NOTE
MySQL Query Executor
Executing an optimizer-selected plan through iterators/operators and storage-engine row access.
This is a historical learning note and may contain outdated or incomplete understanding.
After optimization, the executor runs the chosen plan and requests rows through access methods exposed by the storage engine. Operators perform scans/lookups, joins, filtering, sorting, grouping, and projection as required by the plan.
Execution cost depends on actual cardinalities, cache state, I/O, CPU, locks, and data distribution. EXPLAIN ANALYZE-style runtime measurements are valuable when estimates and reality diverge.