store/mockstore/unistore: fix several issues of coprocessor paging in unistore

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #36145

Problem Summary:

What is changed and how it works?

Fix several bugs.

1. I mistakenly use break ... which should be return

for {
    switch {
        case xxx:
-              break
+             return   // To break the loop, it should be "return"
    }
}

2. To store the key/value in a iterator function, deep copy should be used

dbReader.Scan(keyRange, func(key, value []byte) {
    // key value is borrowed from dbReader 
    // To save the key/value for outside usage, a deep copy is necessary
})

3. When paging size < topN limit count, the cop-pushed-down topN executor should degenerate to no operation.

Take this as example:

mysql> explain select a from trange use index(idx_a) where a > 84 order by a limit 400;
+-----------------------------+---------+-----------+------------------------------+-------------------------------------------------+
| id                          | estRows | task      | access object                | operator info                                   |
+-----------------------------+---------+-----------+------------------------------+-------------------------------------------------+
| TopN_8                      | 400.00  | root      |                              | test.trange.a, offset:0, count:400              |
| └─IndexReader_17            | 400.00  | root      | partition:all                | index:TopN_16                                   |
|   └─TopN_16                 | 400.00  | cop[tikv] |                              | test.trange.a, offset:0, count:400              |
|     └─IndexRangeScan_15     | 3333.33 | cop[tikv] | table:trange, index:idx_a(a) | range:(84,+inf], keep order:false, stats:pseudo |
+-----------------------------+---------+-----------+------------------------------+-------------------------------------------------+
4 rows in set (0.00 sec)

CopDAG: TopN -> IndexScan

IndexScan rows = 1000
TopN count = 400
Paging size = 64

If TopN calculate the top 400, it consume the whole IndexScan rows ...
it returns 64 rows, but it can't determine the corresponding key range!

Check List

Tests

• Unit test

I found the bug when testing #36107
When @@tidb_min_paging_size is set, the current unit tests could cover the change.

• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• XuHuaiyu
• wshwsh12

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/ab2fe819269b8d6c95e3192b416e3b7481fe8fd5

[tiancaiamao]

/run-check_dev_2

[XuHuaiyu]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 4a3a584

[tiancaiamao]

/run-build

[tiancaiamao]

/run-build
/run-mysql-test

[sre-bot]

TiDB MergeCI notify

🔴 Bad News! [1] CI still failing after this pr merged.
These failed integration tests don't seem to be introduced by the current PR.

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci/integration-cdc-test	🔴 failed 1, success 35, total 36	27 min	Existing failure
idc-jenkins-ci-tidb/integration-common-test	🟢 all 11 tests passed	26 min	Existing passed
idc-jenkins-ci-tidb/common-test	🟢 all 12 tests passed	17 min	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	10 min	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	8 min 24 sec	Existing passed
idc-jenkins-ci-tidb/integration-ddl-test	🟢 all 6 tests passed	6 min 33 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	5 min 33 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	3 min 34 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	3 min 24 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed