I have the following table:
CREATE TABLE [Cache].[Marker](
[ID] [int] NOT NULL,
[SubID] [varchar](15) NOT NULL,
[ReadTime] [datetime] NOT NULL,
[EquipmentID] [varchar](25) NULL,
[Sequence] [int] NULL
) ON [PRIMARY]
With the following clustered index:
CREATE UNIQUE CLUSTERED INDEX [IX_Marker_EquipmentID_ReadTime_SubID] ON [Cache].[Marker]
(
[EquipmentID] ASC,
[ReadTime] ASC,
[SubID] 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]
GO
And this query:
Declare #EquipmentId nvarchar(50)
Set #EquipmentId = 'KLM52B-MARKER'
SELECT TOP 1
cr.C44DistId,
cr.C473RightLotId
From Cache.Marker m
INNER JOIN Cache.vwCoaterRecipe AS cr ON cr.MarkerId = m.ID
Where m.EquipmentID = #EquipmentId And m.ReadTime >= '3/1/2013'
ORDER BY m.Id desc
Here is the query plan being generated:
My question is this. Why isn't the clustered index on the Cache.Marker table being used with a seek instead of a scan on another index? Furthermore, SSMS query analyzer is suggesting I add an index on Marker.ReadTime with ID and EquipmentID columns included.
There are roughly 1M rows in the Cache.Marker table.
How many unique equipment ID's do you have? It's probably decided date is a better first lookup (perhaps mistakenly). You can force it to use your index though with the WITH( INDEX() ) statement. FORCESEEK can help as well. I highly recommend this because then index behavior is predictable as databases grow to large sizes.
SELECT TOP 1
cr.C44DistId,
cr.C473RightLotId
From Cache.Marker m
WITH ( INDEX( IX_Marker_EquipmentID_ReadTime_SubID ), FORCESEEK )
INNER JOIN Cache.vwCoaterRecipe AS cr
ON cr.MarkerId = m.ID
Where m.EquipmentID = #EquipmentId And m.ReadTime >= '3/1/2013'
ORDER BY m.Id desc
Related
This is my scenario:
CREATE TABLE [dbo].[tblSMSSendQueueMain](
[ID] [int] IDENTITY(1,1) NOT NULL,
[SendMethod] [int] NOT NULL
CONSTRAINT [PK_tblSMSSendQueueLog] 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] TEXTIMAGE_ON [PRIMARY]
GO
CREATE TABLE [dbo].[tblSMSSendQueueMainSendStatus](
[ID] [bigint] IDENTITY(1,1) NOT NULL,
[QueueID] [int] NULL,
[SendStatus] [int] NULL,
[StatusDate] [datetime] NULL,
[UserID] [int] NULL,
CONSTRAINT [PK_tblSMSSendQueueMainSendStatus] 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]
GO
and some indexes:
CREATE NONCLUSTERED INDEX [IX_tblSMSSendQueueMainSendStatus_SendStatus_Single] ON [dbo].[tblSMSSendQueueMainSendStatus]
(
[SendStatus] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
CREATE NONCLUSTERED INDEX [IX_tblSMSSendQueueMain_SendMethod] ON [dbo].[tblSMSSendQueueMain]
(
[SendMethod] ASC,
[ID] DESC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
each table have about 13m rows
QueueID column of tblSMSSendQueueMainSendStatus is foreign key of ID column in tblSMSSendQueueMain.
The server has an 8 cores Xeon CPU and 8GB RAM.
I use offset and fetch for my paging plan, its perfect and OK for offset under 100k but when the offset going up (more than 100k), the query response is slow, and takes about 5 or 6 seconds to run.
This is my query:
SELECT q.ID
FROM tblSMSSendQueueMain q
INNER JOIN tblSMSSendQueueMainSendStatus qs
ON q.ID = qs.QueueID
WHERE 1 = 1
AND qs.SendStatus = 5
AND [SendMethod] = 19
ORDER BY q.ID desc OFFSET 10 * (1000000 - 1) ROWS
FETCH NEXT 10 ROWS ONLY
Does anyone have any idea where I am going wrong?
