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statistics: mark NDV as deprecated and update variable scopes #18499

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Aug 7, 2024
Merged

statistics: mark NDV as deprecated and update variable scopes #18499

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5e8379d
statistics: mark NDV as deprecated
Rustin170506 Aug 6, 2024
4d53004
statistics: correct the scopes for some variables
Rustin170506 Aug 6, 2024
58d7404
refine wording
Oreoxmt Aug 7, 2024
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4 changes: 2 additions & 2 deletions sql-statements/sql-statement-show-stats-buckets.md
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Expand Up @@ -21,7 +21,7 @@ Currently, the `SHOW STATS_BUCKETS` statement returns the following columns:
| `Repeats` | The occurrence number of the maximum value |
| `Lower_bound` | The minimum value |
| `Upper_bound` | The maximum value |
| `Ndv` | The number of different values in the bucket. When `tidb_analyze_version` = `1`, `Ndv` is always `0`, which has no actual meaning. |
| `Ndv` | The number of distinct values in the bucket. This field is deprecated and always shows `0` due to its inaccurate value. |

## Synopsis

Expand Down Expand Up @@ -61,4 +61,4 @@ This statement is a TiDB extension to MySQL syntax.
## See also

* [`ANALYZE`](/sql-statements/sql-statement-analyze-table.md)
* [Introduction to Statistics](/statistics.md)
* [Introduction to Statistics](/statistics.md)
6 changes: 3 additions & 3 deletions system-variables.md
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Expand Up @@ -1114,7 +1114,7 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a

### tidb_analyze_distsql_scan_concurrency <span class="version-mark">New in v7.6.0</span>

- Scope: GLOBAL
- Scope: SESSION | GLOBAL
- Persists to cluster: Yes
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
- Type: Integer
Expand Down Expand Up @@ -1360,7 +1360,7 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;

### tidb_build_sampling_stats_concurrency <span class="version-mark">New in v7.5.0</span>

- Scope: GLOBAL
- Scope: SESSION | GLOBAL
- Persists to cluster: Yes
- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
- Type: Integer
Expand Down Expand Up @@ -4275,7 +4275,7 @@ mysql> desc select count(distinct a) from test.t;
+-----------------------------------+---------+-----------+-----------------------+---------------------------------+
```

- The second example uses `0`, which assumes that 0% of rows will be scanned before the qualified rows are found.
- The second example uses `0`, which assumes that 0% of rows will be scanned before the qualified rows are found.

```sql
> SET SESSION tidb_opt_ordering_index_selectivity_ratio = 0;
Expand Down
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