入れ子結合を使用する結合計画について
入れ子結合を使用した場合、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 3: 入れ子結合 | (Hashed table_1, index table_2) | 0.24秒間 |
| Spool 1: 行ID結合 | (Hashed spool_3, direct table_2) | 0.22秒間 |
実行時間の見積もりは、0.46秒です。
性能向上係数の見積もりは、9782です。
最適化する結合計画のEXPLAIN出力
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
redistributed by hash code to all AMPs. Then we do a SORT to order
Spool 2 by row hash. The size of Spool 2 is estimated to be 2 rows.
The estimated time for this step is 0.06 seconds.
3) We do a all-AMP JOIN step from Spool 2 (Last Use) by way of an
all-rows scan, which is joined to test.tab2 by way of unique
index #4 "test.tab2.y2 = test.tab1.y1" extracting row ids only.
Spool 2 and test.tab2 are joined using a nested join. The result
goes into Spool 3, which is redistributed by hash code to all AMPs.
Then we do a SORT to order Spool 3 by row hash. The size of Spool 3
is estimated to be 2 rows. The estimated time for this step is 0.18
seconds.
4) We do an all-AMPs JOIN step from Spool 3 (Last Use) by way of an
all-rows scan, which is joined to test.tab2. Spool 3 and test.tab2
are joined using a row id join. 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 0.22 seconds.