The reason this is so slow is that the only way for the server to get the correct starting row is by reading every single row before it.
You are much better off using Keyset Pagination. Instead of paging by starting row-number, pass in a parameter of the starting key.
For this to work, you must return a unique column or columns, and for this to be performant they should be indexed well.
Pass in #startingRow as the previous batch's highest ID, you can get this any way you like. E.g. I have used an ORDER BY so it will be the last row, or your client app will be able to retrieve it from a variable.
SELECT TOP (10)
q.ID
FROM tblSMSSendQueueMain q
INNER JOIN tblSMSSendQueueMainSendStatus qs
ON q.ID = qs.QueueID
WHERE 1 = 1
AND qs.SendStatus = 5
AND q.[SendMethod] = 19
AND qs.ID > #startingRow -- drop this line for the first query
ORDER BY qs.ID;
I must say, your query is somewhat strange. If the foreign key is q.ID = qs.QueueID, then you will get multiple identical results if you are just querying q.ID. I suspect you actually only want q.ID, in which case that is your unique key:
SELECT TOP (10) DISTINCT
q.ID
FROM tblSMSSendQueueMain q
INNER JOIN tblSMSSendQueueMainSendStatus qs
ON q.ID = qs.QueueID
WHERE 1 = 1
AND qs.SendStatus = 5
AND q.[SendMethod] = 19
AND q.ID > #startingRow -- drop this line for the first query
ORDER BY q.ID;
Alternatively, I would prefer an EXISTS/IN as it more clearly states the requirement:
SELECT TOP (10)
q.ID
FROM tblSMSSendQueueMain q
WHERE 1 = 1
AND q.[SendMethod] = 19
AND q.ID IN (
SELECT qs.QueueID
FROM tblSMSSendQueueMainSendStatus qs
WHERE qs.SendStatus = 5
)
AND q.ID > #startingRow -- drop this line for the first query
ORDER BY q.ID;
I'm trying to join data from 3 tables in SQL Servre and display in result:
Alias of an entity
if the entity is virtual
the last date (if known)
the value (if known)
I tried this :
select
sr.alias, c.virtual, max(d.date) date
from
App_references sr
join
Sensor c on (c.id_capteur = sr.id_capteur)
left join
Sensor_data d on (c.id_capteur = d.id_capteur)
group by
d.id_capteur, sr.alias, c.virtual
order by
sr.alias
Here is the database scheme:
CREATE TABLE [dbo].[App_reference]
(
[id_ref] [int] IDENTITY(1,1) NOT NULL,
[alias] [varchar](60) NOT NULL,
[id_capteur] [int] NOT NULL,
CONSTRAINT [PK_App_reference] PRIMARY KEY CLUSTERED
(
[id_ref] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
CREATE TABLE [dbo].[Sensor]
(
[id_capteur] [int] IDENTITY(1,1) NOT NULL,
[name] [varchar](50) NOT NULL,
[virtual] [tinyint] NULL,
[unite] [varchar](5) NULL,
[id_type] [int] NOT NULL,
CONSTRAINT [PK_Sensor] PRIMARY KEY CLUSTERED
(
[id_capteur] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
CREATE TABLE [dbo].[Sensor_data]
(
[id_entry] [int] IDENTITY(1,1) NOT NULL,
[id_capteur] [int] NOT NULL,
[value] [xml] NOT NULL,
[date] [datetime] NOT NULL,
CONSTRAINT [PK_Sensor_data] PRIMARY KEY CLUSTERED
(
[id_entry] ASC,
[id_capteur] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
Supposing each columns like "id_%" are linked by foreign key.
