sql server query running slow from java

Question

I have a java program that runs a bunch of queries against an sql server database. The first of these, which queries against a view returns about 750k records. I can run t

Answer 1

It appears this may not have applied to your particular situation, but I wanted to provide another possible explanation for someone searching for this problem.

I just had a similar problem where a query executed directly in SQL Server took 1 minute while the same query took 5 minutes through a java prepared statemnent. I tracked it down to the fact that it is was done as a prepared statement.

When you execute a query directly in SQL Server, you are providing it a non-parameterized query, in which it knows all of the search criteria at optimization time. In my case, my search criteria included a date range, and SQL server was able to look at it, decide "that date range is huge, let's not use the date index" and then it chose something much better.

When I execute the same query through a java prepared statement, at the time that SQL Server is optimizing the query, you haven't yet provided it any of the parameter values, so it has to make a guess which index to use. In the case of my date range, if it optimizes for a small range and I give it a large range, it will perform slower than it could. Likewise if it optimizes for a large range and I give it a small one, it's again going to perform slower than it could.

To demonstrate this was indeed the problem, as an experiment I tried giving it hints as to what to optimize for using SQL Server's "OPTIMIZE FOR" option. When I told it to use a tiny date range, my java query (which actually had a wide date range) actually took twice as long as before (10 minutes, as opposed to 5 minutes before, and as opposed to 1 minute in SQL Server). When I told it my exact dates to optimize for, the execution time was identical between the java prepared statement.

So my solution was to hard code the exact dates into the query. This worked for me because this was just a one-off statement. The PreparedStatement was not intended to be reused, but merely to parameterize the values to avoid SQL injection. Since these dates were coming from a java.sql.Date object, I didn't have to worry about my date values containing injection code.

However, for a statement that DOES need to be reused, hard coding the dates wouldn't work. Perhaps a better option for that would be to create multiple prepared statements optimized for different date ranges (one for a day, one for a week, one for a month, one for a year, and one for a decade...or maybe you only need 2 or 3 options...I don't know) and then for each query, execute the one prepared statement whose time range best matches the range in the actual query.

Of course, this only works well if your date ranges are evenly distributed. If 80% of your records were in the last year, and 20% percent spread out over the previous 10 years, then doing the "multiple queries based on range size" thing might not be best. You'd have to optimize you queries based on specific ranges or something. You'd need to figure that out through trial an error.