SQL Server partition setup and Batch Inserts with @@identity

websss
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I have a high throughput c# console app which is saving around 250-350 records a sec to a log table (sql server 2017 enterprise)

We are looking at ways to increase performance and sometimes a report is run on the table and things grind to a halt
We are also struggling to keep up at busy times

We currently do not use partitions, and are investigating this
We also insert one record at a time, and attach the insertedID to the c# object of log data for further operations

I'm considering partitions, however i'm not sure what to partition on hour, day, week month
We generally store 6 months of historical data (could also be 1 month or 24 months for smaller number of IOT devices.)
data is usually queried by day, last 7 days, week, month, and sometimes custom date ranges.
I would welcome ideas and comments about what is the best option here, we write more than we query, but queries will span multiple days/weeks

I'm also considering a different approach on inserting, instead of one at a time, do a batch insert.
However i'm unsure how I can then associate the database insertedID with C# object.
Currently its one at a time, so i just add the insertedID to the object, and send that off to messages queues for further processing,

I was looking at the SQL OUTPUT and inserted.ID but not sure when batching 500 records how to marry the inserted ID with the c# object

So in summary I would welcome ideas on
1) partition setup
2) batching inserts and marrying the inserted ID with the c# object (for further processing)
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Ok - here goes....
saving around  250-350 records a sec to a log table
Saving in one statement or running 250 single row inserts?

We also insert one record at a time
There are output tables which could return sets of data if you can architect the application to write in groups.

Partitioning would help with elimination, as in when you query old data you don't block new data, but if your clustered keys are anything like sequential then this shouldn't be an issue. Are you ALWAYS passing a date range to the query? If not, partitioning by date will give a poor result.

What's the smallest datetime range you query on? Do you query by hour, or only by day?

I would suggest that you go with something a little bit complex to set up and maintain, but ultimately very flexible.

Set up partitions to add data into daily partitions - Remember to create a job which keeps 7 days of empty "future" partitions, so that if it fails you don't fall flat. All these partitions can be in the same filegroup.

Decide at what point you are no longer interested in daily data, and merge those daily partitions into weekly partitions, rinse and repeat to monthly partitions.

Finally have a process which truncates the oldest partition and merges it into the 2nd oldest.

To address your problems with row creation -
1) Pre-grow your database so you aren't blocked by the file extension
2) make that transaction as short as possible
3) Turn on snapshot isolation
4) Get the fastest storage you can.
websssCEO

Author

Commented:
saving around  250-350 records a sec to a log table
Saving in one statement or running 250 single row inserts?
Single row - Currently its one save at a time, and 250 saves in 1 sec


Are you ALWAYS passing a date range to the query? If not, partitioning by date will give a poor result.
No not always
This is a typical DATE based query (with other parameters too)


select from MassiveTable
Where GpsDateTime between 'X' and 'X'
and DeviceID = 123
and EventId = 1

But we also query by ID (primary key) too
I.e.
select from MassiveTable
Where PrimaryKeyId = 54321


Partitioning would help with elimination, as in when you query old data you don't block new data, but if your clustered keys are anything like sequential then this shouldn't be an issue
I have the following indexes
1. Clustered Index - Unique - DeviceID, GpsDateTime, PrimaryKeyId(identity)
2. NonClustered Index - Unique - PrimaryKeyId(identity) - and this is a primary key
3. NonClustered Index - Non-Unique - DeviceID, EventId, PrimaryKeyId(identity)

I've just noticed index 3 should be Unique, but thats a side issue.

Does this change things?


What's the smallest datetime range you query on? Do you query by hour, or only by day?
Day - when querying by gpsdatetime, however we do query by primaryKeyID


Decide at what point you are no longer interested in daily data, and merge those daily partitions into weekly partitions, rinse and repeat to monthly partitions.
Does this merge create any downtime or is it instant


To address your problems with row creation -
1) Pre-grow your database so you aren't blocked by the file extension
2) make that transaction as short as possible
3) Turn on snapshot isolation
4) Get the fastest storage you can.
We are on NVME drive, i'll checkout the rest


based on my response do you still think what you suggested is good?
ste5anSenior Developer

Commented:
Single row - Currently its one save at a time, and 250 saves in 1 sec
1) Use batch inserts. Thus use two threshold parameters (maxRows, maxTime). Collect the data in the front-end and post it, when one of the threshold parameters is reached.
2) Use more threads or processes to do the inserts.

To avoid locking cause by inserts, use a different fill factor on your indices. And when you do index maintenance, don't touch the clustered index. Cause this kind of table should have almost no updates, thus the natural filling of the pages should be sufficient. And the partitioning is basically a clustered index maintenance.

