sessionctx/variable: turn off TiDBEnablePseudoForOutdatedStats by default

[xuyifangreeneyes]

What problem does this PR solve?

Issue Number: ref #27601

Problem Summary:

What is changed and how it works?

Change the default value of TiDBEnablePseudoForOutdatedStats from true to false. After the pr, by default, when table stats are outdated, we still use the outdated stats rather than pseudo stats, which is reasonable in most cases.

Check List

Tests

• Unit test
• 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:

• chrysan
• xhebox

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[xhebox]

I think it needs some benchmark to verify...

[xuyifangreeneyes]

I think it needs some benchmark to verify...

In CE Benchmark on stale statistics, when stats healthy is 50%, using outdated stats(tidb_enable_pseudo_for_outdated_stats = off) has a smaller q-error then using pseudo stats(tidb_enable_pseudo_for_outdated_stats = on) when doing cardinality estimation benchmark on IMDB + JOB.

[xhebox]

/merge

[ti-chi-bot]

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

Commit hash: 849128c

[hawkingrei]

@pingcap/tidb-configuration-reviewer PTAL

/hold

[xhebox]

/unhold

[hawkingrei]

/hold

@xuyifangreeneyes Please fix test.

[xuyifangreeneyes]

Found a unreasonable behavior when tidb_enable_pseudo_for_outdated_stats is false:

mysql> create table t(a int,  b int, c int,  index idx_a(a));
Query OK, 0 rows affected (0.02 sec)

mysql> show stats_meta;
+---------+------------+----------------+---------------------+--------------+-----------+
| Db_name | Table_name | Partition_name | Update_time         | Modify_count | Row_count |
+---------+------------+----------------+---------------------+--------------+-----------+
| test    | t          |                | 2022-08-03 23:46:19 |            0 |         0 |
+---------+------------+----------------+---------------------+--------------+-----------+
1 row in set (0.04 sec)

mysql> explain select * from t;
+-----------------------+----------+-----------+---------------+--------------------------------+
| id                    | estRows  | task      | access object | operator info                  |
+-----------------------+----------+-----------+---------------+--------------------------------+
| TableReader_5         | 10000.00 | root      |               | data:TableFullScan_4           |
| └─TableFullScan_4     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-----------------------+----------+-----------+---------------+--------------------------------+
2 rows in set (0.00 sec)

mysql> insert into t values (1,2,2), (3,4,4), (5,6,6), (7,8,8), (9,10,10);
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> show stats_meta;
+---------+------------+----------------+---------------------+--------------+-----------+
| Db_name | Table_name | Partition_name | Update_time         | Modify_count | Row_count |
+---------+------------+----------------+---------------------+--------------+-----------+
| test    | t          |                | 2022-08-03 23:46:19 |            0 |         0 |
+---------+------------+----------------+---------------------+--------------+-----------+
1 row in set (0.00 sec)

mysql> explain select * from t;
+-----------------------+----------+-----------+---------------+--------------------------------+
| id                    | estRows  | task      | access object | operator info                  |
+-----------------------+----------+-----------+---------------+--------------------------------+
| TableReader_5         | 10000.00 | root      |               | data:TableFullScan_4           |
| └─TableFullScan_4     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-----------------------+----------+-----------+---------------+--------------------------------+
2 rows in set (0.00 sec)

mysql> show stats_meta;
+---------+------------+----------------+---------------------+--------------+-----------+
| Db_name | Table_name | Partition_name | Update_time         | Modify_count | Row_count |
+---------+------------+----------------+---------------------+--------------+-----------+
| test    | t          |                | 2022-08-03 23:47:56 |            5 |         5 |
+---------+------------+----------------+---------------------+--------------+-----------+
1 row in set (0.00 sec)

mysql> explain select * from t;
+-----------------------+---------+-----------+---------------+----------------------+
| id                    | estRows | task      | access object | operator info        |
+-----------------------+---------+-----------+---------------+----------------------+
| TableReader_5         | 5.00    | root      |               | data:TableFullScan_4 |
| └─TableFullScan_4     | 5.00    | cop[tikv] | table:t       | keep order:false     |
+-----------------------+---------+-----------+---------------+----------------------+
2 rows in set (0.00 sec)

When modify_count is 5 and row_count is 5, the optimizer doesn't think that it uses pseudo stats. However, it actually does use pseudo stats because it hasn't collected any stats so far.

