scalar subquery costing
Date: Fri, 24 May 2019 15:46:30 +0000
Message-ID: <123977_1558712800_5CE811E0_123977_3376_1_276e6e70afd44f6e8fb06118262cf24a_at_vontobel.com>
The calculated cost for the following query is 262K (on Oracle 12.2):
select /*+ qb_name(QB_MAIN) */
(
select /*+ qb_name(QB_SUBQ) */ count(*)
from t_1k
where t_1k.n1 = t_100k.n1
)
from t_100k ;
Plan Table
--------------------------------------+-----------------------------------+| 3 | TABLE ACCESS FULL | T_100K | 98K | 488K | 69 | 00:00:01 |
| Id | Operation | Name | Rows | Bytes | Cost | Time |
--------------------------------------+-----------------------------------+
| 0 | SELECT STATEMENT | | | | 262K | |
| 1 | SORT AGGREGATE | | 1 | 4 | | |
| 2 | TABLE ACCESS FULL | T_1K | 1 | 4 | 3 | 00:00:01 |
--------------------------------------+-----------------------------------+Predicate Information:
2 - filter("T_1K"."N1"=:B1)
From the optimizer trace we can get more precise cardinalities and costs:
Final cost for query block QB_SUBQ (#0) - All Rows Plan:
Best join order: 1
Cost: 3.009811 Degree: 1 Card: 1.000000 Bytes: 4.000000
Final cost for query block QB_MAIN (#0) - All Rows Plan:
Best join order: 1
Cost: 268174.664510 Degree: 1 Card: 100000.000000 Bytes: 500000.000000
Table: T_100K Alias: T_100K
Card: Original: 100000.000000 Rounded: 100000 Computed: 100000.000000 Non Adjusted: 100000.000000
...
Access Path: TableScan
Cost: 68.697001 Resp: 68.697001 Degree: 0
Cost_io: 68.000000 Cost_cpu: 16737631
In theory, the total cost should be caculated as follows. cardinality(QB_MAIN) * cost(QB_SUBQ) + cost(full table scan T_100K) = 100000 * 3.009811 + 68.69701 = 301049.79701
How the optimizer came up with 268174.664510?
Best regards,
Nenad
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-- http://www.freelists.org/webpage/oracle-lReceived on Fri May 24 2019 - 17:46:30 CEST