Answer
The Cardinality Estimator (CE) predicts row counts used to choose join types, memory grants, and access paths.
Poor estimates ? poor execution plans.
CE depends on statistics, data distribution, and query structure.
Quick Answer
The cardinality estimator predicts how many rows each plan step will produce based on statistics histograms. Wrong predictions lead to wrong join types, bad memory grants, and poor index choices. SQL Server 2014 introduced a new CE (model 120); older compatibility levels use the legacy model. Statistics quality is the root input.
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SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