Why is this stream aggregate necessary?
Check out this query. It's pretty simple (see the end of the post for table and index definitions, and a repro script):
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1 AND 1 = (SELECT 1);
Note: the "AND 1 = (SELECT 1) is just to keep this query from being auto-parameterized, which I felt like was confusing the issue - it actually gets the same plan with or without that clause though
And here's the plan (paste the plan link):
Since there is a "top 1" there, I was surprised to see the stream aggregate operator. It doesn't seem necessary to me, since there is guaranteed to only be one row.
To test that theory, I tried out this logically equivalent query:
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1
GROUP BY Id;
Here's the plan for that one (paste the plan link):
Sure enough, the group by plan is able to get by without the stream aggregate operator.
Notice that both queries read "backwards" from the end of the index and do a "top 1" to get the max revision.
What am I missing here? Is the stream aggregate actually doing work in the first query, or should it be able to be eliminated (and it's just a limitation of the optimizer that it's not)?
By the way, I realize this is not an incredibly practical problem (both queries report 0 ms of CPU and elapsed time), I'm just curious about the internals / behavior being exhibited here.
Here's the setup code I ran before running the two queries above:
DROP TABLE IF EXISTS dbo.TheOneders;
GO
CREATE TABLE dbo.TheOneders
(
Id INT NOT NULL,
Revision SMALLINT NOT NULL,
Something NVARCHAR(23),
CONSTRAINT PK_TheOneders PRIMARY KEY NONCLUSTERED (Id, Revision)
);
GO
INSERT INTO dbo.TheOneders
(Id, Revision, Something)
SELECT DISTINCT TOP 1000
1, m.message_id, 'Do...'
FROM sys.messages m
ORDER BY m.message_id
OPTION (MAXDOP 1);
INSERT INTO dbo.TheOneders
(Id, Revision, Something)
SELECT DISTINCT TOP 100
2, m.message_id, 'Do that thing you do...'
FROM sys.messages m
ORDER BY m.message_id
OPTION (MAXDOP 1);
GO
sql-server group-by aggregate sql-server-2017 database-internals
add a comment |
Check out this query. It's pretty simple (see the end of the post for table and index definitions, and a repro script):
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1 AND 1 = (SELECT 1);
Note: the "AND 1 = (SELECT 1) is just to keep this query from being auto-parameterized, which I felt like was confusing the issue - it actually gets the same plan with or without that clause though
And here's the plan (paste the plan link):
Since there is a "top 1" there, I was surprised to see the stream aggregate operator. It doesn't seem necessary to me, since there is guaranteed to only be one row.
To test that theory, I tried out this logically equivalent query:
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1
GROUP BY Id;
Here's the plan for that one (paste the plan link):
Sure enough, the group by plan is able to get by without the stream aggregate operator.
Notice that both queries read "backwards" from the end of the index and do a "top 1" to get the max revision.
What am I missing here? Is the stream aggregate actually doing work in the first query, or should it be able to be eliminated (and it's just a limitation of the optimizer that it's not)?
By the way, I realize this is not an incredibly practical problem (both queries report 0 ms of CPU and elapsed time), I'm just curious about the internals / behavior being exhibited here.
Here's the setup code I ran before running the two queries above:
DROP TABLE IF EXISTS dbo.TheOneders;
GO
CREATE TABLE dbo.TheOneders
(
Id INT NOT NULL,
Revision SMALLINT NOT NULL,
Something NVARCHAR(23),
CONSTRAINT PK_TheOneders PRIMARY KEY NONCLUSTERED (Id, Revision)
);
GO
INSERT INTO dbo.TheOneders
(Id, Revision, Something)
SELECT DISTINCT TOP 1000
1, m.message_id, 'Do...'
FROM sys.messages m
ORDER BY m.message_id
OPTION (MAXDOP 1);
INSERT INTO dbo.TheOneders
(Id, Revision, Something)
SELECT DISTINCT TOP 100
2, m.message_id, 'Do that thing you do...'
FROM sys.messages m
ORDER BY m.message_id
OPTION (MAXDOP 1);
GO
sql-server group-by aggregate sql-server-2017 database-internals
add a comment |
Check out this query. It's pretty simple (see the end of the post for table and index definitions, and a repro script):
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1 AND 1 = (SELECT 1);
Note: the "AND 1 = (SELECT 1) is just to keep this query from being auto-parameterized, which I felt like was confusing the issue - it actually gets the same plan with or without that clause though
And here's the plan (paste the plan link):
Since there is a "top 1" there, I was surprised to see the stream aggregate operator. It doesn't seem necessary to me, since there is guaranteed to only be one row.
To test that theory, I tried out this logically equivalent query:
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1
GROUP BY Id;
Here's the plan for that one (paste the plan link):
Sure enough, the group by plan is able to get by without the stream aggregate operator.
Notice that both queries read "backwards" from the end of the index and do a "top 1" to get the max revision.
What am I missing here? Is the stream aggregate actually doing work in the first query, or should it be able to be eliminated (and it's just a limitation of the optimizer that it's not)?
By the way, I realize this is not an incredibly practical problem (both queries report 0 ms of CPU and elapsed time), I'm just curious about the internals / behavior being exhibited here.
Here's the setup code I ran before running the two queries above:
DROP TABLE IF EXISTS dbo.TheOneders;
GO
CREATE TABLE dbo.TheOneders
(
Id INT NOT NULL,
Revision SMALLINT NOT NULL,
Something NVARCHAR(23),
CONSTRAINT PK_TheOneders PRIMARY KEY NONCLUSTERED (Id, Revision)
);
GO
INSERT INTO dbo.TheOneders
(Id, Revision, Something)
SELECT DISTINCT TOP 1000
1, m.message_id, 'Do...'
