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Tips for How Oracle Chooses between an Index and a Sort

Don Burleson recently addressed a question on the Oracle Forum about how the Oracle optimizer chooses between performing a sort vs. using an index to deliver ordered results with an ORDER BY clause in the SQL:


He notes:


Oracle will always use the cheapest method for sequence an SQL result set and the CBO will use an index if it consumes fewer resources than a sort.  Remember, a sort that cannot fit into RAM will have to be done in the TEMP tablespace, very slow with lots of disk I/O.


You "could" use an /*+ index */ hint to force index sequencing, but folks rarely do.

The choice depends on several factors:

0 CBO optimizer_mode parameter The all_rows optimizer mode favors non-index sorting while first_rows favors index access and retrieval in index order


1 The cpu_cost parameter favors indexes, the io_cost setting favors sorting

2 - The size of your sort_area_size (or pga_aggregate_target, if used)

3 - The clustering_factor of the index (see dba_indexes view)

4 - The db_block_size and avg_row_len

5 - The estimated size of the sorted result set


6 The quality of your optimizer statistics, as computed by the dbms_stats package


7 The quality of CBO histograms for estimating the size of the final result set
Tip! In some cases Oracle is able to bypass a sort by reading the data in sorted order from the index. Oracle will even read data in reverse order from an index to avoid an in-memory sort.


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