Articles tagged

When AUTO_UPDATE_STATISTICS Doesn’t Happen

When your SQL Server database is set to have its statistics automatically updated, you will probably conclude that, whenever the distribution statistics are out-of-date, they will be updated before the next query is executed against that index or table. Curiously, this isn't always the case. What actually happens is that the statistics only gets updated if needed by the query optimiser to determine an effective query plan.… Read more

Checking the Plan Cache Warnings for a SQL Server Database

How often do you check your query plans during development to see if they contain any warnings? If you're missing them, it means that you're not getting all those hints about missing indexes, join predicates or statistics. Is the query optimiser trying to tell you about implicit conversions? Dennes shows how to view the warnings in plan cache for a particular database using SQL… Read more

Join Reordering and Bushy Plans

Normally, the query Optimiser won't consider 'bushy' plans, where both operands to a join operator could be intermediate results from other joins. This means that it won't produce good query plans for some types of query. Hints, by themselves won't do it. More powerful magic is required.… Read more

Using Optimizer_WhatIF and StatsStream to Simulate a Production Environment

SQL Server's Query optimiser judges the best query plan from the data in the relevant tables and the server's hardware. How, then, can you investigate the query plans being generated for slow-running queries on a customer's production server when you can neither access the server, nor recreate the database from a backup?… Read more

Statistics in SQL Server

SQL Server's Query Optimiser relies on statistics for providing a decent query plan. If the statistics are wrong, outdated or missing then you have little realistic hope of having queries that perform well. It is therefore important to appreciate how SQL Server maintains distribution statistics.… Read more

Fixing Gatekeeper Row Cardinality Estimate Issues

The Query Optimiser needs a good estimate of the number of rows likely to be returned by each physical operator in order to select the best query plan from the most likely alternatives. Sometimes these estimates can go so wildly wrong as to result in a very slow query. Joe Sack shows how it can happen with SQL Queries on a data warehouse with a star schema.… Read more

Statistics on Ascending Columns

It comes as rather a shock to find out that one of the commonest circumstances in an OLTP database, an ascending primary key with most querying on the latest records, can throw the judgement of the Query Optimiser to the extent that perfomance nose-dives. Fabiano once again puts on snorkel and goggles to explore the murky depths of execution plans to find out why. … Read more

Showplan Operator of the Week – Stream Aggregate

Fabiano continues his mission to explain the Query Optimiser in practical terms, describing, one week at a time, all the major Showplan Operators used by SQL Server's Query Optimiser to build the Query Plan. He discusses the Stream Aggregate function and shows how easy it is to get performance gains in GROUP BY clauses by choosing the right index.… Read more

Showplan Operator of the Week – BookMark/Key Lookup

Fabiano continues in his mission to describe the major Showplan Operators used by SQL Server's Query Optimiser. This week he meets a star, the Key Lookup, a stalwart performer, but most famous for its role in ill-performing queries where an index does not 'cover' the data required to execute the query. If you understand why, and in what circumstances, key lookups are slow, it helps greatly with optimising query performance. … Read more

13 Things You Should Know About Statistics and the Query Optimizer

Fabiano launches into a sound technical explanation of the way that the query optimiser works in SQL Server with a mention of Brazilian Soccer stars and young ladies on Copacabana beach. You'll never quite think of statistics, execution plans, and the query optimiser the same way again after reading this, but we think you'll understand them better.… Read more

Controlling Execution Plans with Hints

The Query Optimizer gets it right most of the time, but occasionally it chooses a plan that isn't the best possible. You can give the Query Optimiser a better idea by using Table, Join and Query hints. These come with a risk: Any choices you force on the Optimizer by using hints can turn out to be entirely wrong as the database changes with the addition of data over time. Grant Fritchey, in a chapter from his highly acclaimed book, explains further. … Read more