planner: adjust N used in TopN cost formula based on the total number of rows

[qw4990]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Currently, we use the formula N*row-size*mem-factor to estimate the memory cost of TopN, but actually, min(N, total_rows)*row-size*mem-factor is better.

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

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| tidb_version()                                                                                                                                                                                                                                                                                                  |
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| Release Version: v7.1.0-alpha-299-gee4fb2965d
Edition: Community
Git Commit Hash: ee4fb2965d07a6363711ee942d08d29e14bc1eb4
Git Branch: non-prep-debugx
UTC Build Time: 2023-04-20 09:10:06
GoVersion: go1.20.2
Race Enabled: false
TiKV Min Version: 6.2.0-alpha
Check Table Before Drop: false
Store: unistore |
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