Encountered this problem in production in the form of a deadlock. Figured out that if a transaction was inserting a row on my table, and I wanted to select a totally different row from that table, I would get the following error:
245: Could not position within a file via an index.
144: ISAM error: key value locked
Error in line 1
Near character position 70
My select statement was of the form select * from table where bar = 3 and foo = "CCCC";, where "foo" is a foreign key to a table with 18 rows, and "bar" is the first table's primary key. My insert statement was also inserting a row with foo = "CCCC". Curiously, the select query also returned the desired row before outputting the error.
I tried all this on informix 12.10 with isolation level set to repeatable read. I tried it on production, and in a fresh DB I set up with only the two tables mentioned. The lock mode of both tables is "row".
I investigated by modifying the select statement: select * from table where bar = 3; would not fail. Also, select * from table where bar = 3 and foo = "CCCC" order by ber; would not fail (ber being a random field from the table, ber is not indexed).
I would expect all the select statements I tried to return the desired row without error, OR all of them to fail. My solution in production was to order by a random field in the table, which fixed the deadlock issue
Does anyone know why this issue could have happened ? I suspect it is linked to the indexes on the table, which were all created automatically when adding the primary and foreign keys to the table. But I do not know enough about indexes to understand what happened. Could this be a bug ?
Schema of the tables:
create table options (
foo char(4) not null,
fee int not null)
extent size 16 next size 16
lock mode row;
alter table options add constraint (
primary key (foo)
constraint cons1 );
create table decisions (
bar char(3) not null,
foo char(4) not null,
ber int not null)
extent size 131072 next size 65536
lock mode row;
alter table decisions add constraint (
primary key (bar)
constraint cons2 );
alter table decisions add constraint (
foreign key (foo) references options(foo)
constraint cons3 );
Data I inserted into the "options" table:
AAAA|0|
BBBB|0|
CCCC|1|
DDDD|4|
EEEE|1|
FFFF|8|
Data I inserted into the "decisions" table:
QWE|AAAA|0|
WER|AAAA|9|
ERT|CCCC|2|
RTY|AAAA|32|
TYU|CCCC|1234|
YUI|CCCC|42398|
UIO|AAAA|23178|
IOP|CCCC|1233|
OPA|CCCC|11|
PAS|AAAA|890|
ASD|AAAA|90|
SDF|CCCC|2|
DFG|AAAA|4|
FGH|CCCC|7|
Edit: I used set explain on; for the queries.
select * from decisions where foo = "CCCC" and bar = "QWE" order by foo; returned that the index used was on foo="CCCC". However, for select * from decisions where foo = "CCCC" and bar = "QWE" order by ber;, it's indexed on bar="QWE".
Related
Given the following:
-- This table will have roughly 14 million records
CREATE TABLE IdMappings
(
Id int IDENTITY(1,1) NOT NULL,
OldId int NOT NULL,
NewId int NOT NULL,
RecordType varchar(80) NOT NULL, -- 15 distinct values, will never increase
Processed bit NOT NULL DEFAULT 0,
CONSTRAINT pk_IdMappings
PRIMARY KEY CLUSTERED (Id ASC)
)
CREATE UNIQUE INDEX ux_IdMappings_OldId ON IdMappings (OldId);
CREATE UNIQUE INDEX ux_IdMappings_NewId ON IdMappings (NewId);
and this is the most common query run against the table:
WHILE #firstBatchId <= #maxBatchId
BEGIN
-- the result of this is used to insert into another table:
SELECT
NewId, -- and lots of non-indexed columns from SOME_TABLE
FROM
IdMappings map
INNER JOIN
SOME_TABLE foo ON foo.Id = map.OldId
WHERE
map.Id BETWEEN #firstBatchId AND #lastBatchId
AND map.RecordType = #someRecordType
AND map.Processed = 0
-- We only really need this in case the user kills the binary or SQL Server service:
UPDATE IdMappings
SET Processed = 1
WHERE map.Id BETWEEN #firstBatchId AND #lastBatchId
AND map.RecordType = #someRecordType
SET #firstBatchId += 4999
SET #lastBatchId += 4999
END
What are the best indices to add? I figure Processed isn't worth indexing since it only has 2 values. Is it worth indexing RecordType since there are only about 15 distinct values? How many distinct values will a column likely have before we consider indexing it?