Day - when querying by gpsdatetime, however we do query by primaryKeyID

SELECT * 
FROM yourTable 
WHERE gpsdatetime BETWEEN (Day) AND (Day + 1) 
AND primaryKeyID = primaryKeyID;

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Using the partition column allows partition elimination, which is the key for fast queries.

But we also query by ID (primary key) too
hmm, what's the use-case?

Does this merge create any downtime or is it instant
MERGING partition will require a data move for sure when you have different file groups. But I'm not sure about merging in the same filegroup.. need to test this. Data movement is not instant.

Depending on the exact table definition I would test data compression. And also testing using columnstore index.
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websssCEO

Author

Commented:
But we also query by ID (primary key) too
hmm, what's the use-case?


We have a violations and visitations table which has massivetable.id (FK)
and then on a join we open all the details from that row
ste5anSenior Developer

Commented:
Review your data model.

Add a DATE or DATETIME column to this table and make it also part of the clustered index (maybe also the primary key) and foreign key relationship.
ste5anSenior Developer

Commented:
We generally store 6 months of historical data (could also be 1 month or 24 months for smaller number of IOT devices.)
So about what size do we talk? 2TB max? How big is the current row size (table DDL + indices would be nice to see)?

I would expect that larger partition windows (weekly or monthly sliding) should perform good enough. Just look into distributing them over different IO paths (spindles and controllers) to get better IO throughput.

btw, where and how does @@IDENTITY play a role?
websssCEO

Author

Commented:
Thanks Ste5an

The current byte size per row (given average data stored and data types) is around 764 bytes +
but given page size and storage we are seeing around 1kb per row based on an old calculation we did on rows vs disk space used.
We are currently working on optimizing all these and estimate we can get it to 411 bytes per row by deleting a bunch of legacy stuff, and changing data types to be more efficient.

The MDF is 2TB (was around 1.4TB before changing int/bigint and moving server on weekend)
And only 1.2 billion rows (2 billion on identity as we delete old data), this is likely to sky rocket soon so we are trying to get as lean as possible.
most of the database is small, and then one massive table
This one massive table is 1.2TB and the indexes are 82GB (according to "disk usage by top tables")
I'm not sure how to get at "table DDL and indices"

We do 4 disks (8 on raid1)
2 x Nvme and 2x ssd
one is a little 500gb for OS and 3.4TB on the others for storage.

@@IDENTITY is used to return the ID of the record just inserted to the massivetable.
ste5anSenior Developer

Commented:
I'm not sure how to get at "table DDL and indices"
SSMS, context menu on the table, script as CREATE TABLE script. Same after opening the table node and navigating to the indices,

It would also help to see some high IO/CPU queries and their actual execution plan(from the standard reports or the query store). It would be interesting to see, whether the inserts locks. In this case it maybe also an idea to consider using a staging table. Thus you let your application store the data in a intermediate table. Whereas all other things like your reports run on the normal tables. And you batch move all x minutes the data from the intermediate to your normal tables. But this would require using a sequence as identity provider or a cleanly set identity start value.

When you only append rows, do no updates and do no index maintenance, then using the NOLOCK hint maybe also an acceptable  mitigation for your reports.
websssCEO

