Skip to main content

System.Data.SqlClient.SqlException : The ntext data type cannot be selected as DISTINCT because it is not comparable

After upgrading to NHibernate 5, one of our integration tests started to fail with the following error message:

System.Data.SqlClient.SqlException : The ntext data type cannot be selected as DISTINCT because it is not comparable.

The error itself happened inside a by Nhibernate generated query:

Message: NHibernate.Exceptions.GenericADOException : [SQL: select distinct supplier1_.SupplierID as supplierid1_138_, supplier1_.Address as address2_138_, supplier1_.City as city3_138_, supplier1_.CompanyName as companyname4_138_, supplier1_.ContactName as contactname5_138_, supplier1_.ContactTitle as contacttitle6_138_, supplier1_.Country as country7_138_, supplier1_.Fax as fax8_138_, supplier1_.HomePage as homepage9_138_, supplier1_.Phone as phone10_138_, supplier1_.PostalCode as postalcode11_138_, supplier1_.Region as region12_138_ from Products product0_ inner join Categories category2_ on product0_.CategoryID=category2_.CategoryID, Suppliers supplier1_ where supplier1_.SupplierID=product0_.SupplierID and category2_.CategoryName=@p0] 

The error message pointed us to a ntext data type. A look into the database revealed that indeed we were using the ntext data type.

clip_image002

It is recommended to avoid this data type(it is marked as obsolete) and use nvarchar(MAX) instead.

But why didn’t we got this error before? It turned out that the query generated by NHibernate 5 is different then the one used by NHibernate 4. In NHibernate 5 an extra DISTINCT clause is added to the query, resulting in the error message above.

Labels:

Popular posts from this blog

Podman– Command execution failed with exit code 125

After updating WSL on one of the developer machines, Podman failed to work. When we took a look through Podman Desktop, we noticed that Podman had stopped running and returned the following error message: Error: Command execution failed with exit code 125 Here are the steps we tried to fix the issue: We started by running podman info to get some extra details on what could be wrong: >podman info OS: windows/amd64 provider: wsl version: 5.3.1 Cannot connect to Podman. Please verify your connection to the Linux system using `podman system connection list`, or try `podman machine init` and `podman machine start` to manage a new Linux VM Error: unable to connect to Podman socket: failed to connect: dial tcp 127.0.0.1:2655: connectex: No connection could be made because the target machine actively refused it. That makes sense as the podman VM was not running. Let’s check the VM: >podman machine list NAME         ...
Read more

Cache stampede: when our cache turned against us

While investigating some performance issues, we ran into an ASP.NET Core API that cached a fairly expensive aggregation query for 60 seconds. Under normal load, that was fine: one request rebuilds the cache, everyone else reads from it. Under peak load, dozens of requests would arrive in that same expiry window, all see a cache miss, and all fire the same expensive query in parallel. The database didn't like that. That was the moment when our caching layer stopped helping and started hurting. A burst of requests comes in at the same time, all miss the cache, and all go hammer the database or the downstream API at once. That's a cache stampede . The cache was supposed to protect our backend, and for a few hundred milliseconds it did the opposite. Why this happens IMemoryCache.GetOrCreate (and its async sibling) looks like it protects you, but it doesn't add any locking on its own. Look at the naive version: public async Task<Report> GetReportAsync(string key) ...
Read more

Azure DevOps/ GitHub emoji

I’m really bad at remembering emoji’s. So here is cheat sheet with all emoji’s that can be used in tools that support the github emoji markdown markup: All credits go to rcaviers who created this list.
Read more