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Many SQL Server books and Query Processor references uses these four core terms described above (Selectivity, Density, Cardinality and SARG) to explain the behavior of a query plan, so hopefully this knowledge will make things a little clearer for you in the future.
Early on in the optimization phase, the QO tries to change your query to evaluate an expression, and then change that expression to a constant.
Well basically, you should know to avoid the use of these constructions in your query, as they’ll just make life more difficult.
A constant is a literal value like such as ‘1’, ‘Peter’, ‘10.50’, ‘20090101’, etc. A simple example of Constant Folding is the expression “.
It is good to know where the QO can do this, because that can make our queries a little easier to write.
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. Well, here in Brazil we’ve a famous soccer player and coach called Mario Zagalo who really likes it.
” using the density information we can make the following, 1.0 / 0.0002 = 5000.