What is SQL Server compilation pipeline cost breakdown model?

SQL Server estimates cost based on CPU, IO, and memory usage models for each operator.

The optimizer assigns cost units to each possible execution plan. Costs are derived from estimated IO (logical reads), CPU operations, and memory usage (sort/hash size). Each operator has a cost formula influenced by statistics and cardinality estimates. The optimizer explores multiple join orders and access paths, pruning high-cost branches using heuristics and thresholds to reduce search space complexity.