The request on top pass well, I got value :
alias virtual date
Place 1 (Physique) 0 2017-04-27 14:58:42.423
Place 2 1 NULL
Place 3 1 NULL
But I tried to select the value too by doing this :
select
sr.alias, c.virtual, max(d.date) date, d.value
from
Citopia_test.dbo.Smartparking_reference sr
join
Citopia_test.dbo.Sensor c on (c.id_capteur = sr.id_capteur)
left join
Citopia_test.dbo.Sensor_data d on (c.id_capteur = d.id_capteur)
group by
d.id_capteur, sr.alias, c.virtual
order by
sr.alias
And I got this error :
Column 'Sensor_data.value' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
So I tried several things like adding column in the group by but nothing changes.
You probably want the value from the record with max date. Use ROW_NUMBER to get those records.
select alias, virtual, date, value
from
(
select
sr.alias, c.virtual, d.date, d.value,
row_number() over (partition by sr.alias order by d.date desc) as rn
from Citopia_test.dbo.Smartparking_reference sr
join Citopia_test.dbo.Sensor c on (c.id_capteur = sr.id_capteur)
left join Citopia_test.dbo.Sensor_data d on (c.id_capteur = d.id_capteur)
) numbered
where rn = 1
order by sr.alias;
This gets you one row per sr.alias. If you want one row per sr.alias + c.virtual then change the partition by clause accordingly.
I have a SQL Server query that is performing poorly when retrieving data via pagination using offset/fetch. The earlier pages return results very fast but later ones are extremely slow and creating a bottleneck in our system. It's joining on two temp tables (#A and #T). Here is a pared down version of the query:
Select
I.CustomerID,
I.InvoiceID,
I.ItemID,
TI1.TrackID as TrackID1,
TI1.ItemName as ItemName1,
TI1.TrackCatID as TrackCatID1,
TI1.CategoryName as CategoryName1,
TI2.TrackID as TrackID2,
TI2.ItemName as ItemName2,
TI2.TrackCatID as TrackCatID2,
TI2.CategoryName as CategoryName2,
A.AccID,
A.Name as AccountName,
A.utimestamp as UpdateTimeStamp
FROM
#A A
Inner Join [dbo].[Item] I WITH(FORCESEEK)
On
A.CustomerID = I.CustomerID And
A.AccountID = I.AccountID
Left Join #T TI1 On
I.CustomerID = TI1.CustomerID And
I.TrackID1 = TI1.TrackID
Left Join #T TI2 On
I.CustomerID = TI2.CustomerID And
I.TrackID2 = TI2.TrackID
Order by
A.utimestamp
Offset 0 Rows Fetch Next 1000 Rows Only
Where the temp tables are defined as:
Create table #T (CustomerID uniqueidentifier, TrackCatID uniqueidentifier, TrackID uniqueidentifier, ItemName varchar(100), CategoryName varchar(100),PRIMARY KEY (CustomerID,TrackID))
Create table #A (CustomerID uniqueidentifier, AccountID uniqueidentifier, Name varchar(100), utimestamp timestamp, PRIMARY KEY (CustomerID, AccountID))
Regarding the DB table [dbo].[Item] it is defined as:
CREATE TABLE [dbo].[Item](
[Sequence] [int] IDENTITY(1,1) NOT NULL,
[CustomerID] [uniqueidentifier] NOT NULL,
[ItemID] [uniqueidentifier] NOT NULL,
[AccountID] [uniqueidentifier] NULL,
[TrackID1] [uniqueidentifier] NULL,
[TrackID2] [uniqueidentifier] NULL,
CONSTRAINT [PK_Item] PRIMARY KEY NONCLUSTERED
(
[CustomerID] ASC,
[ItemID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80) ON [PRIMARY],
CONSTRAINT [CX_Item] UNIQUE CLUSTERED
(
[CustomerID] ASC,
[Sequence] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO
And has a number of indexes that each have have several columns:
Item Indexes
1: CusotmerID, Sequence
2: CusotmerID, AccountId, Sequence
3: CusotmerID, TrackId1
4: CusotmerID, TrackId2
5: CusotmerID, ItemID
Is there something I'm missing that's causing the later paginated queries to be slow? Note: The 0 in "Offset 0 Rows" increases by 1000 for every page.