Author

Commented:
thanks

This is the table
CREATE TABLE [dbo].[tblCommonTrackingData](
	[ipkCommanTrackingID] [bigint] IDENTITY(1,1) NOT NULL,
	[vpkDeviceID] [bigint] NULL,
	[vLongitude] [varchar](20) NULL,
	[vLatitude] [varchar](20) NULL,
	[vHeading] [int] NULL,
	[vReportID] [int] NULL,
	[vOdometer] [decimal](12, 3) NULL,
	[vVehicleSpeed] [int] NULL,
	[vTextMessage] [nvarchar](250) NULL,
	[ifkDriverID] [int] NULL,
	[bIsIgnitionOn] [bit] NULL,
	[vEventName] [varchar](80) NULL,
	[bIsEngineOn] [bit] NULL,
	[dGPSDateTime] [datetime] NULL,
	[cInput1] [char](1) NULL,
	[cInput2] [char](1) NULL,
	[cInput3] [char](1) NULL,
	[cInput4] [char](1) NULL,
	[cInput5] [char](1) NULL,
	[cInput6] [char](1) NULL,
	[vTempSensor1] [varchar](6) NULL,
	[vTempSensor2] [varchar](6) NULL,
	[nAltitude] [varchar](7) NULL,
	[iHdop] [int] NULL,
	[iBatteryBackup] [int] NULL,
	[vAnalog1] [varchar](10) NULL,
	[vAnalog2] [varchar](10) NULL,
	[cOutput1] [char](1) NULL,
	[cOutput2] [char](1) NULL,
	[vRoadSpeed] [varchar](4) NULL,
	[bIsGsmLocation] [bit] NOT NULL,
	[vAnalog1Raw] [nvarchar](6) NULL,
	[vAnalog2Raw] [nvarchar](6) NULL,
	[Cell_MMC] [varchar](20) NULL,
	[Cell_MNC] [varchar](20) NULL,
	[Cell_Lac] [varchar](20) NULL,
	[Cell_ID] [varchar](20) NULL,
	[Fuel1_Litres] [smallint] NULL,
	[Fuel1_Raw] [float] NULL,
	[SensorData] [varchar](max) NULL,
	[additionalEventInfo] [varchar](255) NULL,
	[ObdData] [varchar](max) NULL,
	[TripType] [smallint] NULL,
	[Attributes] [varchar](max) NULL,
	[EngineHours] [bigint] NULL,
	[DbDate] [datetime2](7) NULL,
	[RawData] [nvarchar](max) NULL,
 CONSTRAINT [PK_tblCommonTrackingData] PRIMARY KEY NONCLUSTERED 
(
	[ipkCommanTrackingID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 99) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
GO

ALTER TABLE [dbo].[tblCommonTrackingData] ADD  DEFAULT ((0)) FOR [bIsGsmLocation]
GO

ALTER TABLE [dbo].[tblCommonTrackingData] ADD  DEFAULT (getdate()) FOR [DbDate]
GO

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clustered index
/****** Object:  Index [CL__tblCommonTrackingData]    Script Date: 07-Oct-19 12:44:06 PM ******/
CREATE UNIQUE CLUSTERED INDEX [CL__tblCommonTrackingData] ON [dbo].[tblCommonTrackingData]
(
	[vpkDeviceID] ASC,
	[dGPSDateTime] ASC,
	[ipkCommanTrackingID] 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, FILLFACTOR = 98) ON [PRIMARY]
GO

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other index 1
/****** Object:  Index [NonClusteredIndex-20170211-033742]    Script Date: 07-Oct-19 12:44:27 PM ******/
CREATE NONCLUSTERED INDEX [NonClusteredIndex-20170211-033742] ON [dbo].[tblCommonTrackingData]
(
	[vpkDeviceID] ASC,
	[vReportID] ASC,
	[ipkCommanTrackingID] 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

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other index 2
USE [GPSOL]
GO

/****** Object:  Index [PK_tblCommonTrackingData]    Script Date: 07-Oct-19 12:44:49 PM ******/
ALTER TABLE [dbo].[tblCommonTrackingData] ADD  CONSTRAINT [PK_tblCommonTrackingData] PRIMARY KEY NONCLUSTERED 
(
	[ipkCommanTrackingID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 99) ON [PRIMARY]
GO

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I'm not sure how to get this info
It would also help to see some high IO/CPU queries and their actual execution plan(from the standard reports or the query store).
ste5anSenior Developer

Commented:
Okay, what is the exact purpose of this table?

- It has too many nullable columns.
- Storing GPS coordinates in a VARCHAR column is pretty suspect. What precision do you really need? Cause you can store such coordinates as BIGINT (can use the device accuracy) or even as INT (6 decimals ~ 0.11m).
- Same for columns TempSensorX, RoadSpeed, Altitude and maybe also AnalogX and OutputX, if the carry a voltage.
- Analyze the  EventName, TextMessage and AdditionalEventInfo column. Especially the EventName column is a candidate for the domain key normal form.
- the order of your columns in the clustered index is imho one of the performance reasons. In such tables, the first column should be the datetime column. Cause I guess you'll get currently already page splits. Having the DeviceID as first column means that SQL Server need to insert data into existing pages in between of the existing data. Using the GpsDateTime column means that data is only added at the end of the physical table. Also this means, that you primary query condition must be the DeviceID. Here you need to test what is performance wise the better solution. Thus analyze all queries run against your table.
websssCEO

Author

Commented:
Thanks Ste5an that is very good information
I do love it when I see comments from you in particular!

