expression: Add failpoint to force pushdown expression to tikv for debug usage

[yibin87]

What problem does this PR solve?

Issue Number: ref #51876

Problem Summary:
To debug expression behaviors not consistent between tidb and tikv, we need to force push down these projection expressions to tikv ignoring cost consideration. So we add a failpoint here to help debug tikv expression behavior.

What changed and how does it work?

mysql> create table t0(a int, b int);
Query OK, 0 rows affected (0.02 sec)

mysql> insert into t0 values(3, 20);
Query OK, 1 row affected (0.01 sec)

mysql> insert into t0 values(29, 30);
Query OK, 1 row affected (0.01 sec)

mysql> explain select a-b, a+b from t0;
+-------------------------+----------+-----------+---------------+-----------------------------------------------------------------------------+
| id                      | estRows  | task      | access object | operator info                                                               |
+-------------------------+----------+-----------+---------------+-----------------------------------------------------------------------------+
| Projection_3            | 10000.00 | root      |               | minus(test.t0.a, test.t0.b)->Column#4, plus(test.t0.a, test.t0.b)->Column#5 |
| └─TableReader_5         | 10000.00 | root      |               | data:TableFullScan_4                                                        |
|   └─TableFullScan_4     | 10000.00 | cop[tikv] | table:t0      | keep order:false, stats:pseudo                                              |
+-------------------------+----------+-----------+---------------+-----------------------------------------------------------------------------+
3 rows in set (0.00 sec)

Now, after build tidb with failpoint enable: "make failpoint-enable"
export GO_FAILPOINTS="github.com/pingcap/tidb/pkg/expression/forcePushDownTiKV=return(true)"
start tidb server, then a+b and a-b will be pushed down to tikv side:

mysql> explain select a-b, a+b from t0;
+-------------------------+----------+-----------+---------------+-----------------------------------------------------------------------------+
| id                      | estRows  | task      | access object | operator info                                                               |
+-------------------------+----------+-----------+---------------+-----------------------------------------------------------------------------+
| TableReader_8           | 10000.00 | root      |               | data:Projection_4                                                           |
| └─Projection_4          | 10000.00 | cop[tikv] |               | minus(test.t0.a, test.t0.b)->Column#4, plus(test.t0.a, test.t0.b)->Column#5 |
|   └─TableFullScan_7     | 10000.00 | cop[tikv] | table:t0      | keep order:false, stats:pseudo                                              |
+-------------------------+----------+-----------+---------------+-----------------------------------------------------------------------------+
3 rows in set (0.00 sec)

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No need to test

  • I checked and no code files have been changed.

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

[tiprow]

Hi @yibin87. Thanks for your PR.

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[yibin87]

/cc @windtalker @xzhangxian1008

[xzhangxian1008]

LGTM

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 73.8347%. Comparing base (15f3c28) to head (db2fc85).
Report is 9 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #59765        +/-   ##
================================================
+ Coverage   72.9819%   73.8347%   +0.8527%     
================================================
  Files          1698       1699         +1     
  Lines        469046     472951      +3905     
================================================
+ Hits         342319     349202      +6883     
+ Misses       105654     102733      -2921     
+ Partials      21073      21016        -57     

Flag	Coverage Δ
integration	43.2976% <25.0000%> (?)
unit	72.6442% <25.0000%> (+0.4630%)	⬆️

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Components	Coverage Δ
dumpling	52.6910% <ø> (ø)
parser	∅ <ø> (∅)
br	45.0647% <ø> (+0.0394%)	⬆️

[windtalker]

lgtm

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: windtalker, xzhangxian1008

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2025-02-26 02:38:18.217800855 +0000 UTC m=+409846.170959122: ☑️ agreed by xzhangxian1008.
• 2025-02-26 09:30:49.126824951 +0000 UTC m=+434597.079983210: ☑️ agreed by windtalker.