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(Posted first on SO, but its not getting any answers there. Cross-posting to reach a larger audience, since it is cross-discipline.)

In the process of trying to help out an app dev team with some performance issues on a SQL 2000 server, I ran a SQL trace and discovered that all calls to the database are full of API Server Cursor statements (sp_cursorprepexec, sp_cursorfetch, sp_cursorclose).

Looks like they're specifying some connection string properties that force the use of server-side cursors, retrieving only 128 rows of data at a time: (From http://msdn.microsoft.com/en-us/library/Aa172588)

When the API cursor attributes or properties are set to anything other than their defaults, the OLE DB provider for SQL Server and the SQL Server ODBC driver use API server cursors instead of default result sets. Each call to an API function that fetches rows generates a roundtrip to the server to fetch the rows from the API server cursor.

UPDATE: They have multiple Java apps using a JDBC connection to the SQL server with selectMethod=cursor specified (as opposed to selectMethod=direct).

From my DBA perspective, that's just annoying (it clutters the trace up with useless junk), and in all likelihood is resulting in many extra app-to-SQL server round trips, reducing overall performance.

They've apparently tested selectMethod=direct in a very limited way (changed just one application out of ~60) and experienced some issues of some sort (technical details of which I do not have).

So, my questions are:

  • What's the performance impact of selectMethod=cursor vs direct on an already busy SQL 2000 server? (I had speculated that increasing the number of round trips between SQL and APP servers can't do anything good. Was I wrong?)
  • Is selectMethod an application-transparent setting? Could this break their apps if we change it?
  • When should they use cursor vs direct? Are there some types of apps that would benefit from one over the other?

Update: Found out the name of the driver parameter, which have warranted significant edits to title, body, and tags.

Update: Added bounty. Also added bounty to SO question (which is more focused on application behavior). Thanks!

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1 Answer 1

up vote 2 down vote accepted
+100

Briefly,

  1. selectMethod=cursor
    • theoretically requires more server-side resources than selectMethod=direct
    • only loads at most batch-size records into client memory at once, resulting in a more predictable client memory footprint
  2. selectMethod=direct
    • theoretically requires less server-side resources than selectMethod=cursor
    • will read the entire result set into client memory (unless the driver natively supports asynchronous result set retrieval) before the client application can iterate over it; this can reduce performance in two ways:
      1. reduced performance with large result sets if the client application is written in such a way as to stop processing after traversing only a fraction of the result set (with direct it has already paid the cost of retrieving data it will essentially throw away; with cursor the waste is limited to at most batch-size - 1 rows -- the early termination condition should probably be recoded in SQL anyway e.g. as SELECT TOP or window functions)
      2. reduced performance with large result sets because of potential garbage collection and/or out-of-memory issues associated with an increased memory footprint

In summary,

  • Can using selectMethod=cursor lower application performance? -- either method can lower performance, for different reasons. Past a certain resultset size, cursor may still be preferable. See below for when to use one or the other
  • Is selectMethod= an application-transparent setting on a JDBC connection? -- it is transparent, but it can still break their app if memory usage grows significantly enough to hog their client system (and, correspondingly, your server) or crash the client altogether
  • More generally, when should you use cursor vs direct? -- I personally use cursor when dealing with potentially large or otherwise unbounded result sets. The roundtrip overhead is then amortized given a large enough batch size, and my client memory footprint is predictable. I use direct when the size of the result set I expect is known to be inferior to whatever batch size I use with cursor, or bound in some way, or when memory is not an issue.

Cheers, V.

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