The exact purpose of the table is to store the gps co-ordinates and other info related to cars position, this is used for reports, trip replays etc
I also have another table called DEVICES, and one of the columns is LastRecordId which I update on each new record and use for fast lookups of where the car is at the moment (you gave me this idea a long time ago and its worked really well)

To go through your list.
- It has too many nullable columns.
Yes, I also noticed this, and most of them are never null, will this hurt performance? Should I update these to not null?

storing GPS coordinates in a VARCHAR column is pretty suspect
This was a legacy issue, and I had earmarked this to be changed to decimal(9,6) and only store 6 decimal places as this gets down to 11cm as you say, anything further are fleas on dogs back
However, you said I can use INT, I wasn't aware i could use decimals on an INT column (or are you referring to the wider integer/number form?)
I saw that decimal(9,6) would get be to 5 bytes, would you agree this is best?

Same for columns TempSensorX, RoadSpeed, Altitude and maybe also AnalogX and OutputX, if the carry a voltage
I'm keeping roadspeed and will change to tinyint since i dont know of any roads with speed limits > 255, the rest is being dropped as it will be put into attributes column which is often empty and pointless having a dedicated column when rarely has data.

Analyze the  EventName, TextMessage and AdditionalEventInfo column. Especially the EventName column is a candidate for the domain key normal form.
Very good point, the TextMessage is RoadStreetName and is multilingual so annoyingly has to be nvarchar and i've allocated 250 for size.
The EventName column will be removed and either used in lookup table or hardcoded in web app based on the Event/ReportID, this is very pointless and often repeated.

- the order of your columns in the clustered index is imho one of the performance reasons
This is something I did not think about, and my eye's widened when i read this!!!
Almost Every query is basically
DeviceId = 123 and GpsDateTime between 'x' and 'x' and ReportId = 124 

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(or a multiple of reportIds)

From what I understand, each page is 8kb, so you need to fit as many things into these 8kb as possible, and because gpsdatetime appears far down the columns, this is bad, and will cause page splits at points where that data is needed?

So should I be ordering the columns as they appear like so?
GpsDateTime
DeviceId
ReportId
PrimaryKeyID

etc...


If so, this raises the question of HOW? since the data is already present (i guess I'd have to re-create the table and re-structure the columns in the order of most used/queried first, then least used at the end?
ste5anSenior Developer

Commented:
- Nullable columns can affect query plans. Thus it can have an effect, but on your case it should be not the main factor. From the data model view point it's an erroneous implementation when it contains always data or must contain data.

- Storing GPS coordinates: INT has a range from-2,147,483,648 to 2,147,483,647 thus can multiply your GPS coordinates as float by 1,000,000 and store them as INT. You need only -180° to 180° as integer part, leaving 6 digits as decimal places. This would mean 8 byte per coordinate where as a DECIMAL(9, 6) requires 10 bytes. So doing this math on the coordinate saves 20%.

- The order of your columns in the clustered index affects query performance.
The problem is Janus-faced:
For the SELECT performance the optimal order is from most selective column to least selective column. Most selective means that the first column should have the most values  in its domain. So that the first filter step reduces the amount of need data to a small as possible set. The most selective column is (just a guessed) the ReportID. So for the performance when reading data, the optimal order of the columns in your clustered index would be ( ReportID, DeviceID, GpsDateTime ).
But the other side is: every INSERT requires a lock, and the only index minimizing INSERT locks is ( GpsDateTime, DeviceID, ReportID ).
Here you need a test system where you can really test it. Cause I would thing that having the clustered index as ( GpsDateTime, DeviceID, ReportID ) should work for most SELECT queries also well enough. But this must be tested by you.

Changing the clustered index means changing the physical structure of the table on the storage device. Thus it is an IO intensive operation. In your case, this can only be done when there is minimal load on your system. When considering this change, then partition your table also in the same time. This may require a lot of time and resources.
The faster approach would be building a new table but this requires also some coordinate downtime and depending on the complexity of your application you need to test it, cause it involves table renaming, which can lead also some problems.
websssCEO

Author

Commented:
Thanks

Looking at the clustered index
CREATE UNIQUE CLUSTERED INDEX [CL__tblCommonTrackingData] ON [dbo].[tblCommonTrackingData]
(
	[vpkDeviceID] ASC,
	[dGPSDateTime] ASC,
	[ipkCommanTrackingID] 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, FILLFACTOR = 98) ON [PRIMARY]
GO

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I am using the ID since I wanted it to be unique
I'm wondering if i change this clustered index to not be unique and the issues surround this?

i.e.

