入れ子結合を使用しない結合計画について
入れ子結合を使用しない場合は、最適化ルーチンで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 |
|---|---|---|
| Spool 1: 積結合 | duped table_1, direct table_2 |
1時間15分 |
実行時間の見積もりの合計は、1時間15分です。
最適化しない結合計画のEXPLAIN出力
入れ子結合が使用されないときに、次の部分的EXPLAIN出力が生成されます。
2) Next, we do an all-AMPs RETRIEVE step from test.tab1 by way of an all-rows scan with no residual conditions into Spool 2, which is duplicated on all AMPs. The size of Spool 2 is estimated to be 4 rows. The estimated time for this step is 0.08 seconds. 3) We do an all-AMPs JOIN step from Spool 2 (Last Use) by way of an all-rows scan, which is joined to test.tab2. Spool 2 and test.tab2 are joined using a product join, with a join condition of (“Spool_2.y1 = test.tab2.y2"). The result goes into Spool 1, which is built locally on the AMPs. The size of Spool 1 is estimated to be 2 rows. The estimated time for this step is 1 hour and 15 minutes.