scalar subquery costing

From: Noveljic Nenad <>
Date: Fri, 24 May 2019 15:46:30 +0000
Message-ID: <>

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

| 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 |
| 3 | TABLE ACCESS FULL | T_100K | 98K | 488K | 69 | 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,


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Received on Fri May 24 2019 - 17:46:30 CEST

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