Is there any advantage in a composite key if some of the fields are in the WHERE and some are in a JOIN's ON condition? For example:
CREATE INDEX ix_IdMappings_RecordType_OldId
ON IdMappings (RecordType, OldId)
... if I wanted both these fields indexed (I'm not saying I do), does this composite key gain any advantage since both columns don't appear together in the same WHERE or same ON?
Insert time into IdMappings isn't really an issue. After we insert all records into the table, we don't need to do so again for months if ever.
I have a SQL table with a column called [applied], only one row from all rows can be applied ( have the value of 1) all other rows should have the value 0
Is there a check constraint that i can write to force such a case?
If you use null instead of 0, it will be much easier.
Have a CHECK constraint to make sure the (non-null) value = 1. Also have a UNIQUE constraint to only allow a single value 1.
create table testtable (
id int primary key,
applied int,
constraint applied_unique unique (applied),
constraint applied_eq_1 check (applied = 1)
);
Core ANSI SQL, i.e. expected to work with any database.
Most databases support filtered indexes:
create unique index unq_t_applied on t(applied) where applied = 1;
To know exactly how to write trigger that will help you an info of a database you use is needed.
You wil need a trigger where this will be your test control:
SELECT COUNT(APPLIED)
FROM TEST
WHERE APPLIED = 1
If it is > 0 then do not allow insert else allow.
While this can be done with triggers and constraints, they probably require an index. Instead, consider a join table.
create table things_applied (
id smallint primary key default 1,
thing_id bigint references things(id) not null,
check(id = 1)
);
Because the primary key is unique, there can only ever be one row.
The first is activated with an insert.
insert into things_applied (thing_id) values (1);
Change it by updating the row.
update things_applied set thing_id = 2;
To deactivate completely, delete the row.
delete things_applied;
To find the active row, join with the table.
select t.*
from things t
join things_applied ta on ta.thing_id = t.id
To check if it's active at all, count the rows.
select count(id) as active
from things_applied
Try it.
I have an identity column that is also the primary key, of INT datatype. Due to the issue discussed here (cache loss), the identity has gaps and I chose to reseed to the previous value. In concrete terms, I have a situation that looks like this:
Table1
ID_PK Field1
---------------
28 'd'
29 'e'
30 'h'
1029 'f'
1030 'g'
I looked around and couldn't find a clear answer to what happens when I make an insertion and the seed reaches the existent value that would break the constraint. Suppose I were to insert values 'x' and 'y' in two separated queries to the table, I can think of the following possibilities:
The identity will be reseeded before the first insertion and I will have both values inserted correctly.
The first insertion will fail, then the column will be reseeded, and only then the second insertion would succeed.
Neither will work and I will have to explicitly call DBCC CHECKIDENT to reseed before inserting values in the table
So, which is it? Or none of the above? Would this behavior be different if I inserted a multi-row result query into Table1? Thanks in advance
For completeness anyway, here's a script you can use to test:
USE Sandbox;
GO
CREATE TABLE test(ID int IDENTITY(1,1) PRIMARY KEY CLUSTERED, string char(1));
GO
INSERT INTO test (string)
VALUES ('a'),('b'),('c'),('d');
GO
SELECT *
FROM test;
GO
DELETE FROM test
WHERE string IN ('b','c');
GO
SELECT *
FROM test;
GO
DBCC CHECKIDENT ('dbo.test', RESEED, 1);
GO
INSERT INTO test (string)
VALUES ('e'),('f');
GO
SELECT *
FROM test;
GO
INSERT INTO test (string)
VALUES ('g');
GO
SELECT *
FROM test;
GO
DROP TABLE test;
Running this script will give you the answer you need. If you wonder why I have used 1 as the RESEED value, this is explained in the documentation:
The following example forces the current identity value in the
AddressTypeID column in the AddressType table to a value of 10.
Because the table has existing rows, the next row inserted will use 11
as the value, that is, the new current increment value defined for the
column value plus 1.
In my script, this means that the next row to be inserted after the RESEED will have a value of 2 for its IDENTITY, not 1 (as rows already existing in the table (ID's 1 and 4)).
