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It scans every row for finding a particular node or data. If the planner fails to use a proper index, it has to do full Table Scanning.
If table has a less number of records, Table Scan is an excellent choice for query optimizer.
For example, If Table has around 1000 records, Table Scan is faster because unnecessary Index Scanning operation is not required.
But, If the Table has a large number of records, Table Scan requires high CPU and other resources for scanning the whole table.
It is scanning of each row, whenever you are retrieving more than 50% or 70% of the table data, internally planner performing full Index Scan.
Generally, Index pages and Data pages are stored separately.
Index scanning is also dependent on the ordering of the Index data because If matched key found from the top pages, Index Scan is not required.
The Row Lookups also involved in Index Scanning because matched Index key has to fetch data from the data page, so Index Scanning is very costly for the big table.
Index Seek is opposite the Table Scan and Index Scan.
It touches the only qualified row index rather than scanning whole Index Pages, and after that, it fetches only qualified data page.
When we are fetching less number of records with the specific filter, the planner has to choose Index Seek to fetch and scan only qualified data.
Index Seek operation is much faster than Index Scan, and also it does not require more CPU and I/O resources.