Why do incorrect cardinality estimates hurt performance?

Problem Statement

Explanation

The cost model relies on estimated row counts. If estimates are wrong, the optimizer may choose scans, wrong join algorithms, or expensive rehashes. Refreshing statistics, using SARGable predicates, and avoiding functions on columns help the engine estimate more accurately.

Code Solution

SolutionRead Only

-- PostgreSQL
ANALYZE sales;
-- SQL Server
UPDATE STATISTICS sales;

Practice Sets

This question appears in the following practice sets:

Query Optimization & Execution Plans

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Why do incorrect cardinality estimates hurt performance?

Problem Statement

Explanation

Code Solution

Practice Sets

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Master Interviews Anywhere, Anytime

Why do incorrect cardinality estimates hurt performance?

Problem Statement

Explanation

Code Solution

Practice Sets

Related Questions

Which SQL statement retrieves all columns and rows from a table named employees?

Which clause filters rows before aggregation and affects which rows are grouped?

What is the default sort order when ORDER BY is used without ASC or DESC?

Which construct is commonly used to paginate results in SQL engines like Postgres and MySQL?

Which predicate correctly checks for missing values in standard SQL?

More from SQL & Databases

Master Interviews
Anywhere, Anytime