Also, I added a index to the temp table #A and didn't see an improvement to the results:
CREATE INDEX IDX_Timestamp ON #A(utimestamp)
table optin_channel_1 (for each 'channel' there's a dedicated table)
CREATE TABLE [dbo].[optin_channel_1](
[key_id] [bigint] NOT NULL,
[valid_to] [datetime] NOT NULL,
[valid_from] [datetime] NOT NULL,
[key_type_id] [int] NOT NULL,
[optin_flag] [tinyint] NOT NULL,
[source_proc_id] [int] NOT NULL,
[date_inserted] [datetime] NOT NULL
) ON [PRIMARY]
CREATE CLUSTERED INDEX [ix_id] ON [dbo].[optin_channel_1]
(
[key_type_id] ASC,
[key_id] ASC,
[valid_to] ASC,
[valid_from] 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]
table profile_conns
CREATE TABLE [dbo].[profile_conns](
[profile_key_id] [bigint] NOT NULL,
[valid_to] [datetime] NOT NULL,
[valid_from] [datetime] NOT NULL,
[conn_key_id] [bigint] NOT NULL,
[conn_key_type_id] [int] NOT NULL,
[conn_type_id] [int] NOT NULL,
[source_proc_id] [int] NOT NULL,
[date_inserted] [datetime] NOT NULL
) ON [PRIMARY]
CREATE CLUSTERED INDEX [ix_id] ON [dbo].[profile_conns]
(
[profile_key_id] ASC,
[conn_key_type_id] ASC,
[conn_key_id] ASC,
[valid_to] ASC,
[valid_from] 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]
table lu_channel_conns
CREATE TABLE [dbo].[lu_channel_conns](
[channel_id] [int] NOT NULL,
[conn_type_id] [int] NOT NULL,
CONSTRAINT [PK_lu_channel_conns] PRIMARY KEY CLUSTERED
(
[channel_id] ASC,
[conn_type_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]
table lu_conn_type
CREATE TABLE [dbo].[lu_conn_type](
[conn_type_id] [int] NOT NULL,
[default_key_type_id] [int] NOT NULL,
[master_key_type_id] [int] NOT NULL,
[date_inserted] [datetime] NOT NULL,
CONSTRAINT [PK_lu_conns] PRIMARY KEY CLUSTERED
(
[conn_type_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]
view v_source_proc_id_by_group_id
SELECT DISTINCT x.source_proc_id, x.source_proc_group_id
FROM lu_source_proc x INNER JOIN lu_source_proc_group y ON x.source_proc_group_id = y.group_id
There's a dynamic SQL statement going to be executed:
SET #sql_str='SELECT #ret=MAX(o.optin_flag)
FROM optin_channel_'+CAST(#channel_id AS NVARCHAR(100))+' o
INNER HASH JOIN dbo.v_source_proc_id_by_group_id y ON o.source_proc_id=y.source_proc_id AND y.source_proc_group_id=#source_proc_group_id
INNER HASH JOIN profile_conns z ON z.profile_key_id=cast(#profile_key_id AS NVARCHAR(100)) AND z.conn_key_type_id=o.key_type_id AND z.conn_key_id=o.[key_id] AND z.valid_to=''01.01.3000''
INNER HASH JOIN lu_channel_conns x ON x.channel_id=#channel_id AND z.conn_type_id=x.conn_type_id
INNER HASH JOIN lu_conn_type ct ON ct.conn_type_id=x.conn_type_id AND ct.default_key_type_id=o.key_type_id'
SET #param='#channel_id INT, #profile_key_id INT, #source_proc_group_id INT, #ret NVARCHAR(400) OUTPUT'
EXEC sp_executesql #sql_str,#param,#channel_id,#profile_key_id,#source_proc_group_id,#ret OUTPUT
I.e. this gives:
SELECT #ret=MAX(o.optin_flag) AS optin_flag
FROM optin_channel_1 o
INNER HASH JOIN dbo.v_source_proc_id_by_group_id y
ON o.source_proc_id=y.source_proc_id
AND y.source_proc_group_id=5
INNER HASH JOIN profile_conns z
ON z.profile_key_id=1
AND z.conn_key_type_id=o.key_type_id
AND z.conn_key_id=o.[key_id]
AND z.valid_to='01.01.3000'
INNER HASH JOIN lu_channel_conns x
ON x.channel_id=1
AND z.conn_type_id=x.conn_type_id
INNER HASH JOIN lu_conn_type ct
ON ct.conn_type_id=x.conn_type_id
AND ct.default_key_type_id=o.key_type_id
These tables are used for an optin database. optin_flag could be 0 or 1. With the last statement I want to get a 1 as optin_flag from optin_channel_1 for the given channel_id=1 for user with profile_key_id=1, when optin was inserted into database by process belonging to source_proc_group_id=5. I hope this is enough to comprehend what's going on.
