Ayende @ Rahien

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NH Prof New FeatureAlert on bad ‘like’ query

time to read 2 min | 251 words

Originally posted at 12/3/2010

One of the things that are coming to NH Prof is more smarts at the analysis part. We now intend to create a lot more alerts and guidance. One of the new features that is already there as part of this strategy is detecting bad ‘like’ queries.

For example, let us take a look at this


This is generally not a good idea, because that sort of query cannot use an index, and requires the database to generate a table scan, which can be pretty slow.

Here is how it looks like from the query perspective:


And NH Prof (and all the other profilers) will now detect this and warn about this.


In fact, it will even detect queries like this:


More posts in "NH Prof New Feature" series:

  1. (09 Dec 2010) Alert on bad ‘like’ query
  2. (10 Dec 2009) Filter static files
  3. (16 Nov 2009) Exporting Reports
  4. (08 Oct 2009) NHibernate Search Integration
  5. (19 Aug 2009) Multiple Session Factory Support
  6. (07 Aug 2009) Diffing Sessions
  7. (06 Aug 2009) Capturing DDL
  8. (05 Aug 2009) Detect Cross Thread Session Usage
  9. (22 May 2009) Detecting 2nd cache collection loads
  10. (15 May 2009) Error Detection
  11. (12 May 2009) Queries by Url
  12. (04 Feb 2009) View Query Results
  13. (18 Jan 2009) Superfluous <many-to-one> update
  14. (18 Jan 2009) URL tracking
  15. (10 Jan 2009) Detecting distributed transactions (System.Transactions)
  16. (06 Jan 2009) The Query Cache
  17. (05 Jan 2009) Query Duration
  18. (24 Dec 2008) Unbounded result sets
  19. (24 Dec 2008) Row Counts



That makes sense. Is there a "right" way to allow in-string search?

David Neale

Why did you choose to make this alert an error as opposed to an alert? Are there not times when an open query like this is necessary?

Duncan Godwin

Nice feature! I supposed the equivalent for dates would be really easy to implement as well :)

Ayende Rahien


You use the database full text indexes, or Lucene.



Honestly, I've met a lot of developers who really don't understand how SQL works under the hood, so they might not be aware that this type of query does a full table scan. Especially when the code they write is .Contains("EF").

David Neale

Is there any way to use full text indexes without having to resort to HQL? I use LINQ to implement a specification pattern and can't think of a way to introduce HQL to this.

Ayende Rahien


You write a linq extension

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