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Index Full Scan…
Single-block I/O used to read all leaf blocks
Data returned in index sequence
Available when ORDER BY on indexed column
- Additional sort step can be avoided
If CBO doesn’t choose, and you want to try
Notes:
An index full scan uses single-block I/O to read all index leaf blocks. Note that - even though the path name includes the word “full” - this access path does not read all blocks in the index; it reads all leaf blocks, but only enough branch blocks to find the first leaf block. Once the first leaf block is found, it uses single-block I/O to read all leaf blocks from that point forward; it does not use index branch blocks after finding the first leaf block. (Remember that index leaf blocks are linked, i.e. there are forward and backward pointers connecting the leaf blocks.) The ROWIDs are returned to a table access step if necessary.
Index full scans are sometimes chosen when:
A sort can be avoided by accessing table data in index key order.
All the data for the query is stored within the index. In this case, the table itself is never accessed, as demonstrated here:
Notes for this slide continue on the next page…