FROM sys.messages m
ORDER BY m.message_id
OPTION (MAXDOP 1);
INSERT INTO dbo.TheOneders
(Id, Revision, Something)
SELECT DISTINCT TOP 100
2, m.message_id, 'Do that thing you do...'
FROM sys.messages m
ORDER BY m.message_id
OPTION (MAXDOP 1);
GO
sql-server group-by aggregate sql-server-2017 database-internals
Check out this query. It's pretty simple (see the end of the post for table and index definitions, and a repro script):
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1 AND 1 = (SELECT 1);
Note: the "AND 1 = (SELECT 1) is just to keep this query from being auto-parameterized, which I felt like was confusing the issue - it actually gets the same plan with or without that clause though
And here's the plan (paste the plan link):
Since there is a "top 1" there, I was surprised to see the stream aggregate operator. It doesn't seem necessary to me, since there is guaranteed to only be one row.
To test that theory, I tried out this logically equivalent query:
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1
GROUP BY Id;
Here's the plan for that one (paste the plan link):
Sure enough, the group by plan is able to get by without the stream aggregate operator.
Notice that both queries read "backwards" from the end of the index and do a "top 1" to get the max revision.
What am I missing here? Is the stream aggregate actually doing work in the first query, or should it be able to be eliminated (and it's just a limitation of the optimizer that it's not)?
By the way, I realize this is not an incredibly practical problem (both queries report 0 ms of CPU and elapsed time), I'm just curious about the internals / behavior being exhibited here.
Here's the setup code I ran before running the two queries above:
DROP TABLE IF EXISTS dbo.TheOneders;
GO
CREATE TABLE dbo.TheOneders
(
Id INT NOT NULL,
Revision SMALLINT NOT NULL,
Something NVARCHAR(23),
CONSTRAINT PK_TheOneders PRIMARY KEY NONCLUSTERED (Id, Revision)
);
GO
INSERT INTO dbo.TheOneders
(Id, Revision, Something)
SELECT DISTINCT TOP 1000
1, m.message_id, 'Do...'
FROM sys.messages m
ORDER BY m.message_id
OPTION (MAXDOP 1);
INSERT INTO dbo.TheOneders
(Id, Revision, Something)
SELECT DISTINCT TOP 100
2, m.message_id, 'Do that thing you do...'
FROM sys.messages m
ORDER BY m.message_id
OPTION (MAXDOP 1);
GO
sql-server group-by aggregate sql-server-2017 database-internals
sql-server group-by aggregate sql-server-2017 database-internals
edited Dec 26 '18 at 6:19
Joe Obbish
20.8k32881
20.8k32881
asked Dec 18 '18 at 15:42
jadarnel27jadarnel27
3,9971330
3,9971330
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
You can see the role of this aggregate if no rows match the WHERE
clause.
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1
AND 1 = 1 /*To avoid auto parameterisation*/
AND Id%3 = 4 /*always false*/
In that case zero rows go into the aggregate but it still emits one as the correct semantics are to return NULL
in this case.
This is a scalar aggregate as opposed to a vector one.
Your "logically equivalent" query is not equivalent. Adding GROUP BY Id
would make it a vector aggregate and then the correct behaviour would be to return no rows.
See Fun with Scalar and Vector Aggregates for more about this.
add a comment |
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1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
You can see the role of this aggregate if no rows match the WHERE
clause.
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1
AND 1 = 1 /*To avoid auto parameterisation*/
AND Id%3 = 4 /*always false*/
In that case zero rows go into the aggregate but it still emits one as the correct semantics are to return NULL
in this case.
This is a scalar aggregate as opposed to a vector one.
Your "logically equivalent" query is not equivalent. Adding GROUP BY Id
would make it a vector aggregate and then the correct behaviour would be to return no rows.
See Fun with Scalar and Vector Aggregates for more about this.
add a comment |
You can see the role of this aggregate if no rows match the WHERE
clause.
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1
AND 1 = 1 /*To avoid auto parameterisation*/
AND Id%3 = 4 /*always false*/
In that case zero rows go into the aggregate but it still emits one as the correct semantics are to return NULL
in this case.
This is a scalar aggregate as opposed to a vector one.
Your "logically equivalent" query is not equivalent. Adding GROUP BY Id
would make it a vector aggregate and then the correct behaviour would be to return no rows.
See Fun with Scalar and Vector Aggregates for more about this.
add a comment |
You can see the role of this aggregate if no rows match the WHERE
clause.
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1
AND 1 = 1 /*To avoid auto parameterisation*/
AND Id%3 = 4 /*always false*/
In that case zero rows go into the aggregate but it still emits one as the correct semantics are to return NULL
in this case.
This is a scalar aggregate as opposed to a vector one.
Your "logically equivalent" query is not equivalent. Adding GROUP BY Id
would make it a vector aggregate and then the correct behaviour would be to return no rows.
See Fun with Scalar and Vector Aggregates for more about this.
You can see the role of this aggregate if no rows match the WHERE
clause.
SELECT MAX(Revision)
FROM dbo.TheOneders
WHERE Id = 1
AND 1 = 1 /*To avoid auto parameterisation*/
AND Id%3 = 4 /*always false*/
In that case zero rows go into the aggregate but it still emits one as the correct semantics are to return NULL
in this case.
This is a scalar aggregate as opposed to a vector one.
Your "logically equivalent" query is not equivalent. Adding GROUP BY Id
would make it a vector aggregate and then the correct behaviour would be to return no rows.
See Fun with Scalar and Vector Aggregates for more about this.
edited Dec 19 '18 at 10:14
answered Dec 18 '18 at 15:59
Martin SmithMartin Smith
62k10166248
62k10166248
add a comment |
add a comment |
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