Is this the best way to use the CLUSTERED INDEX'es? Or would it be better to remove profile_key_id from index on profile_conns and put z.profile_key_id=1 in a WHERE clause?
May be there's a much better way for optimizing this select (changes in database schema is not possible, only changes on indexes and modifing statement).
Without knowing the size of the tables and the sort of data stored in it them it is difficult to gauge.
Assuming optin_channel_1 has a lot of data and profile_cons has a lot of data I would try the following:
Clustered index on optin_channel_1(key_id) or key_type_id depending on which field has the most distinct values. (since you don't have a covering index)
Clustered index on profile_conns (cons_key_id) or cons_key_type_id depending on what you have chosen in optin_channel_1
etc...
Basically, if your table profile_conns table has not much data, I would put the clustered index on the most fragmented "filter" field (I suspect profile_key_id). If the table has a lot of data I would aim for a hash/merge join and match the clustered index with the clustered index of the optin_channel_1 table.
I would also rewrite the query as such:
SELECT #ret = MAX(o.optin_flag) AS optin_flag
FROM optin_channel_1 o
JOIN dbo.v_source_proc_id_by_group_id y
ON o.source_proc_id = y.source_proc_id
JOIN profile_conns z
ON z.conn_key_type_id = o.key_type_id
AND z.conn_key_id = o.[key_id]
JOIN lu_channel_conns x
ON z.conn_type_id = x.conn_type_id
JOIN lu_conn_type ct
ON ct.conn_type_id = x.conn_type_id
AND ct.default_key_type_id=o.key_type_id
WHERE y.source_proc_group_id = 5
AND z.profile_key_id = 1
AND x.channel_id = 1
AND z.valid_to = '01.01.3000'
The query changed this way because:
Putting the filter conditions in the where clause shows you what are relevant fields to aim for a hash/merge join
Putting join hints is rarely a good idea. It is very hard to beat the query governor to determine the best query plan. A bad plan usually indicates you have an issue with your indexes/statistics.
So as summary:
small table joined to big table ==> go for nested loops & focus your clustered index on the "filter" field in the small table & the join field in the big table.
big table joined to big table => go for hash/merge join and put the clustered index on the matching field on both sides
multi-field indexes usually only a good idea when they are "covering", this means all the fields you query are included in the index. (or are included with the include() clause)
I need some pointer on how to debug the following problem.
Environment: SQL Server 2005 Enterprise.
I have an indexed view with contains clustered index and multiple non-unique, non-clustered index. However when I execute the query, SQL server always perform Clustered index scan instead of index seek on my key.
Here is a simplify version.
CREATE VIEW MyIndexedView WITH SCHEMABINDING
SELECT a.Col1, b.Col2, c.Col3, d.Col4
FROM a JOIN b on a.id = b.id
JOIN c on a.id = c.id
JION d on c.id = d.id
There is a clustered index on Col1, and non-unique, non-clustered on Col2, Col3.
