再帰的SELECTの例
次の例では再帰的問合わせを示しています。
EXPLAIN WITH RECURSIVE temp_table (employee_number, depth) AS (SELECT root.employee_number, 0 as depth FROM Employee root WHERE root.manager_employee_number = 801 UNION ALL SELECT indirect.employee_number, seed.depth+1 as depth FROM temp_table seed, Employee indirect WHERE seed.employee_number = indirect.manager_employee_number AND depth <= 20) SELECT employee_number, depth FROM temp_table;
EXPLAINではこのリクエストに対して次の応答を生成します。
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2) Next, we lock PERSONNEL.root for read.
3) We do an all-AMPs RETRIEVE step from PERSONNEL.root by
way of an all-rows scan with a condition of (
"PERSONNEL.root.manager_employee_number = 801") into
Spool 3 (all_amps), which is built locally on the AMPs. The size
of Spool 3 is estimated with no confidence to be 1 row. The
estimated time for this step is 0.06 seconds.
4) We do an all-AMPs RETRIEVE step from Spool 3 by way of an all-rows
scan into Spool 2 (all_amps), which is built locally on the AMPs.
The size of Spool 2 is estimated with no confidence to be 1 row.
The estimated time for this step is 0.07 seconds.
5) We execute the following steps in parallel.
1) We do an all-AMPs RETRIEVE step from Spool 3 (Last Use) by
way of an all-rows scan with a condition of ("(DEPTH <= 20)
AND (NOT (EMPLOYEE_NUMBER IS NULL ))") into Spool 4
(all_amps), which is duplicated on all AMPs. The size of
Spool 4 is estimated with no confidence to be 8 rows. The
estimated time for this step is 0.03 seconds.
2) We do an all-AMPs RETRIEVE step from
PERSONNEL.indirect by way of an all-rows scan with a
condition of
("NOT (PERSONNEL.indirect.manager_employee_number IS NULL)")
into Spool 5 (all_amps), which is built locally on the AMPs.
The size of Spool 5 is estimated with no confidence to be 8
rows. The estimated time for this step is 0.01 seconds.
6) 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) by way of an
all-rows scan. Spool 4 and Spool 5 are joined using a single
partition hash join, with a join condition of ("EMPLOYEE_NUMBER =
manager_employee_number"). The result goes into Spool 6
(all_amps), which is built locally on the AMPs. The size of Spool
6 is estimated with no confidence to be 3 rows. The estimated
time for this step is 0.08 seconds.
7) We do an all-AMPs RETRIEVE step from Spool 6 (Last Use) by way of
an all-rows scan into Spool 3 (all_amps), which is built locally
on the AMPs. The size of Spool 3 is estimated with no confidence
to be 4 rows. The estimated time for this step is 0.07 seconds.
If one or more rows are inserted into spool 3, then go to step 4.
8) We do an all-AMPs RETRIEVE step from Spool 2 (Last Use) by way of
an all-rows scan into Spool 7 (all_amps), which is built locally
on the AMPs. The size of Spool 7 is estimated with no confidence
to be 142 rows. The estimated time for this step is 0.07 seconds.
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ステップ3は再帰問合わせ内のシード文の処理を指定し、最初の一時的な結果セットを作成します。
ステップ4～7は、再帰問合わせ内の再帰的文の処理と一時結果セットに挿入する新しい行がなくなるまでの繰り返しに相当します。ステップ4～7は静的計画を示しますが、各繰り返しではさまざまなカーディナリティを持つスプールが生成される可能性があります。このため、これらのステップではスプール サイズの信頼度のレベルは信頼度なしに設定されます。
ステップ8ではユーザーに送信される最終結果の処理を指します。