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问题:
I'm having newsequentialid() learning problems in sql server management studio. Create a table with a uniqueidentifier column 'UniqueID', and set the default to newsequentialid().
Step 1. saving the design:
'Table_1' table - Error validating the default for column 'UniqueID'.
Save it anyway.
Step 2. view the sql:
CREATE TABLE [dbo].[Table_1]( [ID] [int] IDENTITY(1,1) NOT NULL, [Name] [nvarchar](50) NOT NULL, [UniqueID] [uniqueidentifier] NOT NULL ) ON [PRIMARY] GO ALTER TABLE [dbo].[Table_1] ADD CONSTRAINT [DF_Table_1_UniqueID] DEFAULT (newsequentialid()) FOR [UniqueID] GO
Looks reasonable.
Step 3. add some rows:
1 test 72b48f77-0e26-de11-acd4-001bfc39ff92 2 test2 92f0fc8f-0e26-de11-acd4-001bfc39ff92 3 test3 122aa19b-0e26-de11-acd4-001bfc39ff92
They don't look very sequential. ??
Edit: I have gotten it to work somewhat if the inserts are all done at once, then the unique id is sequential. On later inserts, sql server seems to forget the last sequential id, and starts a new sequence.
Running this in ssms results in squential guids:
insert into Table_1 (Name) values('test13a'); insert into Table_1 (Name) values('test14a'); insert into Table_1 (Name) values('test15a'); insert into Table_1 (Name) values('test16a'); insert into Table_1 (Name) values('test17a');
回答1:
newsequentialid is primarily to solve the issue of page fragmentation when your table is clustered by a uniqueidentifier. Your table is clustered by an integer column. I set up two test tables, one where the newsequentialid column is the primary key and one where it is not (like yours), and in the primary key the GUIDs were always sequential. In the other, they were not.
I do not know the internals/technical reasons why it behaves that way, but it seems clear that newsequentialid() is only truly sequential when your table is clustered by it. Otherwise, it seems to behave similarly to newid() / RowGuid.
Also, I'm curious as to why you would want to use newsequentialid() when you don't have to. It has many downsides which newid() does not, and none of the benefits - the biggest being that newid() is not practically predictable, whereas newsequentialid() is. If you are not worried about fragmentation, what's the point?
回答2:
Those values are actually "sequential" as per the definition of NEWSEQUENTIALID():
Creates a GUID that is greater than any GUID previously generated by this function on a specified computer since Windows was started.
It doesn't say there can't be any gaps in the GUIDs, it's just that any new GUID should be greater than the previous one.
Try this:
create table #test(id int, txt varchar(50), gid uniqueidentifier) insert into #test select 1 ,'test','72b48f77-0e26-de11-acd4-001bfc39ff92' union select 2, 'test2', '92f0fc8f-0e26-de11-acd4-001bfc39ff92' union select 3, 'test3', '122aa19b-0e26-de11-acd4-001bfc39ff92' select * from #test order by gid asc
As you can see, the records are ordered 1, 2, 3 which is as expected.
回答3:
THEY ARE SEQUENTIAL!
1 test 72b48f77-0e26-de11-acd4-001bfc39ff92 2 test2 92f0fc8f-0e26-de11-acd4-001bfc39ff92 3 test3 122aa19b-0e26-de11-acd4-001bfc39ff92
77 < 8f < 9b !!! You have to see the highest value byets, not the lowest (from right to left)
回答4:
I'm not familiar with newsequentialid(), for uniqueidentifier types I call newid().
回答5:
There definitely can be gaps in NewSequentialId() sequences - I've found the following causes gaps:
- As soon as another call is made by another table needing a NewSequentialId()
- Failed inserts
- Rollbacks
(2 and 3 are similar to identity() in this respect)
For example, given 2 tables using NewSequentialId()
create table XXX(someGuid uniqueidentifier DEFAULT NEWSEQUENTIALID(), x INT) create table YYY(someGuid uniqueidentifier DEFAULT NEWSEQUENTIALID(), y DateTime) GO insert into XXX(x) values(1) insert into XXX(x) values(2) insert into XXX(x) values(3) GO insert into YYY(y) values(current_timestamp) insert into YYY(y) values(current_timestamp) insert into YYY(y) values(current_timestamp) GO insert into XXX(x) values(4) insert into XXX(x) values(5) insert into XXX(x) values(6) GO SELECT * FROM XXX 6A6E85CB-CCA3-E111-9E8E-005056C00008 1 6B6E85CB-CCA3-E111-9E8E-005056C00008 2 6C6E85CB-CCA3-E111-9E8E-005056C00008 3 **CCEA7AF2-CCA3-E111-9E8E-005056C00008 4** Gap here because we 'switched' to y CDEA7AF2-CCA3-E111-9E8E-005056C00008 5 CEEA7AF2-CCA3-E111-9E8E-005056C00008 6 SELECT * FROM YYY 8F9438E1-CCA3-E111-9E8E-005056C00008 2012-05-22 07:13:35.503 909438E1-CCA3-E111-9E8E-005056C00008 2012-05-22 07:13:41.210 919438E1-CCA3-E111-9E8E-005056C00008 2012-05-22 07:13:41.220
Also, NewSequentialId()s aren't returned to the sequence in the case of a failed insert, e.g.
insert into XXX(x) values(1) insert into XXX(x) values(2) BEGIN TRAN insert into XXX(x) values(3) insert into XXX(x) values(4) ROLLBACK TRAN insert into XXX(x) values(5) insert into XXX(x) values(6) GO 686EFE5B-CDA3-E111-9E8E-005056C00008 696EFE5B-CDA3-E111-9E8E-005056C00008 6C6EFE5B-CDA3-E111-9E8E-005056C00008 6D6EFE5B-CDA3-E111-9E8E-005056C00008 i.e. a Gap of 2 Guids rolled back and insert into XXX(x) values(1) insert into XXX(x) values(2) insert into XXX(x) values(3) GO insert into XXX(x) values(99999999999999) -- overflow GO insert into XXX(x) values(4) insert into XXX(x) values(5) insert into XXX(x) values(6) go select * from xxx AC613611-CFA3-E111-9E8E-005056C00008 1 AD613611-CFA3-E111-9E8E-005056C00008 2 AE613611-CFA3-E111-9E8E-005056C00008 3 **B0613611-CFA3-E111-9E8E-005056C00008 4** Gap of 1 - overflow failure B1613611-CFA3-E111-9E8E-005056C00008 5 B2613611-CFA3-E111-9E8E-005056C00008 6
回答6:
NEWSEQUENTIALGUID (as every guid generated in a way that warrant their sequence) includes a part of the Guid calculated via a time stamp. So if you run the inserts at different time you'll see some gaps.
But the important part is that the Guid are "ordered" in a way that do not cause page splits (if the Guid is used in a index) and this is what happens when using the new sequential guid.
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