"Lost Inserts"

[Tony]

Is there any known SQL Server bug whereby a record can be successfully
inserted and committed, but then later be found not to be in the
database? For example, if there was a server crash just after the
commit, could committed data be lost?

I'm sure the answer must be "no", but a client is telling me this is
happening, and I said I'd enquire.

[Erland Sommarskog]

Tony (an******@onetel.net.uk) writes:

Is there any known SQL Server bug whereby a record can be successfully
inserted and committed, but then later be found not to be in the
database? For example, if there was a server crash just after the
commit, could committed data be lost?

I'm sure the answer must be "no", but a client is telling me this is
happening, and I said I'd enquire.

Of course, in case of a crash or a hardware error, the database could
become corrupt, in which case you could lose data. But in such cases
you should run DBCC CHECKDB to investigate.

If data appears to be lost, but the database appears to be safe and sound,
there are two possibilities:
1) Data was never committed, but was believed to because of an application
error. (There are a few gotchas that could lead to this.)
2) There is some other process that deletes the rows for one reason or
another.

In case of the latter, Lumgients Log Explorer (www.lumigent.com) can
be invaluable tool to track down what is happening.

--
Erland Sommarskog, SQL Server MVP, so****@algonet.se

Books Online for SQL Server SP3 at
http://www.microsoft.com/sql/techinf...2000/books.asp

[Greg D. Moore \(Strider\)]

"Tony" <an******@onetel.net.uk> wrote in message
news:c0*************************@posting.google.co m...

Is there any known SQL Server bug whereby a record can be successfully
inserted and committed, but then later be found not to be in the
database? For example, if there was a server crash just after the
commit, could committed data be lost?

Very unlikely, but possible.

I'm sure the answer must be "no", but a client is telling me this is
happening, and I said I'd enquire.

What makes them think this is happening?

[Tony Andrews]

> If data appears to be lost, but the database appears to be safe and
sound,

there are two possibilities:
1) Data was never committed, but was believed to because > of an application error. (There are a few gotchas that could lead to this.) 2) There is some other process that deletes the rows for one reason or

another.

Yes, my suspicion is that it is possibility (1). The application is
written in ASP using ODBC to connect to the database. Autocommit is
used. The logic is something like this:
1) Insert record into table
2) Redirect to another page to print out a certificate

Allegedly, there are a few people holding certificates where no
corresponding record exists in the database. There is no code in the
app. that deletes this data, though of course it could be deleted
manually.

I have verified that if the insert fails (e.g. if I add a check
constraint that will always be violated), the ASP page aborts and does
not redirect to the next page.

Are there any particular scenarios I should look out for?

Thanks for your quick response.

Tony

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[Erland Sommarskog]

Tony Andrews (an******@onetel.net.uk) writes:

Yes, my suspicion is that it is possibility (1). The application is
written in ASP using ODBC to connect to the database. Autocommit is
used. The logic is something like this:
1) Insert record into table
2) Redirect to another page to print out a certificate

Allegedly, there are a few people holding certificates where no
corresponding record exists in the database. There is no code in the
app. that deletes this data, though of course it could be deleted
manually.

I have verified that if the insert fails (e.g. if I add a check
constraint that will always be violated), the ASP page aborts and does
not redirect to the next page.

Are there any particular scenarios I should look out for?

There are a few nasty situations, which must be handled properly in
application code.

Say that you call a stored procedure which starts a user-defined
transaction. That stored procedure runs for a longer time, because of
the query or because of blocking. By default, all client libraries
have a timeout which sets in after 30 seconds (if no data have been
seen), and cancels the query. Contrary to what you may think, this DOES
NOT rollback the transaction! Thus, in your error-handling code you
need to issue "IF @@trancount > 0 ROLLBACK TRANSACTION".

Note here that even you don't use stored procedures but issue plain
INSERT statements, this can happen if there is a trigger on the table.
A trigger alwyas executes in the context of a transaction. (You may
get an automatic rollback when you cancel execution in a trigger; I
have not tested this case, but I would not trust on getting a ROLLBACK.)

There are some variations of this theme. If a stored procedure refers
to a non-existing table, the procedure will abort on that statement,
without rollback any transactions it may have started. Again, the
client could as a matter of routine always issue rollback in case of
an error.

--
Erland Sommarskog, SQL Server MVP, so****@algonet.se

Books Online for SQL Server SP3 at
http://www.microsoft.com/sql/techinf...2000/books.asp