As several have said in the comments though, there's really no need to use RESEED on an IDENTITY column. If you need to maintain a sequence, you should (unsurprisingly) be using a SEQUENCE: CREATE SEQUENCE (Transact-SQL)
It depends:
Scenario 1
You get duplicates in the IDENTITY column, as no unique index or PK constraint.
create table I (
id int identity(1,1) not null,
i int null
)
Scenario 2
You get the following error as the inserted value conflicts with the Primary Key constraint:
Msg 2627, Level 14, State 1, Line 1 Violation of PRIMARY KEY
constraint 'PK__I__3213E83FE0B0E009'. Cannot insert duplicate key in
object 'dbo.I'. The duplicate key value is (11). The statement has
been terminated.
create table I (
id int identity(1,1) not null primary key,
i int null
)
This proves that IDENTITY on it's own does not guarantee uniqueness, only a UNIQUE CONSTRAINT does that.
To close, turns out it's (2).
First insertion fails, reseed is automatic to the highest value, and only next insertion suceeds. Multi-value insertions behave the same if any of the values would break the primary key constraint.
Here are two tables that I have, with Table B referencing Table A:
CREATE TABLE TableA
(
[Id_A] [bigint] IDENTITY(1,1) NOT NULL,
...
CONSTRAINT [PK_TableA_Id_A] PRIMARY KEY CLUSTERED
(
[Id_A] ASC
)
)
CREATE TABLE TableB
(
[Id_B] [bigint] IDENTITY(1,1) NOT NULL,
[RefId_A] [bigint] NOT NULL
...
CONSTRAINT [PK_TableB_Id_B] PRIMARY KEY CLUSTERED
(
[Id_B] ASC
)
)
ALTER TABLE [dbo].[TableB] WITH CHECK ADD CONSTRAINT [FK_Id_A] FOREIGN KEY([RefId_A])
REFERENCES [dbo].[TableA] ([Id_A])
These two tables are part of 2 databases.
Table A and Table B in database 1;
Table A and Table B in database 2.
I need to merge the rows of Table A from database 1 into Table A of database 2 and the rows of Table B from database 1 into Table B of database 2.
I used the SQL Data Import and Export Wizard , checked the Enable Identity Insert option but it fails:
An OLE DB record is available. Source: "Microsoft SQL Server Native
Client 11.0" Hresult: 0x80004005 Description: "Violation of PRIMARY
KEY constraint 'PK_TableB_Id_B'. Cannot insert duplicate key in object
'dbo.TableB'. The duplicate key value is (1).". (SQL Server Import and
Export Wizard)
Which seems to make sense. There are rows in Table B of database 1 that have the same auto-generated PK as rows of Table B in database 2.
QUESTION
In this scenario, how can I merge the tables content from database 1 to the tables of database 2 while maintaining the foreign key constraints?
You can try something like the following. In here we assume that you need to insert all records as new ones (and not compare if some already exist or not). I wrapped both operations in a transaction to ensure that both go OK or none at all.
BEGIN TRY
IF OBJECT_ID('tempdb..#IdentityRelationships') IS NOT NULL
DROP TABLE #IdentityRelationships
CREATE TABLE #IdentityRelationships (
OldIdentity INT,
NewIdentity INT)
BEGIN TRANSACTION
;WITH SourceData AS
(
SELECT
OldIdentity = A.Id_A,
OtherColumn = A.OtherColumn
FROM
Database1.Schema.TableA AS A
)
MERGE INTO
Database2.Schema.TableA AS T
USING
SourceData AS S ON 1 = 0 -- Will always execute the "WHEN NOT MATCHED" operation
WHEN NOT MATCHED THEN
INSERT (
OtherColumn)
VALUES (
S.OtherColumn)
OUTPUT
inserted.Id_A, -- "MERGE" clause can output non-inserted values
S.ID_A
INTO
#IdentityRelationships (
NewIdentity,
OldIdentity);
INSERT INTO Database2.Schema.TableB (
RefId_A,
OtherData)
SELECT
RefId_A = I.NewIdentity,
OtherData = T.OtherData
FROM
Database1.Schema.TableB AS T
INNER JOIN #IdentityRelationships AS I ON T.RefID_A = I.OldIdentity
COMMIT
END TRY
BEGIN CATCH
DECLARE #v_ErrorMessage VARCHAR(MAX) = CONVERT(VARCHAR(MAX), ERROR_MESSAGE())
IF ##TRANCOUNT > 0
ROLLBACK
RAISERROR (#v_ErrorMessage, 16, 1)
END CATCH
This is too long for a comment.
There is no simple way to do this. Your primary keys are identity columns that both start at "1", so the relationships are ambiguous.
