スター結合最適化を使用する結合計画およびファクト テーブルのsubquery結合に使用されるクエリー
この例では、次のクエリーを使用します。
SELECT … WHERE widgets.color=COLOR.code AND widgets.size=SIZE.code AND widgets.options IN (SELECT OPTIONS.code);
最適化される結合計画
結合を最適化した場合には、大さなテーブルの非固有セカンダリ インデックスが結合列の集まり(color、size、およびoptions)で構成されていると、次の結合計画が生成されます。
| 操作 | 結合テーブル | 処理時間合計(秒) |
|---|---|---|
| Spool 4:プロダクト ジョイン | コピーcolor、直接size | 0.31 |
| Spool 6:積結合 | ローカル4、コピーoptions | 4.46 |
| Spool 1:入れ子結合 | コピー6、インデックスwidgets | 22.73 |
完了時間
実行時間の見積もりは、27.40秒です。
性能向上係数の見積もりは、245です。
最適化する結合計画のEXPLAIN出力
この最適化された結合計画のEXPLAIN出力の一部を次に示します。
2) Next, we execute the following steps in parallel. a) We do an all-AMPs RETRIEVE step from TEST.Options by way of an all-rows scan with no residual conditions into Spool 2, which is redistributed by hash code to all AMPs. Then we do a SORT to order Spool 2 by the sort key in spool field1 eliminating duplicate rows. The size of Spool 2 is estimated to be 10 rows. The estimated time for this step is 0.19 seconds. b) We do an all-AMPs RETRIEVE step from TEST.Color by way of an all-rows scan with no residual conditions into Spool 3, which is duplicated on all AMPs. The size of Spool 3 is estimated to be 4 rows. The estimated time for this step is 0.08 seconds. 3) We execute the following steps in parallel. a) We do an all-AMPs JOIN step from Spool 3 (Last Use) by way of an all-rows scan, which is joined to TEST.Size. Spool 3 and TEST.Size are joined using a product join. The result goes into Spool 4, which is built locally on the AMPs. The size of Spool 4 is estimated to be 20 rows. The estimated time for this step is 0.23 seconds. b) We do an all-AMPs RETRIEVE step from Spool 2 (Last Use) by way of an all-rows scan into Spool 5, which is duplicated on all AMPs. The size of Spool 5 is estimated to be 20 rows. The estimated time for this step is 0.27 seconds. 4) We do an all-AMPs JOIN step from Spool 4 (Last Use) by way of an all-rows scan, which is joined to Spool 5 (Last Use). Spool 4 and Spool 5 are joined using a product join. The result goes into Spool 6, which is duplicated on all AMPs. The size of Spool 6 is estimated to be 400 rows. The estimated time for this step is 4.19 seconds. 5) We do an all-AMPs JOIN step from Spool 6 (Last Use) by way of an all-rows scan, which is joined to TEST.Widgets by way of index # 4. Spool 6 and TEST.Widgets are joined using a nested join. The result goes into Spool 1, which is built locally on the AMPs. The size of Spool 1 is estimated to be 200 rows. The estimated time for this step is 22.73 seconds.