The Subquery Scan node reads the results of a subquery
SubqueryScan is for scanning the output of a sub-query in the range table. An extra plan node is often needed above the sub-query's plan to perform expression evaluations (which can't be pushed into the sub-query without risking changing its semantics).
- PostgreSQL Documentation - Using Explain
- pgAnalyze - Explain
- eTutorials - Understanding How PostgreSQL Executes a Query
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