You have two options:
A composite primary key, identifying the database source of the records.
A new primary key. You can preserve the existing primary key values from one database.
Your question doesn't provide enough information to say which is the better approach: "merge" is not clearly defined.
I might suggest that you just recreate all the tables. Insert all the rows from table A into a new table. Add a new identity primary key. Keep the original primary key and source.
Then bring the data from Table B into a new table, looking up the new primary key in the new Table A. At this point, the new Table B is finished, except for defining the primary key constraint.
Then drop the unnecessarily columns in the new table A.
How could I set a constraint on a table so that only one of the records has its isDefault bit field set to 1?
The constraint is not table scope, but one default per set of rows, specified by a FormID.
Use a unique filtered index
On SQL Server 2008 or higher you can simply use a unique filtered index
CREATE UNIQUE INDEX IX_TableName_FormID_isDefault
ON TableName(FormID)
WHERE isDefault = 1
Where the table is
CREATE TABLE TableName(
FormID INT NOT NULL,
isDefault BIT NOT NULL
)
For example if you try to insert many rows with the same FormID and isDefault set to 1 you will have this error:
Cannot insert duplicate key row in object 'dbo.TableName' with unique
index 'IX_TableName_FormID_isDefault'. The duplicate key value is (1).
Source: http://technet.microsoft.com/en-us/library/cc280372.aspx
Here's a modification of Damien_The_Unbeliever's solution that allows one default per FormID.
CREATE VIEW form_defaults
AS
SELECT FormID
FROM whatever
WHERE isDefault = 1
GO
CREATE UNIQUE CLUSTERED INDEX ix_form_defaults on form_defaults (FormID)
GO
But the serious relational folks will tell you this information should just be in another table.
CREATE TABLE form
FormID int NOT NULL PRIMARY KEY
DefaultWhateverID int FOREIGN KEY REFERENCES Whatever(ID)
From a normalization perspective, this would be an inefficient way of storing a single fact.
I would opt to hold this information at a higher level, by storing (in a different table) a foreign key to the identifier of the row which is considered to be the default.
CREATE TABLE [dbo].[Foo](
[Id] [int] NOT NULL,
CONSTRAINT [PK_Foo] PRIMARY KEY CLUSTERED
(
[Id] ASC
) ON [PRIMARY]
) ON [PRIMARY]
GO
CREATE TABLE [dbo].[DefaultSettings](
[DefaultFoo] [int] NULL
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[DefaultSettings] WITH CHECK ADD CONSTRAINT [FK_DefaultSettings_Foo] FOREIGN KEY([DefaultFoo])
REFERENCES [dbo].[Foo] ([Id])
GO
ALTER TABLE [dbo].[DefaultSettings] CHECK CONSTRAINT [FK_DefaultSettings_Foo]
GO
You could use an insert/update trigger.
Within the trigger after an insert or update, if the count of rows with isDefault = 1 is more than 1, then rollback the transaction.
CREATE VIEW vOnlyOneDefault
AS
SELECT 1 as Lock
FROM <underlying table>
WHERE Default = 1
GO
CREATE UNIQUE CLUSTERED INDEX IX_vOnlyOneDefault on vOnlyOneDefault (Lock)
GO
You'll need to have the right ANSI settings turned on for this.
I don't know about SQLServer.But if it supports Function-Based Indexes like in Oracle, I hope this can be translated, if not, sorry.
You can do an index like this on suposed that default value is 1234, the column is DEFAULT_COLUMN and ID_COLUMN is the primary key:
CREATE
UNIQUE
INDEX only_one_default
ON my_table
( DECODE(DEFAULT_COLUMN, 1234, -1, ID_COLUMN) )
This DDL creates an unique index indexing -1 if the value of DEFAULT_COLUMN is 1234 and ID_COLUMN in any other case. Then, if two columns have DEFAULT_COLUMN value, it raises an exception.
The question implies to me that you have a primary table that has some child records and one of those child records will be the default record. Using address and a separate default table here is an example of how to make that happen using third normal form. Of course I don't know if it's valuable to answer something that is so old but it struck my fancy.
