Sometimes people speak “SQL” and expect you know all the terminology. In recent a conversation about query plans I was ranting on and on about how the predicates were written when the person stopped me and asked a very simple question. Like a light bulb going off, it reminded me that not everyone knows what the other person is talking about when terminology is mentioned without clarification. So, this post will answer that persons very valid simple question. What is a SQL Predicate? Predicates are expressions that evaluate to TRUE, FALSE, UNKNOWN. There are two types of predicates Filtered Predicates and Join Predicates. Filtered Predicates cover your WHERE or HAVING…
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In SQL Server the query optimizer uses a cardinality estimator to answer data SELECTIVITY questions like the ones below. The optimizer uses this cardinality data, which relies heavy on statistics to get the answers and calculate a cost estimate. It takes the #of Rows to Satisfy a Predicate/Total # of Input Rows. How many rows will satisfy a single filter predicate? Multiple? WHERE How many rows will satisfy a join predicate between two tables? JOIN How many distinct values do we expect from a specific column? DISTINCT GROUP BY From this estimate the optimizer is able to find an efficient execution plan that fulfills a query request. You will note that…
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One of the things I’ve been able to implement to help with performance is changing from Update Statistics Synchronous to Auto Update Statistics Asynchronously. It’s a simple change that can have a big impact when implemented in highly transactional OLTP environments. Notice I said OLTP not OLAP, since data in an OLAP environment tends to not be as dynamic, so it’s rare to enable this in a data warehouse. So, what’s the difference between the two and why does it help? Synchronous (defaulted as AUTO_UPDATE_STATISTICS =TRUE) By default, when Auto Update Statistics is set to True, the SQL Server Query Optimizer will automatically update statistics when data has met a threshold…
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OK So, I am doing some digging and peaking around again in SQL Server and came across a database option called Date Correlation Optimization Enabled = False. Honestly, I had no clue what it did, so I took it as a learning opportunity to look into it and do a little research. Who knows, it may actual help me solve one of the many problems I run into day to day for clients. Syntax ALTER DATABASE DEMO SET DATE_CORRELATION_OPTIMIZATION ON; So, What Does It Do? According to MSDN – The DATE_CORRELATION_OPTIMIZATION database SET option improves the performance of queries that perform an equi-join between two tables whose date or datetime columns are correlated,…
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How many of you check the amount of Virtual Log Files (VLFs) your transaction logs have? Working as a consultant now, I see this as something that is often ignored by DBAs. This is an easy thing maintain and yet so many don’t know how to. Keeping these in check can give you a performance boost not only on startup but with your insert/update/delete as well as backup/restore operations. SQL Server performs better with a smaller number of right sized virtual log files. I highly recommend you add this to your server reviews. What is a VLF? Every transaction log is composed of smaller segments called virtual log files. Every…
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I am honored to be a Guest Blogger at SQLPerformance.com. As a member of the SentryOne Product Advisory Council (PAC) I will be writing occasionally for the site. You can catch my first blog post It’s Not You It’s Me (I/O Troubleshooting) through simple changes. SQLPerformance.com is about providing innovative and practical solutions for improving SQL Server performance. Whether you are running a 3rd party application database where very little can be changed, or you are a DBA at a site where getting the application developers to change anything is next to impossible, they cover both the “how” and the “why.”