CREATE NOT-A-UNIQUE CLUSTERED INDEX [CL__tblCommonTrackingData] ON [dbo].[tblCommonTrackingData]
(
	[vpkDeviceID] ASC,
	[dGPSDateTime] ASC,
	[ReportId] 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, FILLFACTOR = 98) ON [PRIMARY]
GO

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Because I dont really need ID, it was just there for unique-ness
But I do need ReportId in the above

I'm not sure what the most selective means/needs to be
Every query selects a Device, then GpsDateTime, then ReportId (EventId)
The above sometimes is not unquie as there are often 2 or more records with same timestamp etc but with maybe a battery voltage difference

An example of reportId is
1 = position
2 = ignition on
3 = ignition off
4 = overspeed
5 = main power disconnected
6= idling
7 = tow
.....etc etc

A trip report just deals with ignition on/ off, but a full movement report deals with 1,2,3
So when querying a car(s) for report/info, you always use deviceId, gpsdatetime (between dates) and then reportId


I'm also not sure if ASC on the index should be DESC for gpsdatetime or if that makes any difference
Senior Developer
Commented:
I am using the ID since I wanted it to be unique
 I'm wondering if i change this clustered index to not be unique and the issues surround this?
I guess you have here a little misunderstanding. The primary key (PK) and the clustered index (CIX) are two completely different things. The PK is a logical declaration to enforce uniqueness for a candidate key in the relation (table), It is the primary, thus most used key for foreign key relationships. E.g. for foreign key relationships it is sufficient to use an unique constraint (UQ). They (PK & UQ) are implemented by the engine using indices. Indices are the physical representation of the PK and UQ. Indices as physical detail can be clustered or non-clustered as attribute. Only one index can be clustered. This CIX defines the physical order of data and pages stored on disk. Now the point  I guess you missed:
The CIX can be any index. It must not be the PK.
It's just because in the majority of cases being the PK also the CIX gives the best performance. Thus SSMS creates the PK per default as CIX. But this is not necessary. A further thing about the CIX as any index, they don't need to be unique. Especially the CIX should order the data in a way that the majority of use-cases (or first-class use-cases) have best performance.

So in your case the existing PK

ALTER TABLE dbo.tblCommonTrackingData
ADD CONSTRAINT PK_tblCommonTrackingData
    PRIMARY KEY NONCLUSTERED ( ipkCommanTrackingID ASC );

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is sufficient for the uniqueness of ipkCommanTrackingID.
 
But the existing CIX is suboptimal:

CREATE UNIQUE CLUSTERED INDEX CL__tblCommonTrackingData
    ON dbo.tblCommonTrackingData (
    vpkDeviceID ASC ,
    dGPSDateTime ASC ,
    ipkCommanTrackingID ASC );

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It means that INSERT statements are inserting all over the physical space of your table. This also means that this lead to more pages being locked then it's good for your use-cases. Due to your locking issues and my experience I think

CREATE UNIQUE CLUSTERED INDEX CL__tblCommonTrackingData
    ON dbo.tblCommonTrackingData (
    dGPSDateTime ASC ,
    vpkDeviceID ASC ,
    ipkCommanTrackingID ASC );

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is the best CIX.

But you need to test it. Cause I don't know all of your use-cases, so I may have made some wrong assumptions. And much more important:
Changing the CIX of such a large table will take some time and resources. Thus you need to evaluate the impact on the running system against the gain in performance.

Thus I would start by tackling the problem at the INSERT side first, to see how much lesser locking you get by doing batch INSERTs from your front-end instead of single row INSERTs. Cause this can be a mitigation to the locking problem, so that you can test the CIX change without pressure.

I'm not sure what the most selective means/needs to be
Every query selects a Device, then GpsDateTime, then ReportId (EventId)
A simple visualization:
 
Capture.PNG
Consider a WHERE Column1=1 AND Column2=1 AND Column3=1 and think of the rows in the image as a data page. Index usage means that the first column in the index is processed frist, then the others.
Then in least selective case the engine needs to read for pages for Column1 then it need to look up the Column2 values before it can select the actual page by Column3. When the index is now vice versa, then we only need a single read to get the wanted data.
websssCEO

Author

Commented:
Sorry for delay was on vacation

I had to read your message many times for it to sink in (Most selective) but I get it now
I have a dev server I can try this new index on, but not sure how I test it against old index?
ste5anSenior Developer

Commented:
For comparing such scenarios, I use normally two instances. Cause you need two tables and ideally two databases two run your existing queries, views and procedures.

Instances are useful, cause you can run tests in parallel. Otherwise you need to clear the buffer pool before testing (CHECKPOINT; DBCC DROPCLEANBUFFERS;).

One with the original tables one with the test scenario. For many tests it is sufficient, that both instances have the same amount of physical RAM assigned ( machine RAM - 4GB ) / 2. So that both instances can buffer the same amount of pages. Then the runtime and IO are comparable.

For running load test you run only one instance and give it the entire memory ( machine RAM - 4GB ) or the rule you use on your production server.

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