Cardinality Estimates

External Article

Fixing Gatekeeper Row Cardinality Estimate Issues

  • Article

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.

2012-12-10

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Question of the Day

ORDER BY alias

There is a table tmp_tab: 

CREATE TABLE tmp_tab (
id int,
val int
);
INSERT INTO tmp_tab VALUES
(1, 1),
(2, NULL),
(3, 3),
(4, 4),
(5, 5);
You want to order the rows ids by the following expression: 
    ISNULL(val, id) + 1
Which of the following queries produces the expected ordering and why? (Select all correct)

See possible answers