Why are both of these MSSQL indexes useful? - sql

Question

I have a MSSQL database table Events. I am worried that performance could be improved.
EventId
LocationId
Start
End
Quantity
Price
Currency
1
4
2022-08-31 22:00:00.0000000 +02:00
2022-08-31 23:00:00.0000000 +02:00
7.50000
2.0
EUR
2
2
2022-04-04 19:00:00.0000000 +01:00
2022-04-04 20:00:00.0000000 +01:00
1.50000
7.5
EUR
3
2
2022-04-04 19:00:00.0000000 +01:00
2022-04-04 20:00:00.0000000 +01:00
4.00000
8.2
EUR
I already have the following index:
CREATE NONCLUSTERED INDEX [IDX__Events__Location_Start_End] on [Events]
(
[LocationId] asc,
[Start] asc,
[End] asc
)
But Azure suggests that I create this index (medium impact):
CREATE NONCLUSTERED INDEX [IDX__Events__Location_End] ON [dbo].[Events] ([LocationId], [End]) INCLUDE ([Currency], [Price], [Quantity], [Start]) WITH (ONLINE = ON)
Hint: I do a lot of queries where I select Events greater than a start time and less than an end time.
Why is this extra index useful? Should I change my first index instead?
EDIT:
I run this code (EF Core) very often:
var relevantEvents = await _events.Where($#"
[{nameof(Events.LocationId)}] = #locationId
and [{nameof(Events.End)}] > #start
and [{nameof(Events.Start)}] < #end
", args);
Besides that, I upsert to the table often as well.

Answer 1

If you always do queries of the form shown at the end of your question with an equality predicate on LocationId and an inequality predicate on both End and Start then both LocationId, End and LocationId, Start would be viable indexing choices.
Note there is no benefit of adding the third column in as a key column because it will only be able to do a range seek for one or the other of them but the other one should be added as an included column.
My suspicion is that for typical scenarios LocationId, Start will generally involve reading more rows in the range seek than LocationId, End would (as the table accumulates years worth of data Start < #end will still need to read all the old rows from years ago).
The reason your existing index might not be being used and it feels moved to suggest an additional one is due to the INCLUDE ([Currency], [Price], [Quantity]) in the suggested one. If you add these included columns to your existing one you may well see the recommendation go away but you should consider which of LocationId, End and LocationId, Start will typically be able to narrow down the rows better (see "Numbers of rows read" in the execution plan).
CREATE TABLE dbo.Events
(
EventId INT IDENTITY PRIMARY KEY,
LocationId INT,
Start DATETIME2,
[End] DATETIME2,
Quantity DECIMAL(10,6),
Price DECIMAL(10,2),
Currency CHAR(3),
INDEX IDX__Events__Location_Start_End(LocationId, Start, [End]),
INDEX IDX__Events__Location_End(LocationId, [End]) INCLUDE (Start)
)
INSERT INTO dbo.Events
(LocationId, Start,[End], Quantity, Price,Currency)
SELECT LocationId = 1,
Start = DATEADD(SECOND, -Num, GETDATE()),
[End] = DATEADD(SECOND, 60-Num, GETDATE()),
Qty = 7.5,
Price = 2,
Currency = 'EUR'
FROM
(
SELECT TOP 1000000 Num = ROW_NUMBER() OVER (ORDER BY ##SPID)
FROM sys.all_columns c1, sys.all_columns c2
) Nums
DECLARE #Start DATE = GETDATE(), #End DATE = DATEADD(DAY, 1, GETDATE())
SELECT COUNT(*)
FROM dbo.Events WITH (INDEX = IDX__Events__Location_Start_End)
WHERE LocationId = 1 AND [End] > #Start AND Start < #End
OPTION (RECOMPILE)
SELECT COUNT(*)
FROM dbo.Events WITH (INDEX = IDX__Events__Location_End)
WHERE LocationId = 1 AND [End] > #Start AND Start < #End
OPTION (RECOMPILE)