Answer
Table scans read every row, while index scans use index trees to quickly locate matching rows. Index scans improve performance when filtering selective values.
Quick Answer
Index scan: MySQL uses the index B-Tree to find rows. Can be range scan (range of index values) or full index scan (all index values in order). Faster than table scan for selective queries. Table scan (full scan): reads every row in the table sequentially - used when no suitable index exists or when the optimizer estimates it's cheaper than the index (e.g., retrieving 70% of rows). Seen in EXPLAIN as type=ALL.
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SugharaIQ Editorial Team
Verified Answer
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Source: SugharaIQ