The hash GROUP BY method creates the aggregates without sorting, and is almost always faster than sort-based GROUP BY. The min and max will be easy to find for each department. The index was scanned in ascending order.. 22 Rows from table SATURN.SFRSTCR were accessed using rowid got from an index.
SORT GROUP BY NOSORT. It happens to be one of the simplest transformations in the Oracle Optimizer’s repertoire and I know that some of you are very well-informed and know about it already.
26 Rows were returned by … 23 SORT GROUP BY NOSORT 24 WINDOW SORT 25 The rows from step 24 were sorted to eliminate duplicate rows.
Snippet Name: INDEXES: SORT and NOSORT. However, if the rows are already stored in the database in ascending order you can use the NOSORT clause to skip the sorting process.
by SSWUG Research (Oren Nakdimon) In the previous post we saw that the Oracle Optimizer has a special operation – SORT GROUP BY ROLLUP – for performing several aggregations of the same population in a single scan, as long as the grouping keys are in a “rollup form”; for example: group by句に従って、並べ替え済みの 結果セットをまとめます。この処理は中間結果をバッファせず、パイプライン化して実行します。「インデックスを使ったgroup by」も参照して下さい。 HASH GROUP BY Description: When creating an index, the default method Oracle uses to sort the index is in ascending order (SORT). SORT GROUP BY NOSORT ROLLUP. If Oracle reads the index in order, it doesn't have to sort anything and doesn't need to write any temporary space to find the min and max. Unfortunately, when you include an ORDER BY clause on the same columns as those in the GROUP BY Oracle reverts to the older sort-based GROUP BY with a corresponding drop in performance. In Oracle Database 12.1.0.2, we added a new transformation called Group-by and Aggregation Elimination and it slipped through any mention in our collateral. Oracle doesn't have that exact operation, but it has something close with a combination of INDEX FULL SCAN followed by a SORT GROUP BY NOSORT.