When I run the following query
SELECT a.Col1, b.Col2, c.Col3 FROM MyIndexedView WITH(NOEXPAND) WHERE b.Col2='blah'
and look at execution plan, I see SQL server run Clustered index scan on a.Col1 instead of perform index seek on Col2.
I tried to recreate the view and index.
Updated:
I did some additional testing and running these two queries side by side in Query Analyzer.
a) SELECT a.Col1, b.Col2, c.Col3
FROM MyIndexedView WITH(NOEXPAND) WHERE b.Col2='blah'
b) SELECT a.Col1, b.Col2, c.Col3
FROM MyIndexedView WHERE b.Col2 = 'blah'
Query 'a' will take 95% of the time and use Cluster Indexed scan. Query 'b' will only take 5% of the time and use Index Seek on col2. I try to swap the order of queries (run b first and a later) yield the same percentage.
This little experiment confirm that if sql use index seek it will be faster then cluster index scan.
Second I though if I don't include "WITH(NOEXPAND)" then SQL server will not use index on Indexed view. (Maybe I should start another question on the exact step to create indexed view).
I reproduced your sample and came up with the expected results with the index seek on Col2. The only way I was able to get it to do the clustered index scan was if I disabled the index. So first try rebuilding the index on Col2 to make sure it is actually enabled (or check the "Use Index" checkbox in index properties - options).
Here are the scripts I used to create the tables, view & indexes
CREATE TABLE [dbo].[a](
[id] [int] IDENTITY(1,1) NOT NULL,
[Col1] [varchar](100) NOT NULL,
CONSTRAINT [PK_a] 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]
GO
CREATE TABLE [dbo].[b](
[id] [int] IDENTITY(1,1) NOT NULL,
[Col2] [varchar](100) NOT NULL,
CONSTRAINT [PK_b] 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]
GO
CREATE TABLE [dbo].[c](
[id] [int] IDENTITY(1,1) NOT NULL,
[Col3] [varchar](100) NOT NULL,
CONSTRAINT [PK_c] 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]
GO
CREATE TABLE [dbo].[d](
[id] [int] IDENTITY(1,1) NOT NULL,
[Col4] [varchar](100) NOT NULL,
CONSTRAINT [PK_d] 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]
GO
CREATE VIEW [dbo].[MyIndexedView] WITH SCHEMABINDING
AS
SELECT a.Col1, b.Col2, c.Col3, d.Col4
FROM dbo.a JOIN dbo.b on a.id = b.id
JOIN dbo.c on a.id = c.id
JOIN dbo.d on c.id = d.id
GO
/****** Object: Index [IX] Script Date: 11/13/2009 21:50:01 ******/
CREATE UNIQUE CLUSTERED INDEX [IX] ON [dbo].[MyIndexedView]
(
[Col1] 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]
GO
/****** Object: Index [IX2] Script Date: 11/13/2009 21:50:39 ******/
CREATE NONCLUSTERED INDEX [IX2] ON [dbo].[MyIndexedView]
(
[Col2] ASC,
[Col3] 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]
GO
And I populated the tables like this:
declare #x int
SET #x = 0
while #x < 10
begin
INSERT INTO a (Col1 ) VALUES (newid())
INSERT INTO b (Col2 ) VALUES (newid())
INSERT INTO c (Col3 ) VALUES (newid())
INSERT INTO d (Col4 ) VALUES (newid())
SET #x=#x+1
end
Executing your query
SELECT Col1, Col2, Col3 FROM MyIndexedView WITH(NOEXPAND) WHERE Col2='blah'
shows an index seek on IX2
but if I disable that index
ALTER INDEX [IX2] ON [dbo].[MyIndexedView] DISABLE
and rerun, I see the clustered index scan on MyIndexedView.IX
How many records are there in your view?
If the result of the join is small then it is most cost efficient to scan the clustered index than seek another.