pingcap / pingcap/tidb

A performance bug about impossible WHERE query

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#60,757 4 comments 0 reactions 0 assignees View on GitHub
severity/minor type/bug
Dominant language
Go
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Description

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
If you have a query of the form:
Q1 INTERSECT Q2 ... INTERSECT Qn,
and you know that query Qn always returns an empty set(e.g., a query with WHERE 1=2), then the entire intersection will always be empty.
I think that the query should be eliminated during optimization, as it will always return an empty set and should never consume execution time.

```
mysql> use INFORMATION_SCHEMA;
mysql> SELECT * FROM TABLES CROSS JOIN COLUMNS ON TABLES.TABLE_NAME = COLUMNS.TABLE_NAME
INTERSECT
SELECT * FROM TABLES CROSS JOIN COLUMNS ON TABLES.TABLE_NAME != COLUMNS.TABLE_NAME
INTERSECT
SELECT * FROM TABLES CROSS JOIN COLUMNS ON TABLES.TABLE_NAME > COLUMNS.TABLE_NAME
INTERSECT
SELECT * FROM TABLES CROSS JOIN COLUMNS ON TABLES.TABLE_NAME < COLUMNS.TABLE_NAME WHERE 1=2;
Empty set (1 min 7.93 sec)
```
### 2. What did you expect to see? (Required)
Return Empty set quickly

### 3. What did you see instead (Required)
Empty set (1 min 7.93 sec)

### 4. What is your TiDB version? (Required)

| Release Version: v8.5.1
Edition: Community
Git Commit Hash: fea86c8e35ad4a86a5e1160701f99493c2ee547c
Git Branch: HEAD
UTC Build Time: 2025-01-16 07:38:34
GoVersion: go1.23.4
Race Enabled: false
Check Table Before Drop: false
Store: tikv |

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