16

I have a query that is currently taking an average of 2500ms to complete. My table is very narrow, but there are 44 million rows. What options do I have to improve performance, or is this as good as it gets?

The Query

SELECT TOP 1000 * FROM [CIA_WIZ].[dbo].[Heartbeats]
WHERE [DateEntered] BETWEEN '2011-08-30' and '2011-08-31'; 

The Table

CREATE TABLE [dbo].[Heartbeats](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [DeviceID] [int] NOT NULL,
    [IsPUp] [bit] NOT NULL,
    [IsWebUp] [bit] NOT NULL,
    [IsPingUp] [bit] NOT NULL,
    [DateEntered] [datetime] NOT NULL,
 CONSTRAINT [PK_Heartbeats] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

The Index

CREATE NONCLUSTERED INDEX [CommonQueryIndex] ON [dbo].[Heartbeats] 
(
    [DateEntered] ASC,
    [DeviceID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]

Would adding additional indexes help? If so, what would they look like? The current performance is acceptable, because the query is only run occasionally, but I'm wondering as a learning exercise, is there anything I can do to make this faster?

UPDATE

When I change the query to use a force index hint, the query executes in 50ms:

SELECT TOP 1000 * FROM [CIA_WIZ].[dbo].[Heartbeats] WITH(INDEX(CommonQueryIndex))
WHERE [DateEntered] BETWEEN '2011-08-30' and '2011-08-31' 

Adding a correctly selective DeviceID clause also hits the 50ms range:

SELECT TOP 1000 * FROM [CIA_WIZ].[dbo].[Heartbeats]
WHERE [DateEntered] BETWEEN '2011-08-30' and '2011-08-31' AND DeviceID = 4;

If I add ORDER BY [DateEntered], [DeviceID] to the original query, I am in the 50ms range:

SELECT TOP 1000 * FROM [CIA_WIZ].[dbo].[Heartbeats]
WHERE [DateEntered] BETWEEN '2011-08-30' and '2011-08-31' 
ORDER BY [DateEntered], [DeviceID];

These all use the index I was expecting (CommonQueryIndex) so, I suppose my question is now, is there a way to force this index to be used on queries like this? Or is the size of my table throwing off the optimizer too much and I must just use an ORDER BY or a hint?

1
15

Why the the optimiser doesn't go for your your first index:

CREATE NONCLUSTERED INDEX [CommonQueryIndex] ON [dbo].[Heartbeats] 
(
    [DateEntered] ASC,
    [DeviceID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]

Is a matter of selectivity of the [DateEntered] Column.

You told us that your table has 44 million rows. the row size is:

4 bytes, for the ID, 4 bytes for the Device ID, 8 bytes for the date, and 1 byte for the 4 bit columns. that's 17 bytes + 7 bytes overhead for (tags, Null bitmap, var col offset,,col count) totals 24 Bytes per row.

That would rougly translate to 140k pages. To store those 44 million rows.

Now the optimiser can do two things:

  1. It could scan the table (clustered index scan)
  2. Or it could use your index. For every row in your index, it would then need to do a bookmark lookup in the clustered index.

Now at a certain point it just becomes more expensive to do all these single lookups in the clustered index for every index entry found in your non clustered index. The threshold for that is generally the total count of lookups should exceed 25% tot 33% of the total table page count.

So in this case: 140k/25%=35000 rows 140k/33%=46666 rows.

(@RBarryYoung, 35k is 0.08% of the total rows and 46666 is 0.10 %, so I think that is where the confusion was)

So if your where clause will result in somewhere between 35000 and 46666 rows.(this is underneath the top clause!) It's very likely that your non clustered will not be used and that the clustered index scan will be used.

The only two ways to change this are:

  1. Make your where clause more selective. (if possible)
  2. Drop the * and select only a few columns so you can use a covering index.

now sure you could create a covering index even when you use a select *. Hoever that just creates a massive overhead for your inserts/updates/deletes. We would have to know more about your work load (read vs write) to make sure if that's the best solution.

Changing from datetime to smalldatetime is a 16% reducion in size on clustered index and a 24% reduction in size on your non clustered index.

0
8

Is there a particular reason that your PK is clustered? Many people do this because it defaults that way, or they think that PKs must be clustered. No so. Clustered indexes are usually best for range queries (like this one) or on the foreign key of a child table.

An effect of a clustering index is that it bunches all of the data together because the data is stored on the leaf nodes of the cluster b tree. So, assuming that you are not asking for 'too wide' of a range, the optimizer will know exactly what part of the b tree contains the data and it won't have to find a row identifer and then hop over to where the data is (like it does when dealing with a NC index). What is 'too wide' of a range? A ridiculous example would be asking for 11 months of data from a table that only has a year's worth of records. Pulling one day of data should not be a problem, assuming that your statistics are up to date. (Though, the optimizer may get into trouble if you are looking for yesterday's data and you haven't updated stats for three days.)

Since you are running a "SELECT *" query, the engine will need to return all of the columns in the table (even if someone adds a new one that your app doesn't need at that moment) so a covering index or an index with included columns won't help much, if at all. (If you are including every column from the table in an index, you are doing something wrong.) The optimizer will probably ignore those NC indexes.

So, what to do?

My suggestion would be to drop the NC index, change the clustered PK to nonclustered and create a clustered index on [DateEntered]. Simpler is better, until it is proved otherwise.

0
4

As long as you've got that "*" in there, then the only thing that I could imagine that would make much difference would be to change your index definition to this:

CREATE NONCLUSTERED INDEX [CommonQueryIndex] ON [dbo].[Heartbeats] 
(
    [DateEntered] ASC,
    [DeviceID] ASC
)INCLUDE (ID, IsWebUp, IsPingUp, IsPUp)
 WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]

As I noted in the comments, it should use that index, but if it doesn't you can persuade it to with either an ORDER BY or an index hint.

0
3

I'd look at this a bit differently.

  • Yes, I know it's an old thread but I'm intrigued.

I'd dump the datetime column - change it to an int. Have a lookup table or do a convert for your date.

Dump the clustered index - leave it as a heap and create a non-clustered index on the new INT column which represents the date. i.e. today would be 20121015. That order is important. Depending on how frequently you load the table, look at creating that index in DESC order. Maint cost will be higher and you will want to introduce a fill factor or partitioning. Partitioning would also help decrease your run time.

Lastly, if you can use SQL 2012, try using SEQUENCE - it will outperform identity() for inserts.

0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.