Azure DB/MSSQL 2017 query performance regression - sql

Question

I have this pretty simple table with 17m records in it:
CREATE TABLE [dbo].[LineNumbers](
[Id] [int] IDENTITY(1,1) NOT NULL,
[LineDescriptionId] [int] NOT NULL,
[ProtocolId] [int] NULL,
[Value] [int] NULL,
CONSTRAINT [PK_LineNumbers] 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]
Query to the table with additional join works fine, if there is no ProtocolId in it:
select top 1
ln.LineDescriptionId
from LineNumbers ln
join LineDescriptions ld on ld.Id = ln.LineDescriptionId and ld.ProtocolSetId = 25
-- Elapsed time: 00:00:00.1718750
Execution plan: https://www.brentozar.com/pastetheplan/?id=rJV34gvR7
But when I try to add ProtocolId to the field list, query time grows dramatically:
select top 1
ln.ProtocolId
from LineNumbers ln
join LineDescriptions ld on ld.Id = ln.LineDescriptionId and ld.ProtocolSetId = 25
-- Elapsed time: 00:02:19.6464843
Execution plan: https://www.brentozar.com/pastetheplan/?id=SkG-hyDCQ
Also, this works smooth:
select top 1
(select ProtocolId from LineNumbers where LineNumbers.Id = ln.Id) as ProtocolId
from LineNumbers ln
join LineDescriptions ld on ld.Id = ln.LineDescriptionId and ld.ProtocolSetId = 25
-- Elapsed time: 00:00:00.1718750
Tried this queries and variations on Azure DB and local MSSQL 2017. Results are the same. As long as I keep ProtocolId out of the field list everything is fine.
Is there some mistake in my data scheme (everything was created via migrations of Entity Framework)?
CREATE TABLE [dbo].[LineNumbers](
[Id] [int] IDENTITY(1,1) NOT NULL,
[LineDescriptionId] [int] NOT NULL,
[ProtocolId] [int] NULL,
[Value] [int] NULL,
CONSTRAINT [PK_LineNumbers] 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
/****** Object: Index [IX_LineNumbers_LineDescriptionId] Script Date: 21.11.2018 10:47:09 ******/
CREATE NONCLUSTERED INDEX [IX_LineNumbers_LineDescriptionId] ON [dbo].[LineNumbers]
(
[LineDescriptionId] 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
/****** Object: Index [IX_LineNumbers_LineDescriptionId_Value] Script Date: 21.11.2018 10:47:09 ******/
CREATE NONCLUSTERED INDEX [IX_LineNumbers_LineDescriptionId_Value] ON [dbo].[LineNumbers]
(
[LineDescriptionId] ASC,
[Value] 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
/****** Object: Index [IX_LineNumbers_ProtocolId] Script Date: 21.11.2018 10:47:09 ******/
CREATE NONCLUSTERED INDEX [IX_LineNumbers_ProtocolId] ON [dbo].[LineNumbers]
(
[ProtocolId] 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
ALTER TABLE [dbo].[LineNumbers] WITH NOCHECK ADD CONSTRAINT [FK_LineNumbers_LineDescriptions_LineDescriptionId] FOREIGN KEY([LineDescriptionId])
REFERENCES [dbo].[LineDescriptions] ([Id])
ON DELETE CASCADE
GO
ALTER TABLE [dbo].[LineNumbers] CHECK CONSTRAINT [FK_LineNumbers_LineDescriptions_LineDescriptionId]
GO
ALTER TABLE [dbo].[LineNumbers] WITH NOCHECK ADD CONSTRAINT [FK_LineNumbers_Protocols_ProtocolId] FOREIGN KEY([ProtocolId])
REFERENCES [dbo].[Protocols] ([Id])
GO
ALTER TABLE [dbo].[LineNumbers] CHECK CONSTRAINT [FK_LineNumbers_Protocols_ProtocolId]
GO

Answer 1

Eventually, I solved it by adding nonclustered index on field LineNumbers.LineDescriptionId with inclusion of LineNumbers.ProtocolId
CREATE NONCLUSTERED INDEX [IX_LineNumbers_LineDescriptionId_ProtocolId] ON
[dbo].[LineNumbers]([LineDescriptionId] ASC)
INCLUDE ([ProtocolId])
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF,
SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF,
ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON)
Result:
SELECT TOP 1
ln.ProtocolId
FROM LineNumbers ln
JOIN LineDescriptions ld ON ld.Id = ln.LineDescriptionId AND ld.ProtocolSetId = 25
-- Elapsed time: 00:00:00.1403155
Execution plan: https://www.brentozar.com/pastetheplan/?id=Syywn1wRQ
Why does it work that way?
For example, if I'd do similar use case with PostgreSQL then there is no need in any additional indexes at all (beside obvious FK indexes on ProtocolId and LineDescriptionId fields).