[hawkingrei]

/unhold

[hawkingrei]

/merge

[ti-chi-bot]

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

Commit hash: 70abc97

[sre-bot]

TiDB MergeCI notify

✅ Well Done! New fixed [1] after this pr merged.

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/integration-common-test	🔴 failed 1, success 16, total 17	21 min	Existing failure
idc-jenkins-ci-tidb/integration-ddl-test	✅ all 6 tests passed	5 min 1 sec	Fixed
idc-jenkins-ci/integration-cdc-test	🟢 all 37 tests passed	25 min	Existing passed
idc-jenkins-ci-tidb/common-test	🟢 all 11 tests passed	13 min	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	6 min 2 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	5 min 0 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	4 min 49 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	4 min 41 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	3 min 41 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed

[xuyifangreeneyes]

Found a unreasonable behavior when tidb_enable_pseudo_for_outdated_stats is false:

mysql> create table t(a int,  b int, c int,  index idx_a(a));
Query OK, 0 rows affected (0.02 sec)

mysql> show stats_meta;
+---------+------------+----------------+---------------------+--------------+-----------+
| Db_name | Table_name | Partition_name | Update_time         | Modify_count | Row_count |
+---------+------------+----------------+---------------------+--------------+-----------+
| test    | t          |                | 2022-08-03 23:46:19 |            0 |         0 |
+---------+------------+----------------+---------------------+--------------+-----------+
1 row in set (0.04 sec)

mysql> explain select * from t;
+-----------------------+----------+-----------+---------------+--------------------------------+
| id                    | estRows  | task      | access object | operator info                  |
+-----------------------+----------+-----------+---------------+--------------------------------+
| TableReader_5         | 10000.00 | root      |               | data:TableFullScan_4           |
| └─TableFullScan_4     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-----------------------+----------+-----------+---------------+--------------------------------+
2 rows in set (0.00 sec)

mysql> insert into t values (1,2,2), (3,4,4), (5,6,6), (7,8,8), (9,10,10);
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> show stats_meta;
+---------+------------+----------------+---------------------+--------------+-----------+
| Db_name | Table_name | Partition_name | Update_time         | Modify_count | Row_count |
+---------+------------+----------------+---------------------+--------------+-----------+
| test    | t          |                | 2022-08-03 23:46:19 |            0 |         0 |
+---------+------------+----------------+---------------------+--------------+-----------+
1 row in set (0.00 sec)

mysql> explain select * from t;
+-----------------------+----------+-----------+---------------+--------------------------------+
| id                    | estRows  | task      | access object | operator info                  |
+-----------------------+----------+-----------+---------------+--------------------------------+
| TableReader_5         | 10000.00 | root      |               | data:TableFullScan_4           |
| └─TableFullScan_4     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-----------------------+----------+-----------+---------------+--------------------------------+
2 rows in set (0.00 sec)

mysql> show stats_meta;
+---------+------------+----------------+---------------------+--------------+-----------+
| Db_name | Table_name | Partition_name | Update_time         | Modify_count | Row_count |
+---------+------------+----------------+---------------------+--------------+-----------+
| test    | t          |                | 2022-08-03 23:47:56 |            5 |         5 |
+---------+------------+----------------+---------------------+--------------+-----------+
1 row in set (0.00 sec)

mysql> explain select * from t;
+-----------------------+---------+-----------+---------------+----------------------+
| id                    | estRows | task      | access object | operator info        |
+-----------------------+---------+-----------+---------------+----------------------+
| TableReader_5         | 5.00    | root      |               | data:TableFullScan_4 |
| └─TableFullScan_4     | 5.00    | cop[tikv] | table:t       | keep order:false     |
+-----------------------+---------+-----------+---------------+----------------------+
2 rows in set (0.00 sec)

When modify_count is 5 and row_count is 5, the optimizer doesn't think that it uses pseudo stats. However, it actually does use pseudo stats because it hasn't collected any stats so far.

It is solved by #37444