.POI(ID_POI, Latitude, Longitude) 
  INCLUDE(
          Address, 
          City, 
          Country, 
          gmtOffset, 
          Name, 
          Region
          )

This will help you avoid RID lookup operation and improve the overall performance of the query.