--drop table dev.defaultAddress;
--drop table dev.addresses;
--drop table dev.people;
CREATE TABLE [dev].[people](
[Id] [int] identity primary key,
name char(20)
)
GO
CREATE TABLE [dev].[Addresses](
id int identity primary key,
peopleId int foreign key references dev.people(id),
address varchar(100)
) ON [PRIMARY]
GO
CREATE TABLE [dev].[defaultAddress](
id int identity primary key,
peopleId int foreign key references dev.people(id),
addressesId int foreign key references dev.addresses(id))
go
create unique index defaultAddress on dev.defaultAddress (peopleId)
go
create unique index idx_addr_id_person on dev.addresses(peopleid,id);
go
ALTER TABLE dev.defaultAddress
ADD CONSTRAINT FK_Def_People_Address
FOREIGN KEY(peopleID, addressesID)
REFERENCES dev.Addresses(peopleId, id)
go
insert into dev.people (name)
select 'Bill' union
select 'John' union
select 'Harry'
insert into dev.Addresses (peopleid, address)
select 1, '123 someplace' union
select 1,'work place' union
select 2,'home address' union
select 3,'some address'
insert into dev.defaultaddress (peopleId, addressesid)
select 1,1 union
select 2,3
-- so two home addresses are default now
-- try adding another default address to Bill and you get an error
select * from dev.people
join dev.addresses on people.id = addresses.peopleid
left join dev.defaultAddress on defaultAddress.peopleid = people.id and defaultaddress.addressesid = addresses.id
insert into dev.defaultaddress (peopleId, addressesId)
select 1,2
GO
You could do it through an instead of trigger, or if you want it as a constraint create a constraint that references a function that checks for a row that has the default set to 1
EDIT oops, needs to be <=
Create table mytable(id1 int, defaultX bit not null default(0))
go
create Function dbo.fx_DefaultExists()
returns int as
Begin
Declare #Ret int
Set #ret = 0
Select #ret = count(1) from mytable
Where defaultX = 1
Return #ret
End
GO
Alter table mytable add
CONSTRAINT [CHK_DEFAULT_SET] CHECK
(([dbo].fx_DefaultExists()<=(1)))
GO
Insert into mytable (id1, defaultX) values (1,1)
Insert into mytable (id1, defaultX) values (2,1)
This is a fairly complex process that cannot be handled through a simple constraint.
We do this through a trigger. However before you write the trigger you need to be able to answer several things:
do we want to fail the insert if a default exists, change it to 0 instead of 1 or change the existing default to 0 and leave this one as 1?
what do we want to do if the default record is deleted and other non default records are still there? Do we make one the default, if so how do we determine which one?
You will also need to be very, very careful to make the trigger handle multiple row processing. For instance a client might decide that all of the records of a particular type should be the default. You wouldn't change a million records one at a time, so this trigger needs to be able to handle that. It also needs to handle that without looping or the use of a cursor (you really don't want the type of transaction discussed above to take hours locking up the table the whole time).
You also need a very extensive tesing scenario for this trigger before it goes live. You need to test:
adding a record with no default and it is the first record for that customer
adding a record with a default and it is the first record for that customer
adding a record with no default and it is the not the first record for that customer
adding a record with a default and it is the not the first record for that customer
Updating a record to have the default when no other record has it (assuming you don't require one record to always be set as the deafault)
Updating a record to remove the default
Deleting the record with the deafult
Deleting a record without the default
Performing a mass insert with multiple situations in the data including two records which both have isdefault set to 1 and all of the situations tested when running individual record inserts
Performing a mass update with multiple situations in the data including two records which both have isdefault set to 1 and all of the situations tested when running individual record updates
Performing a mass delete with multiple situations in the data including two records which both have isdefault set to 1 and all of the situations tested when running individual record deletes
#Andy Jones gave an answer above closest to mine, but bearing in mind the Rule of Three, I placed the logic directly in the stored proc that updates this table. This was my simple solution. If I need to update the table from elsewhere, I will move the logic to a trigger. The one default rule applies to each set of records specified by a FormID and a ConfigID:
ALTER proc [dbo].[cpForm_UpdateLinkedReport]
#reportLinkId int,
#defaultYN bit,
#linkName nvarchar(150)
as
if #defaultYN = 1
begin
declare #formId int, #configId int
select #formId = FormID, #configId = ConfigID from csReportLink where ReportLinkID = #reportLinkId
update csReportLink set DefaultYN = 0 where isnull(ConfigID, #configId) = #configId and FormID = #formId
end
update
csReportLink
set
DefaultYN = #defaultYN,
LinkName = #linkName
where
ReportLinkID = #reportLinkId