参考文档：
https://docs.oracle.com/database/121/CNCPT/indexiot.htm#CNCPT1181
Clustering Factor (Doc ID 39836.1)
Index Clustering Factor
The index clustering factor measures row order in relation to an indexed value such as employee last name. As the degree of order increases, the clustering factor decreases.
The clustering factor is useful as a rough measure of the number of I/Os required to read an entire table using an index:
If the clustering factor is high, then Oracle Database performs a relatively high number of I/Os during a large index range scan. The index entries point to random table blocks, so the database may have to read and reread the same blocks over and over again to retrieve the data pointed to by the index.
If the clustering factor is low, then Oracle Database performs a relatively low number of I/Os during a large index range scan. The index keys in a range tend to point to the same data block, so the database does not have to read and reread the same blocks over and over.
The clustering factor is relevant for index scans because it can show:
Whether the database will use an index for large range scans
The degree of table organization in relation to the index key
Whether you should consider using an index-organized table, partitioning, or table cluster if rows must be ordered by the index key
Example 3-4 Clustering Factor
Assume that the employees table fits into two data blocks. Table 3-1 depicts the rows in the two data blocks (the ellipses indicate data that is not shown).
Table 3-1 Contents of Two Data Blocks in the Employees Table
Data Block 1Data Block 2 100 Steven King SKING ... 156 Janette King JKING ... 115 Alexander Khoo AKHOO ... . . . 116 Shelli Baida SBAIDA ... 204 Hermann Baer HBAER ... 105 David Austin DAUSTIN ... 130 Mozhe Atkinson MATKINSO ... 166 Sundar Ande SANDE ... 174 Ellen Abel EABEL ... 149 Eleni Zlotkey EZLOTKEY ... 200 Jennifer Whalen JWHALEN ... . . . 137 Renske Ladwig RLADWIG ... 173 Sundita Kumar SKUMAR ... 101 Neena Kochar NKOCHHAR ...Rows are stored in the blocks in order of last name (shown in bold). For example, the bottom row in data block 1 describes Abel, the next row up describes Ande, and so on alphabetically until the top row in block 1 for Steven King. The bottom row in block 2 describes Kochar, the next row up describes Kumar, and so on alphabetically until the last row in the block for Zlotkey.
Assume that an index exists on the last name column. Each name entry corresponds to a rowid. Conceptually, the index entries would look as follows:
Abel,block1row1 Ande,block1row2 Atkinson,block1row3 Austin,block1row4 Baer,block1row5 . . .Assume that a separate index exists on the employee ID column. Conceptually, the index entries might look as follows, with employee IDs distributed in almost random locations throughout the two blocks:
100,block1row50 101,block2row1 102,block1row9 103,block2row19 104,block2row39 105,block1row4 . . .The following statement queries the ALL_INDEXES view for the clustering factor for these two indexes:
SQL> SELECT INDEX_NAME, CLUSTERING_FACTOR 2 FROM ALL_INDEXES 3 WHERE INDEX_NAME IN ('EMP_NAME_IX','EMP_EMP_ID_PK'); INDEX_NAME CLUSTERING_FACTOR -------------------- ----------------- EMP_EMP_ID_PK 19 EMP_NAME_IX 2The clustering factor for EMP_NAME_IX is low, which means that adjacent index entries in a single leaf block tend to point to rows in the same data blocks. The clustering factor for EMP_EMP_ID_PK is high, which means that adjacent index entries in the same leaf block are much less likely to point to rows in the same data blocks.
MOS上的解释：
If the count is close to the number of blocks in the table then the index is well ordered. This is true because the counter only gets incremented when the actual data is found in a different block from the last piece of row data. What this means is that when an index block is read, the table lookups required based on that block are likely to be in the same table blocks. So, the lower the Clustering Factor the less I/O is likely to be used by the statement and so the optimizer is more likely to choose that index. If the count is close to the number of rows in the table then the index is less well ordered. In this case adjacent index entries do not tend to point to the same block in the table, thus more block reads are likely to be required. An index with a higher clustering factor is more likely to have to revisit Data blocks than with a lower Clustering factor . The clustering factor can be used by the optimizer to adjust the potential number of blocks that will be accessed by a particular predicate or query. This is useful for determining the number of base table blocks that will be visited when accessed via the index. The clustering factor is effectively a count of the number of data blocks visited as the result of an index lookup. Multiplying the clustering factor by the selectivity will give the cost of the operation. It is predominately used to calculate costs for index range scans.
END
