I always get a kick out of when a query needs to perform a look up on the last 4 digits of a column because it usually ends up being something radioactive like a credit card number or a social security number. Either way, trying to do a lookup by the RIGHT most characters of […]
Read MoreQuery Tuning
Perform RTRIM and LTRIM in your code not in your queries
On occasion I’ll see queries trim off whitespace from both the input parameters as well as against the columns in the predicate (WHERE clause). If you’ve been reading my most recent posts I’ll start to sound like a broken record – applying functions to columns in a predicate will make a query unsargable (unable to […]
Read MorePerformance of parameterized LIKE queries with new parameter sniffing option
A while back I wrote a blog post on how parameterized queries with a LIKE in the predicate (WHERE) performed slower than the non-parameterized version. In a nutshell, the parameterized version of the query can’t tell for certain what parameters will be passed and does not generate an optimal plan. While attempting to set up […]
Read MoreDon’t use UPPER in WHERE unless you really need it
In SQL Server when I run into a problematic query doing an UPPER on a column in the predicate (WHERE clause), I check the collation scheme of the database. If you have a Case Insensitive (CI) collation scheme you don’t need to use UPPER. As with most functions applied to a column, it is unsargable […]
Read MorePerformance implications of using DATEDIFF function in WHERE
These might be narrow cases I’m describing, but as I’ve seen it happen often enough I figured it is worth writing a post about. In this post I’ll describe two different scenarios where the DATEDIFF function caused a query to perform poorly due to DATEDIFF not being sargable. The first scenario I’ll be describing is […]
Read MorePerformance implications of using ISNULL vs IS NULL
Quite often I see queries that are not sargable due to the use of the ISNULL function in the query’s predicate (WHERE clause). If you’re not familiar with sargable it means whether or not a condition in the predicate can take advantage of an index. I won’t dive too deep into sargability here, but I’ll […]
Read MoreFixing an implicit type conversion without a code change
If you have ever run into the problem of implicit type conversions you will be aware that the fix for it involves a code change or a table change. In a previous post I showed a common cause of such implicit conversions. In this post I will show a fix for this problem that does […]
Read MoreHow to remove a hint from a query without code changes
On occasion I’ve been in situations where I’ve identified a query with a hint applied that was causing degraded performance. In the case of vendor code this can be a problem since removal or changes to hints would require a code change. This can also be a problem in shops where there are strict processes […]
Read MoreJoin Hints – Careful, They Force Order!
Recently I was looking at a query generating deadlocks as a result of a clustered index scan. I saw that someone forced a LOOP JOIN on one of the offending queries. At first glance it appeared as if the LOOP JOIN should have made an index seek more likely; however, after remembering a side effect […]
Read MoreWhat are wait events and why are they important for performance analysis?
While trying to get to the bottom of a SQL Server performance issue, on many occasions I’ve seen engineering teams and in some cases DBA’s start focussing on perfmon counters to determine where a SQL performance issue is coming from. Performance counters quite often will report symptoms of a performance issue and can many times […]
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