Clustered Index Scan and Clustered Index Seek for Clustered Index :
Index Seek means that SQL Server will traverse the index from the root down to the leaf level, comparing
the values in the SARG to the key values in the index rows to determine which page to look at next. Seeking
through an index typically means a root-to-leaf traversal.
Index scan, the alternative to Index Seek, means that SQL Server will stay at the leaf level and traverse just that
one level of the index from the first page to the last. I like to think of a seek as a vertical traversal of an
index and a scan as a horizontal traversal. Remember that for a clustered index, the leaf level of the index is
the table data itself, so scanning the clustered index really means scanning the table. In fact, the only time you'll
see Table Scan as an operator in the plan is for heaps.
Clustered Index Scan 相当于全表扫描
Clustered Index Seek 相当于索引扫描
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