The Index-Only Scan node scans only the specified indexes and will not visit parent tables
The Index Only Scan node scans only indexes and will not visit parent tables. This makes it potentially faster than the Index Scan node, but with the caveats that the index must support index scanning, and it must contain all the columns to required by this stage of the query.
- PostgreSQL Documentation - Using Explain
- pgAnalyze - Explain
- eTutorials - Understanding How PostgreSQL Executes a Query
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